1 Paper As a Substrate and an Active Material in Paper Electronics

Paper as a Substrate and an Active Material in Paper Electronics

Item Type Article

Authors Khan, Sherjeel M.; Nassar, Joanna M.; Hussain, Muhammad Mustafa

Citation Khan, S. M., Nassar, J. M., & Hussain, M. M. (2020). Paper as a Substrate and an Active Material in Paper Electronics. ACS Applied Electronic Materials. doi:10.1021/acsaelm.0c00484

Eprint version Post-print

DOI 10.1021/acsaelm.0c00484

Publisher American Chemical Society (ACS)

Journal ACS Applied Electronic Materials

Rights This document is the Accepted Manuscript version of a Published Work that appeared in final form in ACS Applied Electronic Materials, copyright © American Chemical Society after peer review and technical editing by the publisher. To access the final edited and published work see https://pubs.acs.org/doi/10.1021/ acsaelm.0c00484.

Download date 03/10/2021 19:29:59

Link to Item http://hdl.handle.net/10754/665989 Paper as a substrate and an active material in paper electronics

Sherjeel M. Khan1, Joanna M. Nassar2, and Muhammad M. Hussain1,3*

1 mmh labs, Electrical Engineering, Computer Electrical Mathematical Science and Engineering Division, King Abdullah University of Science and Technology (KAUST), Thuwal, Saudi Arabia 23966-6900

2 Hopkins Marine Station, Biology and Chemical Engineering Department, Stanford University, CA, USA 94305

3 Department of Electrical Engineering and Computer Sciences, University of California, Berkeley, CA USA 94720

Corresponding author’s e-mail: [email protected] or, [email protected]

Keywords: paper; electronics; flexible; sensor; point-of-care.

Abstract

Paper is an essential part of our daily life in many different ways. It is made by compressing cellulose fibers sourced from wood into thin sheets. Paper is an inherently flexible material which can transport liquids through its medium by capillary action without the need of external force. The mesh network of cellulose in paper gives it a unique set of mechanical properties. Owing to its exclusive and advantageous properties, paper is being used as an active material and a substrate in electronics. Paper as an active material means that paper is utilized in its intrinsic form without modifications. Activated (or functionalized) paper has been widely exploited in many applications but in order to take true advantage of all the beneficial properties of paper, it needs to be used in its natural produced form. Notably, paper is employed in humidity sensors, pressure sensors, and MEMS devices in its natural form. Additionally, paper is used as a substrate in additively manufactured and origami inspired electronic devices. Here, we present an overview of how paper is used to make fully flexible and low-cost devices. Furthermore, the emergence of paper-based point- of-care devices is briefly discussed.

1. Introduction

Generally, paper is created by pressing together cellulose fibers while they are still moist. Cellulose is a widely available material and it constitutes one-third of all plant matter1. The cellulose fibers in paper

are thinner than a micron but their length can be as much as tens of millimeters. Paper is a ubiquitous

material. Widely established as a recyclable material, paper products have a recovery rate of about 70

percent. According to a report by the United States Environmental Protection Agency on municipal solid

waste (MSW), in the United States, paper waste constitutes 27.4% of the total MSW. However, the MSW

recovery is dominated by paper at 51%2. The paper recycling process has significantly matured over the

past decades. It saves tremendous amount of energy and reduces deforestation. Paper has been utilized by

researchers and scientists alike to create sensors going back as far as the early 19th century when Gay-

Lussac created the famous litmus paper test to detect acids which was followed by several chemists

developing paper test sticks to detect dry chemicals3, 4. By the early 20th century, paper was being used to

detect several markers in urine5. In 1952, Martin and Synge received the Nobel Prize in chemistry for their invention of paper chromatography. By 1964, the first commercial paper testing device was launched by

Ames that was intended to be used as an instant test of blood glucose level, followed by the launch of the first commercial paper-based pregnancy test kits in 19886. By the early 21st century, research interest in

paper is booming as paper continually finds uses in many different kinds of sensing systems.

While paper has been used in many areas for centuries, over time scientists and researchers have

developed a keen interest in using paper in sensing devices. Besides recyclability, paper presents several

other advantages that make it a suitable material to be used in sensing devices. It is easy to manufacture at

much lower costs compared to the processing costs of semiconductor material7, 8. Since it is comprised of cellulose fibers compressed together, paper has a porous structure. The porous nature of paper allows it to be used as filter paper where fluids or particles smaller than the pore size can pass through and trap the larger sized particles. Scientists have predominantly used filter paper in colorimetric assays and specific chemical substance detection devices9, 10. Paper as a dielectric material can be used in numerous sensing mechanisms specially in capacitive sensing devices11. Paper also experiences capillary action, a process in

which liquid is transported across the surface of the paper without the need of external forces12. Capillarity

of papers allows transfers of liquids from its source to the point of testing without using syringe pumps,

electric field generation, microfabrication, or any external equipment13. Most importantly, capillarity of

papers is utilized in paper based microfluidic devices which are low-cost, easy-to-use, disposable, and equipment-free14. Paper-based microfluidic systems are especially prudent providing low cost healthcare

and disease screening in the developing world where infrastructure and trained medical staff are lacking15.

In such devices, paper not only acts as the channel to transport water but also as a substrate material. In

fact, paper is abundantly used as a substrate material in many sensing devices due to flexibility and good

mechanical, thermal and chemical properties.

Flexible and stretchable electronics have been gaining popularity over the years to complement the conventional rigid electronics. Rigid electronics pose integration challenges in certain applications where space is limited, or the electronics needs to be installed on curved surfaces like a pipe. Flexible and stretchable electronics convert the rigid electronics into flexible form, thus, making it possible to integrate them in tight curved spaces16, 17. As a result, paper (which is inherently flexible) has garnered a considerable

attention of the researchers working in the field of flexible electronics. Paper is compatible with certain

CMOS based material deposition processes that can be used to deposit sensing films on top of the paper

substrate to form paper-based sensors18. The properties of paper can be further enhanced by activating the surface of the paper using activation materials or by mixing the cellulose fibers with the active material in its structure19. Additionally, paper is used in energy storage elements such as paper-based supercapacitors and fuel cells20-23. Following the increased popularity and interest in electronic skin for plants and marine

species, paper has also been to create an electronics skin for humans24-28. In such a skin, paper not only acts

as the flexible biocompatible substrate, but also as a sensing material29.

In this review, the focus will be on the paper-based sensors where the paper is being used in its commonly available form. We will cover the various ways paper is being used in sensing devices but most importantly, we will present a new emerging area where (inactivated) paper itself is used as an active sensing material.

Being an active material means that the paper itself is part of the sensing structure or is responsible for the

detection of the desired stimulus. By using paper as an active medium, a new added value can be generated

for this material. Regarding the desirable properties of paper, there are other readily available materials

around us that can be similarly used in the sensing devices. Metal foils exhibit similar mechanical properties

to that of paper while enjoying a much higher thermal stability. They are low-cost, recyclable, and easy to

manufacture. Thus, in this review article we will consider materials such as metal foils under the same

umbrella of paper-based sensors.

2. Properties of Paper

Paper in general is made up of cellulose fibers but the mechanical properties of each paper type differ from each other due to differences in the material composition and manufacturing processes. It is hard to generalize a set of properties for paper as the constituents of paper vary among different types of paper. Each type of paper serves a different specific function based on its structure and composition.

Printing paper has some cellulose fiber with a large amount of filler material. The filler material can either be natural materials (limestone, clay, and talc) or synthetic alternatives (precipitated calcium carbonate, titanium dioxide, and gypsum). The quantity and type of filler materials define the structure, thickness, and appearance of the paper30. The filler dictates the; cost of production, refractive index, paper strength, brightness, energy required for drying, friction, pore size, and burn rate of the paper31-33. Fillers can

negatively affect the strength, retention, abrasion, dusting, and sheet two-sidedness.

Thus, the diversity in the types and quantity of filler material used in each type of paper is what

makes the properties of paper (in general) so diverse. This has remained a challenge for researchers to

theoretically identify the properties of the paper which they used in their research. Paper is a non-continuous

material, as it is a pressed mixture of cellulose and fillers, to which conventional stress strain models cannot

be applied. The structure, dominated by threads of cellulose fibers and filler material, overall remains non-

uniform and random in all types of paper. The elastic modulus of paper is anisotropic in nature and it

increases as the density of paper increases34. The humidity content of the paper also dominates the elastic

modulus. Larger amount of moisture content softens the material, thereby, reducing the elastic modulus of

the paper34, 35. While the mechanical strength, flexibility, and customizability of paper are the desired traits

in an engineering material, the presence of moisture in the paper and the large pore size results in a poor

thermal performance36. Thus, paper cannot be used in high temperature processes, limiting the types of

deposition processes that can be applied on paper and the number of materials that can be used (especially

those with higher curing temperatures). Cellulose itself is prone to degradation at temperatures around 100

°C which is considered a low temperature in the electronics manufacturing industry. Cellulose degradation

results in the reduction in mechanical strength of the paper37. Thus, in this review article, we will observe

that the researchers use paper only in low temperature environments and perform low temperature

deposition processes, if any.

3. Paper as a Substrate

Flexible electronics makes it possible to integrate large sized devices into miniaturized electronics systems38. Paper, inherently flexible, is widely used as the substrate for sensor, actuators, and energy storage

elements39-44. This property has been the driving force in the emergence of the field of flexible paper electronics. It is worth noting that despite having such a diverse set of properties across different types of paper, the flexibility of paper remains consistent throughout them. It is difficult to derive theoretical models of paper flexibility due to its nonuniform structure and randomness. The lack of mathematical models diminishes the possibility of using Finite Element Methods (FEM) simulations that are predominantly used to verify the theoretical models. Nevertheless, researchers have attempted to generalize some critical mechanical properties of paper by making some assumptions. In their book, Niskanen et al. have provided a comprehensive overview of the challenges associated with the developments of products that use wood- fibers (cellulose)34. Stress-strain relations are critical to studying the flexibility of a material, but they can

only be defined for continuous materials. Niskanen et al. have derived these relations from macroscopic

experiments by considering paper as a continuous material because the fiber network structure can be

considered continuous at the centimeter scale. However, when paper is being used as a substrate, different

materials are deposited or printed on it making the mechanical models ever so complex, as the interactions

between a web of cellulose fibers and commonly used deposited materials has not been studied.

To understand the feasibility of using paper as a substrate in flexible electronics, it is important to study the bending mechanics of a sensor with a paper substrate and the effect of bending on the performance. Nassar et al. used paper as a substrate to produce flexible paper-based temperature sensors45.

The temperature sensors were drawn on a post-it sticky note paper using a silver ink pen. The paper-based temperature sensor is aimed to be used in a wearable skin. Thus, it would experience bending when the subject is moving. A paper-based temperature sensor in the bent form can be seen in Figure 1a. The web like rough structure of the paper results in a strong adhesion as a result of which no peeling off of the silver ink was observed46. Nonetheless, the film of silver can potentially undergo two types of strain: tensile and

compressive strain, as depicted by Figures 1b and 1c. Before looking into the effect of stress in the film on paper, we need to understand the effects of in-plane tensile loading on paper. There are very little changes in the elastic modulus until it reaches the point of peak stress. At the point of peak stress, plastic deformations start to show in the microscopic fiber network, but the elastic stiffness of the fibers is not reduced. The elastic modulus starts to reduce after the peak stress because the long fibers start to show ductility. Thus, the stress starts to reduce after the point of peak stress. If the strain is continually applied, at some point, the network of fiber breaks and paper tears apart (Figure 1d). Although paper may appear

as a plastic or brittle material, it does not typically follow the stress-strain curve of either plastic or brittle

materials. The stress-strain curve of paper has a significant plastic region after the elastic region (Figure

1e)47-49. There is no sharp curve or peak at the point of yield stress, as is the case of typical plastic and brittle

materials (Figure 1f). These characteristics make it a feasible choice for paper to be used as a substrate, as

it is strong and flexible. Other desirable properties of paper are its high specific stiffness, and small

thickness50, 51. The thickness of paper can be on the order of tens of micrometers which further reduces its

cost52.

As established in the previous paragraph, paper is a feasible substrate in terms of providing a platform with optimal mechanical strength under tensile strain. However, when compressive strain is applied on the paper substrate, it will buckle or bend (Figure 1c). It is commonly observed that rolled paper can regain its original shape if it is not pinched. However, the same cannot be said for the silver ink

deposited on the paper. Figures 1b and 1c show that the silver ink experiences either tensile or compressive strain depending upon the direction of bend. Thus, it is important to study the changes in electrical and topographical properties caused by the induced strain in silver metal film. Tensile stress in the silver ink film induces strain at the point of bending leading to an increase in resistance. Alternatively, under compressive strain, the silver particles are brought closer together leading to an increase in resistance.

Nassar et al. showed that the change in resistance appears to be linear with the change in bending radius45.

On the other hand, silver also undergoes linear increase in resistance with a rise in temperature, which is

the underlying working principle of a resistive temperature sensor. Here, although paper possesses several

desired features for a flexible low-cost biofriendly skin, the changes in the resistance due to bending would

lead to false results as the subject moves around in daily life. In order to counteract such issues, thicker

films can be deposited which reduce the changes in resistance due to bending53.

Besides being used as a substrate for devices, paper can also be used to make patterned shapes of other materials. Hydrogels are extensively used in industrial and environmental applications due to their higher water absorption capacity, long service life, and wide varieties of raw chemical resources54. It is difficult to make complex shapes of hydrogel films due to the difficulty in adding solutions of crosslinking ions to millimeter-sized layers of uncured polymer. Brachel et al. devised a procedure to creating complex

shapes in hydrogel films as illustrated in Figure 2a55. Templates are wetted with solutions containing

multivalent by immersing them in solutions of uncross-linked linear polymer. A glass slide applied with

1.5% w/w solution of sodium alginate is pressed against the paper template cut in the desired shapes.

Hydrogel is formed by the ions that are leached out of the paper. Excess (uncross-linked) polymer is washed

away by a stream of deionized water. A metal spatula is used to gently pry off and release the films from

the template. This procedure enabled by paper provides a manufacturing method, as the paper templates

can be reused for large scale production. In addition, complex shapes, as seen in Figure 2b, were formed

using paper templates that would have been otherwise very difficult to make.

3.1. Pencil Drawn Components

Using a pencil to draw sensors can help leverage the key benefits that paper offers-ease of manufacturing, low cost, and absorb liquids. Sun et al. demonstrated a unique and innovative Near Infrared

(NIR) detection sensor while taking advantage of these beneficial properties of paper56. They created

composite pencil leads comprised of thermoresponsive pyrene-based ionic liquid [Pyrmim]+[Br]−. They

drew lines of sensor on paper using the composite pencil which presents a low cost and flexible

manufacturing approach. The resulting paper chip was tested to detect NIR light without making any

contact to the object that was emitting the radiation (Figure 3a). In addition, they were able to accurately

read temperature of a warm object by placing the sensor on its surface. Furthermore, due to the flexibility

of the paper used, they saw that as the paper was folded, the current changed with the folding angle. This

behavior further opens the possibility of using this sensor as an angle goniometer for electronic robots

(Figure 3b). This particular property of changing electric signals of pencil lead drawn pattern on paper has

also been used to create a cheap disposable sensor that can sense deformation, damage and heat transfer of

materials for industrial applications57.

Similarly, Phan et al. developed a tactile sensor with paper as a substrate and graphite as the resistive sensing elements followed by a plastic lamination58. The sensors were drawn on the paper using a

2B pencil into U-shaped carbon resistors (Figure 3c). The plastic lamination provides the necessary

stiffness for a cantilever shaped structure to stay erect and provide insulation from atmospheric humidity.

Nassar et al. also used an HB graphite pencil to make a pH sensor, as seen in Figure 3d29. The layer of

graphite deposited over the silver ink pen electrodes acts as the pH sensitive layer. Redox reactions occur

between the graphite and hydroxyl ions in the corresponding aqueous solutions. Acids have larger

concentration of hydrogen ions H+ than neutral water, and bases have larger concentration of hydroxide

- + - ions OH . Hydroxonium ions H3O and hydroxyl ions OH get adsorbed on the surface of the paper. In

alkaline solutions, the carbonyl functional group undergoes a reduction step (gaining electrons e-) to transform into methane (CH4), which is its highly reduced state, leading to a decrease in the resistance with respect to the resistance of a neutral solution. On the other hand, in acidic solution, the carbon-based film

− undergoes oxidation (loses e ) to form CO2, which is its highly oxidized state, leading to an increase in the

resistance. Paper has also been used to create Ultraviolet (UV) sensors. Gimenez et al. drew an interdigitated pattern of UV sensitive material electrodes on a piece of paper59. A small quantity of ZnO

was then dropped on the electrodes and left to cure at 120 °C for five minutes. The porous matrix of fibers

is used to trap ZnO (A UV sensitive material) crystals to create an evenly distributed layer of ZnO crystals

on the surface59. Since the porous structure of the paper binds with ZnO without the need of any surface activation or treatment, the production cost is lowered and this process algins well with the focus of this article to present paper-based electronics that use the paper in its original form. UV light creates electron hole pairs from ZnO on the surface of the paper. The generated electrons are attracted by oxygen molecules when the sensor comes in contact with oxygen in air which results in reducing the mobility of free electrons60. The reduced mobility, consequently, increases the resistance of the structure. Hence, the

resistance increases according to the amount of oxygen present in air. The paper-based UV sensors have been demonstrated to show enhanced photoconductive current when compared to sensors made on glass substrates, which results in higher sensitivity. Thus, evidently in certain cases, the unique structure of paper creates high performance sensors compared with their semiconductor counterparts60. Seemingly challenging sensors, like oxygen sensors, have also been fabricated on paper using a pencil. Zhang et al. present a rapid prototyping technique of creating NO2 gas sensor by rubbing an 8B pencil on printed silver

interdigitated electrodes to form a thin and uniform graphitic coating61. The silver nanoparticles exfoliate into the graphene film to form the paper based NO2 gas sensors that offer higher sensitivity and better

reproducibility in comparison to traditional rigid substrate gas sensors (Figure 3e). Similarly, rubbing pencil on palladium electrodes forms a hydrogen gas sensor with high sensitivity and response rate62. As

discussed before, such paper-based sensors result in reduced production costs and simplify manufacturing

processes

3.2. Printed Components

Electrodes in sensors are generally made using CMOS based fabrication methods that involve electro polymerization or lithography. However, these processes require complex equipment, mostly run at high temperatures, and are costly to operate. A lower cost alternative to print electrodes is to use inkjet-

printing technology. This is an additive process where ink is deposited in the desired areas, thus removing

the need of using masks, and the thickness of the deposited materials can be precisely controlled63. In addition, many materials are compatible with inkjet-based deposition methods and can be deposited at room temperature. However, one of the crucial benefits of this technology to produce low cost sensors is the ability to print on a variety of substrates, including paper. Inkjet printing produces much lower wastage in comparison to conventional fabrication methods64.

It is possible to make several unique types of sensors due to the flexibility of printing various kinds

of materials on paper. Huang et al. created paper-based flexible ammonia gas (NH3) sensor with silver and poly(m-aminobenzene sulfonic acid) functionalized single-walled carbon nanotubes (SWNT-PABS)65. The

process involves inkjet printing of silver dispersion on a photo-paper to form electrodes followed by

printing of SWNT-PABS dispersion on the silver electrodes. The deposition of both layers completed the

stack for ammonia sensor. The sensors showed short response and recovery times to different

concentrations of NH3 at the ppm level. In addition to the desired response, the sensor showed stability over

extended periods of time and at increased temperatures. Liu et al. further demonstrated the viability of a mm-Wave radar, made using carbon nano-tubes deposited on paper using inkjet printing66. Backscattered signals received from the passive sensors were used to calculate the quantities of gases. Alternatively, Kan et al. created gas sensors on paper by spray coating PbS nanowires on the paper at room temperature. Also, in this case, there are immense advantages of using paper, besides its lower cost as compared to the alternative substrates. The paper provides inherent flexibility for superior mechanical bendability67. The

spray coating is undertaken at room temperature that results in a low-cost and simple fabrication method

that is scalable for large scale production. Most important is depositing PbS nanowires on a paper that

exhibits a porous network microstructure which forms an effective pathway for gas adsorption and

diffusion. Taking advantage of the flexibility of paper, the sensor was shown to perform well at a bending

angle as high as 60 degrees.

Beduk et al. were able to create a hydrazine sensor by using inkjet printed electrodes of poly(ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) sol-gel on paper68. Hydrazine is a

toxic substance used as a precursor in pharmaceutical and pesticide industry. Due to its toxicity, it is critical

to detect the amount of hydrazine exposure. The PEDOT:PSS electrodes were functionalized with zinc

oxide (ZnO) to create an amperometric sensor that can detect small amounts of hydrazine. Amperometric

methods are simple, low cost, and often do not require pre-treatments of the sample. Since paper is used as

the substrate, the samples can directly be introduced to the sensors and the resulting change in current can

be attributed to detected material. The fabricated sensor was tested in tap, sea, and mineral water samples.

The results showed increased current output in the electrodes with the rise in hydrazine concentration giving

a linear response in the 10–500 μM hydrazine concentration range and a 5 μM detection limit. The ZnO

improves the stability and sensitivity of the sensor. Another amperometric ∼sensor was created by depositing

a conductive ink containing Cu nanoparticles, graphite, and polystyrene onto paper. The sensor was used to evaluate the amount of carbohydrates in food69. Here again, due to the use of paper as a substrate, the sample in liquid form is dropped directly on the paper around the sensor (Figure 4a). In response to contact with glucose in the sample, the oxidation current increases. Visual color changes in electrochromic materials have also been utilized to detect changes in resistance. A gold nanoparticle film coated with

Prussian blue/polyaniline (electrochromic film) was deposited on paper by Liana et al.70. As the voltage through the film increases to reach the reduction voltage of the electrochromic film, the green/blue film starts to become transparent. Thus, an image sensor can detect the changes in resistance of a paper sensor from a distance. This is particularly useful for extracting data from sensors that may be immersed in a fluid medium as the radiowave frequency based wireless communication methods do not function inside liquids.

The color-based detection methods on paper can be further utilized to detect other stimuli. Khiabani et al.

71 deposited titanium dioxide (TiO2), polyvinylpyrrolidone (PVP), and food dye on paper via inkjet printing .

TiO2 acts as a photocatalyst which decomposes the food dyes that become discolored with respect to UV exposure. The extent of discoloration of the film can be used to create cheap disposable UV detection sensors to be used on human skin in places of UV exposure risks, like beaches.

Graphene, as a sensing material, has garnered tremendous interest in the research community over the past decades. The ability to deposit graphene using inkjet printing can open the doors to the possibility

of duplicating graphene-based sensors on paper. Panraksa et al. demonstrated a graphene-based inkjet printed sensor that can be used for rapid, selective, and sensitive detection of acetylcholinesterase72. The

working principle of the sensor is based on the change in current due to the formation of thiocholine (TCh)

by the hydrolysis of acetylthiocholine chloride (ATCh) by AChE. The sensor was able to detect AChE

levels as low as 0.1 U/mL up to 15 U/mL and can be used to detect AChE levels in human blood. Thus,

inkjet printed electrodes on paper present low-cost alternative methods to detect chemicals in air or bodily

fluids with a potential to create low-cost Point of Care (PoC) devices.

Paper has the innate property to absorb and hold extremely small amounts of liquids in its surface.

Researchers have taken advantage of this property to detect chemicals that change colors in response to

certain stimuli. In one such example, Ali et al. created a simple all-in-one paper-based sensor for E. coli detection using a composite ink made of a fluorogenic DNAzyme probe73. When the sensor encounters the

E. coli in a concentration of greater than 100 cells/mL, it generates a fluorescence signal after 5 minutes.

The additional advantages of this technique were that the sensors remained stable for months at room temperature and the sample of bacterium does not require any pretreatment. Similar phenomenon of changing color was used by Donato et al. to create a sensor for the detection of iron [Fe(II)]74. The sensor comprised of 2,2-bipyridyl dye inkjet printed on a piece of paper. As the dye comes in contact with Fe, its color changed from white to red and the redness increased with increased concentration of Fe (Figure 4b).

The color changes were read by a simple Red-Blue-Green (RGB) sensor (Figure 4c). Diacetylene (DA) monomers is another inkjet printable chemical that has the tendency to change color when exposed to UV radiation. Yoon et al. deposited DA ink on paper using a common inkjet printer75. The color of the ink

changed to blue when exposed to UV with the possibility to return to its original color under specific

temperature ranges (Figure 4d). The reversible reaction allowed them to use the ink to create QR codes on

a parking ticket to act as a dual anticounterfeiting system, by combining QR decoding and the color

changing ability of the DA ink to validate authenticity of the tickets. Such paper-based devices can help us

make low cost temperature sensors and create anticounterfeiting barcodes. On another occasion, inkjet

printable color changing dyes have been demonstrated to detect the presence of certain surfactant, further

showing the compatibility of paper with inkjet printing that can lead to the formation of numerous unique sensor types76.

3.3. Origami Inspired Energy Storage/Harvesting Devices

The ease of cutting paper into complex shapes using laser cutters, paper cutter or even simple scissors allows fabrication of complex 3D structures that do not break or tear when bent. Furthermore, many researchers have taken advantage of this property of paper to make complex origami inspired devices. Such devices are not possible with either semiconductor or polymer-based substrates. Cybulski et al.

demonstrated one such example where they created an origami-based paper microscope named Foldscope77.

Using paper and origami technique, they were able to show a method for large scale production of

microscopes at low costs. Brightfield, Darkfield, and Fluorescence microscopes were demonstrated using

this method. The whole process involves the following steps, cut shapes in paper, fold into the microscope

structure, insert illumination setup, and mount sample (Figure 5a). The essential components include a

spherical ball lens (or other micro-lenses), lens-holder apertures, an LED with diffuser or condenser lens, a

battery, and an electrical switch (Figure 5b). Folding provides a passive alignment mechanism that is used

here to align the micro-lens with the light source. The alignment accuracy during the folding process is

improved through elastic averaging within kinematic constraints, which is achieved by folding features that

form a closed structural loop between the optics stage and the illumination stage (Figure 5c). To test the alignment accuracy, the researchers created 20 independent Foldscopes using 350 mm thick cardstock paper. They manually folded and unfolded the Foldscopes multiple times while measuring the absolute X–

Y alignment. The results showed some accuracy errors (less than 20% of the paper thickness) in both directions that are a result of the design which can be improved in the future. The results from the various microscope structures can be seen in Figure 6a-6d. The microscope was easily able to identify structures as small as 5 µm. This device summarizes the discussion so far that paper has several innate advantages. It is low-cost and has a special ability to be crafted into shapes that are otherwise impossible with most of the conventional materials. The processing and manufacturing costs can be kept low while the possibility of recyclability reduces plastic and electronic waste pollution. Furthermore, the advantages might not have to

sacrifice the performance, as shown with a microscope whose structure is made of paper that can zoom into

objects as small as a few microns.

There are several other applications that take advantage of the origami techniques. In essence, the origami technique allows paper to be folded and stacked to form complex structures. There has been one field where researchers have repeatedly utilized origami and that is to make energy storage and energy harvesting devices on paper. The growing need of paper-based energy storage elements comes from the increase in development of devices and sensors that are built on paper and thus, the need for homogeneous integration of energy storage and harvesting elements. One way to harvest electrical energy on paper is by making Microbial Fuel Cells (MFC) which convert biological energy from biomass directly into electrons via microbial metabolism20. In an MFC, paper not only acts as the substrate but as an active material in

which liquid agents rapidly absorb due to capillary action. The rapid redox reaction allows for rapid power

generation using a simple fabrication method on a piece of paper. Although the energy density is low,

researchers are working to increase the energy output78. Fraiwan et al. took advantage of origami techniques

to fold an array of four MFCs to increase the number of MFCs in a given area (Figure 7a). As previously

emphasized, paper-based devices are capable of being mass produced due to their compatibility with large scale manufacturing processes. For this MFC, paper was cut using a laser cutter. The surfaces are coated with a spray adhesive before the folding process is undertaken by cardboard folding equipment, as used in cardboard packaging companies. In the process of folding, each MFC makes an electrical connection with the MFCs above and below it, which is kept intact by the adhesive. They were able to harvest 1.2 µW/cm2

of energy that is twice as much as the latest research was able to achieve at that time79. In this setup, the anolyte contains whole bacteria and the catholyte contains ferricyanide. When droplets of each solution are poured onto their respective spaces, they travel through the paper under the influence of capillary forces to reach the MFC reservoir. In the reservoir, the bacterial metabolism generates electrons that are transferred

to an external electrode through the cell membrane, which results in the current flow between anode and cathode. Lee et al. further showed that in a similar structure, air acts as the activator for the cathode80.

Bacteria loaded on the paper-based origami MFC stack generate electrons while the cathodes, when

exposed to air, make use of the oxygen molecules in air to act as the electron acceptor. Shitanda et al. used

a different origami folding technique to create the MFC stack, as shown in Figure 7b81. While most paper

based MFCs employ spray deposition of metals, Shitanda et al. used screen printing, for the first time, to

create a printable origami array-type MFC which showed a much higher power density of 180 µW/cm2.

Taking advantage of screen printing, Choi et al. used a novel graphite-polymer composite and graphite ink with activated carbon as anodic materials to enhance performance as compared to a conventional graphite ink or gold anode82. They further developed a novel technique to pattern a hydrophobic wax on the paper, which acted as an ion exchange membrane, coupled with use of specialized anodic materials that enhanced the adherence of bacteria. The wax membrane is cheaper than the commonly used commercial membrane while being thin and lightweight. Furthermore, the wax membrane is compatible with screen printing and laser cutting to allow large scale batch production. Being deposited on paper lets the membrane take advantage of the porosity of the paper to enhance electron manipulation and ion transport. Choi et al. further demonstrated a MFC that functions as a low-cost and disposable diagnostic device for resource-limited regions83. A drop of bacteria- containing liquid coming from wastewater resources acts as a renewable and

sustainable source of power for paper-based devices. This further strengthens the case of using paper based

sustainable and low-cost devices to spread electronics to people across the world, especially in the

developing countries. Moreover, Rojas et al. showed that flexible thermoelectric nanogenerators (TEGs)

can be made on paper substrates for applications where conventional rigid TEGs find limited use84. Strips

of four thermopiles (Bi2Te3-Sb2Te3 pairs) are deposited on paper via sputtering. They created a TEG on

standard paper that can generate 0.5 nW power at a temperature difference of 50 K (Figure 7c). The paper

is then folded to create a TEG stack. The thermopiles, being deposited on paper, can be folded into 3D

structures using origami techniques to form the TEG. Such origami based TEGs present a unique benefit

of being able to control the temperature difference by varying the gaps between the folds of paper, which

is an easy task when dealing with paper. Thus, by using paper to harvest energy, it will be possible to

provide essential technologies to the broad population by inclusion of everyday materials and simple

processes.

In addition to energy harvesting devices, origami has been widely used to create paper-based energy storage devices, mainly lithium ion batteries. Such techniques allow the formation of deformable batteries over conventionally used materials like elastomers. 3D stacking of multiple energy storage elements increases the energy density in a given 2D area. A concept of paper-based origami lithium ion batteries can be seen in Figure 885. Paper coated with carbon nanotubes served as the current collecting layers on top of which

further layers of the lithium ion battery are deposited, as depicted in Figure 8a. Figure 8b further illustrates

how a paper-based battery is folded using origami into a Miura-Ori pattern. ‘Mountain’ and ‘valley’ creases

connect equal-sized parallelogram faces. Miura-Ori pattern allows the structure to be compressible in either

one direction or collapsible in two directions (Figure 8b). Since the main faces remain rigid while the

folded paper creases along the folds, the battery can retain its functionality while being subject to such a

high level of deformation (Figure 8c). The paper itself does not undergo any significant strain except at the

creases. The fabrication involves processes like slurry mixing, coating, and packaging that are in line with

conventional industrial processing methods to allow large scale fabrication of these origami enabled paper-

based batteries. The battery was used to power LEDs as a demonstration, first maintained in the folded form

and then subjected to 50 cycles of folding and unfolding while it retained the power output (Figure 8d).

Similar to lithium ion batteries, an origami enabled paper-based fluidic battery has been demonstrated by

Chen et al.86. Wax printing was used to create the microfluidic channels followed by gluing copper and aluminum sheets on the paper, to serve as the electrodes for the redox reaction, separated by a membrane.

Origami techniques were used to create stacked three-dimensional structures held together by paper glue.

The battery was able to provide an open-circuit potential from 0.82 V and a current of 500 μA. This amount of energy is enough to power an LED. Electrophoresis is a technique in microfluidics where a voltage is used to separate two chemicals87. A paper based fluidic battery allows homogeneous integration of energy storage devices that can perform electrophoresis in microfluidic devices (Figure 8e). Energy storage on paper is not only limited to origami inspired devices. Energy storage for nanoelectronics is a challenge since commonly used lithium ion batteries cannot be used at such extremely small size. Supercapacitors

(SCs) have become the viable alternative to provide power for self-powered nanosystems. They are safer

to use and have higher power density, life cycles, and environmental benignancy compared to batteries88.

Yuan et al. successfully fabricated all-solid-state polyaniline-based (PANI-based) flexible SCs on paper

substrates to store energy generated by a piezoelectric generator or a solar cell. They further demonstrated

the functionality of the battery by using it to power a strain sensor89. Further building upon the concept of

using cellulose fiber-based energy harvesters, they presented a self-powered breathing monitor using a

similar strain sensor90. Thus, paper is a versatile substrate used to create a variety of energy storage and energy harvesting elements which can even be utilized in self-powered nanosystems.

4. Paper as an Active Material

Besides being used as a substrate, paper is also found as an active material in sensors, meaning the paper is

used as the sensing material. Extensive research has been carried out on how the surface of paper can be

functionalized/activated, or the composition of paper varied by mixing it with sensing elements, to form

paper-based sensors91-93. Some examples include paper surface coated with a sensing material like carbon-

nanotubes94-99, or paper chemically modified by soaking in chemical reagents to create a sensing material100-

106. Functionalized paper has also been used to create flexible electronics 107-113. However, most often the process of functionalization involves high temperature processes which further increase the cost of fabrication and deteriorates the properties of the paper as high temperatures result in degradation of cellulose114-116. Thus, in order to truly take advantage of paper for low cost and easy-to-manufacture applications, it is highly beneficial that the paper is used in its naturally produced form. The following sections will discuss research where non-functionalized common paper is used as an active material.

4.1. Humidity Sensors

Paper can absorb, store, and deliver precise amount of liquids through its surface which allows chemists to deal with reagents without touching them117-119. Furthermore, reagents can be added in small amounts with multiple iterations to further allow enrichment of the reagent in a given area120. The porous

structures permit air (or gases) to permeate (or diffuse) through its surface121. This property is particularly useful in microfluidics as it does not allow bubble formation, which is an issue in micromachined channels122. Moreover, since the pores are of a particular size, the paper acts as a filter to selectively remove

particles. This phenomenon is predominantly used in filter paper to selectively separate particles from

fluids123. Capillary action of paper can transport fluids from one point to another avoiding the use of pumps,

which is used for analyte separation in chromatography124, 125.

Paper is most widely used as an active material in humidity or moisture sensors as it has an acute

affinity to adsorb and desorb moisture due to its porous cellulose-fiber nature. Thus, high-performance capacitive humidity sensors can be formed by using paper as the dielectric material126, 127. In the humidity sensor, paper not only acts as the active sensing material but also as a substrate. To take advantage of using paper as a substrate, the interdigitated electrodes for the humidity sensors are made using a silver ink pen

(Figure 9a). The process can be accomplished by hand at the small scale, or automated pencil sketch machines at a larger scale. In the interdigitated capacitive electrodes, the effective dielectric is a combination of the dielectric constants of both air and paper in between the electrodes. Cellulose paper is hygroscopic in nature. When the paper comes in contact with a humid environment, water molecules adsorb on its surface by making loose bonds with the hydroxyl groups29. As a result, the effective dielectric constant increases. The change in dielectric constant is much greater in paper than air due to concentrated adsorption of water molecules on paper surface11. Thus, the overall capacitance of the sensor changes in response to humidity changes (Figure 9b). Humidity sensors can also be formed on paper by creating sensing electrodes

using pencils in conjunction with a layer of oxidized multiwalled carbon nanotubes ink128, or by drop casting

CdS nanoparticles129. These sensors have shown to exhibit good reproducibility and stability during dynamic measurements. Alternatively, inkjet-printing has been used to form interdigitated electrodes for a wide range of relative humidity sensing from 10% to 90%130. For most paper-based humidity sensors, the

adsorption transient is an exponential function while desorption follows exponential law. It is evident that

paper not only presents a cheap alternative to other humidity sensors but also its porous structure results in

high sensitivity and fast response rate. Combined with the inherent mechanical flexibility, paper has shown

to be a viable option for integration into surgical masks to acts as respiration sensors131. Combined with electronics, there is potential to convert common things like surgical masks into smart sensing objects.

4.2. Pressure Sensors

A capacitive sensor can be made to act as a pressure sensor. As discussed in the introduction, we will add readily available materials that exhibit similar advantageous properties as paper to the scope of paper-based materials. These properties are low-cost, availability, ease of fabrication process, and recyclability. A couple such materials are aluminum foil, sponges, and wipes. They are readily available at a low cost and can be cut in the same way as paper to create sensing structures. A pressure sensor can be formed by making a parallel plate capacitive structure with a sponge or wipe sandwiched between two metal foils. The metal foils act as top and bottom electrodes while the sponge or wipe acts as the pressure sensitive dielectric (Figure 9c). When pressure is applied on the top metal foil, the sponge or wipe will compress, thus changing the distance between the electrodes which in turn results in an increase in capacitance. Both sponges and wipes are porous materials which allow them to be compressed even with application of a small force (Figure 9d). However, the compression range is different so the choice can be made depending upon the applications. When compressed, sponges give a larger range of operation due to their increased thickness. The microfiber wipes, although much thinner than sponges, have a higher sensitivity due to increased deformation under mechanical stimuli29. Alternatively, pressure sensors can be formed using a pen and a piece of paper. An aqueous solution FeCl3 is drawn on a piece of paper that is folded in such a way that it acts as a cantilever132. Pressure applied on the free end of the cantilever bends

the structure and ultimately changes the resistance of the ink trace (Figure 9e). Such a structure attached to

finger joints can result in highly sensitive movement sensors (Figure 9f). Thus, we see that paper provides

numerous possibilities of creating sensors that can be integrated into our daily lives as useful tools. In order

to fully realize the economic and social impact of using low-cost materials, Khan et al. have demonstrated

the use of paper-based temperature and humidity sensors in the healthcare industry. The paper-based

temperature and humidity sensors monitored the ambient conditions inside a prescription container (Figure

10a). Studies show that stability and efficacy of pharmaceutical medications, like pills, can be affected by

storage conditions where high levels of temperature and humidity can greatly reduce potency and quality133.

However, creating low-cost sensors is only one side of the coin, low-cost integration strategies in conjunction with low-cost sensors are needed to make it a practical and viable solution for the healthcare

industry to adopt. Khan el al. eventually demonstrated a smart lid that not only interfaced with these paper- based sensors but also housed a pill counter134. The smart cap can attach to the common prescription

container, like any normal cap, and the sensor, being made of flexible materials, lines the inside of the

prescription bottle (Figure 10b). As discussed before, there are other alternative materials to paper that

share the same advantages of paper as an active sensing material. One such key material is aluminum foil.

Aluminum foil has been used as the active sensing material in pressure sensors for a variety of applications.

By using air as the dielectric material, Khan et al. create acoustic sensors by creating a parallel plate

capacitive sensor with aluminum foil plates135. Air is highly sensitive to pressure, as compared to other dielectric materials like foam136, and thus, it is able to respond to low pressures of sound. In order to

demonstrate an application, the structure of the sensor was made in such a way that it responded to sounds

in the range of wheezing137. Wheezing is an early symptom of asthma and by designing a sensor that only responds to wheezing, they were able to create a low-cost asthma sensor that could be installed on the chest of the subject (Figure 10c and 10d). However, the usage of such materials is not just limited to healthcare applications. Such metal foil-based pressure sensors have been shown to act as inputs to control robots or used as touchless proximity sensors138. Furthermore, a security tag was made using pieces of metal and

metal foil (Figure 10e). The tag, attached to an object, generates an alert when moved (Figure 10f)139.

Since the sensor is made from very primitive and low-cost materials, it can even be used to secure everyday objects, like a vase or a painting. Such deployment of security tags would otherwise be impossible as the current electronic sensing tags cost upwards of fifty dollars, which makes it unreasonable to use them as security tags for objects that cost less than that. The key advantage here is that low-cost and widespread availability opens up the prospect of deploying sensors for everyone in the world. There is a dire need to spread electronics to low-income countries in the developing world so that they can leverage the power of technology to improve their living standard and get access to quality healthcare.

4.3. MEMS Sensors

Paper can be cut into any desired shape to form complex structure which allows it to be used to create Microelectromechanical systems (MEMS) based devices. Usually, MEMS devices are formed with

silicon but due to the complexity of the structures, the energy, and financial costs of an extensive number

of processes becomes very high. Paper is compatible with some deposition processes to realize sensors,

such as force and oxygen sensors. Liu et al. deposited a piezoresistive material on top of a cantilever shaped

paper, as shown in Figure 11a140. Carbon based resistors act as the force sensing material on the structure.

Force application causes the cantilever to bend which induces stress in the carbon resistor resulting in a

change in its resistance. The change in resistance then provides information about the amount of force

applied on the cantilever structure. The contacts are made up of large sized silver ink pads. Both the carbon

ink and silver ink are deposited using screen printing. In addition to the ease of cutting paper into desired

shapes using a laser cutter, screen printing allows easy deposition of different materials on top of paper.

Furthermore, these two industrial scalable processes can be used to manufacture such sensors at much larger

scale (Figure 11b). Environmental humidity is a cause of concern for such sensors as the paper is hydrophilic and moisture tends to adsorb on its surface, which affects the mechanical properties of the sensor. To make the sensor hydrophobic, the researchers functionalized the surface hydroxyl groups of the paper (cellulose fibers) with trichlorosilane vapor to form surface silanol linkages to generate a fluorinated, highly textured, hydrophobic surface. This surface treatment minimizes the effect of environmental humidity on the mechanical and electrical properties of the sensor. The sensor showed a linear output response for deflection on either side of the cantilevers. They further performed bending tests on the cantilever for 1200 cycles and the resulting change in beam stiffness was less than 4%. A disadvantage of using paper as a substrate was evident from the buckling of the paper due to the thin and long structure of the cantilever and the gravitational force. To increase the stiffness of the beam, it is folded across the two lines illustrated in Figures 11c and 11d. Similarly, Akter et al. used spray deposition technique to have a controlled deposition of piezoelectric graphite ink on paper141. Such paper-based force sensors can be used to measure the mechanical properties of soft and thin materials. There are MEMS sensors that have employed paper in its natural form to create sensors. Salim et al. created a kirigami-inspired split-ring resonator (SRR) strain sensor142. In an SRR, the split gap dictates its resonance frequency. The sensor works

in a way that the changes in the gap, in response to tensile stress, can be quantitatively identified by noting

the changes in the resonance frequency. The sensor structure was created using two sheets of paper with

cuts that are part of the kirigami process (Figure 11e). On the fixed end, a film of silver nanoparticles is deposited using inkjet printing and a stretchable film is deposited on the end which is supposed to be stretched (Figure 11f). As strain is applied on the sensor and increased to 17%, the resonance frequency changes from 4 to 4.64 GHz. Such a strain sensor is relatively easy to manufacture, low cost, and disposable due to the use of paper and commonplace inkjet printing.

Based on these examples, we observe several advantages of paper based MEMS technology140. In contrast to paper, which can be folded into three-dimensional structures with high stiffness and anisotropic responses, silicon-based devices cannot be folded. Paper-based MEMS devices can be manufactured with simpler low-cost tooling and has the potential for mass production (by automatic paper cutting and screen printing). Electric circuits can be readily integrated with the paper-based sensors to form monolithic paper- based chips. Chemicals can be used to modify the surface of paper, and its high ratio of surface area to weight, provide ways of surface treatments to generate varying sensitivities. On the other hand, paper-based

MEMS force sensors present some limitations143. The performance (i.e., measurement range, resolution, and sensitivity) is lower than silicon-based force sensors. However, a case can be made that performance- to-cost ratio can be higher for the paper-based sensors. Such sensors can be targeted towards applications that do not require high performance and can greatly benefit from lower costs. Paper has a lower Young's modulus (2 GPa) than silicon (130–170 GPa), which results in low natural resonant frequency ( 25 Hz), limiting the use to low frequency or static force measurements 143. Paper has much lower tolerance∼ than silicon-based devices for high levels in temperature and atmospheric components (e.g. water vapor, ozone, prolonged exposure to dioxygen or peroxides). Paper can be made into an electro-active material by mixing cellulose with chemicals during paper production. An electro-activated paper product has been utilized to create a MEMS sensor where the modified paper acts a piezoelectric sensor to monitor vibration in a cantilever beam144. However, this type of paper is not readily available and has to be specially made, which

greatly increases the cost, and use of high temperatures can potentially degrade its quality.

5. Functionalized paper-based Point-of-Care (POC) Devices

We have discussed in great detail how paper can be aptly utilized to create low-cost high performing devices. Here, we would like to further discuss the practicality of paper in real life applications. Paper- based Point-of-care (POC) devices have been a critical driver for the widespread adoption of paper in electronics. However, in POC devices, paper is mostly used in activated form93. Smartphone-based point-

of-care (POC) devices are rapidly emerging as testing devices due to their cost-effectiveness and widespread availability in resource-limited areas. A smartphone, combined with a paper-based biosensor, can offer enough accuracy and sensitivity for monitoring and diagnostics, opening doors for the development of rapid, simple, and cost-effective connected devices and wearables for medical diagnostics, environmental testing, and food safety monitoring. Smartphones can act as cheap, portable, analytical laboratory devices to selectively detect and analyze analytes. Advancements in smartphones, electronics, and new mobile applications (app) developments facilitate their use as a smart detector for POC devices145.

There is a large gap between availability and demand of medical care services, especially in

developing countries where inadequate healthcare budgets give rise to frequent outbreaks of chronic

infectious diseases that have limited medical solutions. In such circumstances, developing inexpensive,

effective, rapid, and portable diagnostic devices as a replacement for traditional laboratory-based ones

becomes even more imperative. Most of the existing laboratory detection techniques of analytes are costly

and/or time consuming, owing to the process involving expert driven sophisticated analysis techniques.

Therefore, smartphone-enabled colorimetric testing techniques can lead the way towards cost-effective,

widespread, and fast paper-based POC monitoring. Paper-based POC devices can deliver the following

advantages: (1) simpler operation without the need for professionally trained operators; (2) reduced analysis

time and quicker test results; (3) simpler and cost-effective fabrication; and (4) convenience of use,

especially in limited-resource areas. In smartphone based POC diagnostic devices, samples such as blood,

urine, sweat, saliva, or tears can be tested and analyzed for the detection of multiple biomarkers of serious

diseases. Colorimetric, ﬂuorescent, brightﬁeld, and electrochemical methods are different techniques that

can be utilized to examine these samples and identify different diseases, as summarized in Figure 12145.

The technique used depends on the application and resources of testing. The focus of this section is to shed

light on current methods in smartphone-enabled devices that have been developed for point-of-care applications in clinical diagnostics, food safety, and environmental protection. We will narrow our discussion to POC devices developed using only paper-based sensors, as they exhibit the desired low-cost and high manufacturability for democratizing the technology.

5.1. POC Devices for Basic Healthcare Monitoring

Widely studied high-quality and low-cost smartphone-based POC devices tend to feature paper- based platforms. Specifically, microfluidic paper-based analytical devices (µPADs) have increasingly showed the potential of using such low-cost platforms for the detection of urine metabolites, blood glucose, pH levels, liver function, and infectious agents146. µPADs have numerous advantages over the alternative

conventional lab-on-a-chip devices (fabricated on materials like polymers, glass, and silicon), such as low

cost, easy and fast fabrication, and disposability147. The integration of paper-based analytical devices with paper microfluidics has enabled several works on calorimetric based glucose monitoring, capable of detecting various enzymatic and biological reactions with an electronic readout. Cai et al. demonstrated a

ﬂower-shaped µPAD for glucose detection using artiﬁcial urine samples that exhibited good performance and feasibility as a quantitative analysis device147. Davaji et al. used a µPAD POC device to detect biotin in DNA samples and estimate glucose levels via temperature change and entropy estimations148.

Calorimetric POC devices also demonstrated accurate glucose level detection in blood samples using

commercially available glucometer paper strips combined with an image processing algorithm149. Image

processing plays a vital role in determining glucose concentration but one central challenge is to accurately

detect color in an image under presence of varying illumination149. An RGB correction algorithm is

generally developed and used for adjusting luminosity of images.

5.2. POC Devices for Severe Disease Detection

More recently, paper based POC devices have been further advanced by coupling biological systems with paper electronic devices for disease diagnostics and detection, such as cancer, pneumonia, and kidney diseases. One study demonstrated the use of functionalized conductive paper (CP) to detect the conjugation of the anti-carcinoembryonic antigen (CEA) protein for quantitative estimation of

this cancer biomarker150. In another study, Bhattacharjee et al. developed an economical, fast, reliable,

portable, and biocompatible lung function monitoring point-of-care-testing device (LFM-POCT) consisting

of a mouthpiece, paper-based humidity sensor, micro-heater assemblage, and real-time monitoring unit

(Figure 13a)151. The device was capable of measuring the frequency of breathing and peak ﬂow rate of

human exhalation, which could be necessary to detect severe diseases such as asthma, bronchitis, or

pneumonia . Creatinine level in urine is also one of the most important indicators for kidney diseases, giving

early insight about potential kidney failure or malfunction. Traditional methods of creatinine analysis are

expensive, time-consuming, and impractical. By using low-cost paper, and coupling it with widely available

smartphone apps, Tambaru et al. developed a creatinine detector using an inexpensive and instrument-free

method152.

5.3. Expanding POC Devices for Blood Cell Diagnostics and Infectious Diseases

For the first time, using the power of paper technology and smartphone integration, blood type identification is possible using simple image processing and a portable phone. A study designed a smart “Paper Barcode” platform for blood type detection through ABO/RhD blood grouping based on the principle of hemagglutination reaction between RBCs and antibodies153. As shown in Figure 13b, the platform uses 3 paper strips of sensing channels each functionalized to react to different types of blood, such as group A, B, or O, with selectivity towards negative and positive genetic modification. The barcode- like design enables a bar length reading app to interpret the results and identify the blood type among 8 types of ABO/RhD combinations without need for further analysis. The app is user friendly and provides on-screen notification of blood type (Figure 13b)153. Results are then easily saved in e-form and transferred between users and professionals for further medical investigation when needed.

Providing a powerful strategy for the development of low-cost multi-diagnostic POC biosensors, a one-touch-activated blood multi-diagnostic system (OBMS) has been advanced154 by integrating a hollow

microneedle and a paper-based sensor (Figure 13c) which provides a number of unique characteristics for

simplifying the design of microsystems and enhancing user performance. Li et al. prepared a device that

performed functions related to blood collection, serum separation, and detection using a one touch

operation. This system has been successfully demonstrated in living organisms with great potential for

human clinical applications and commercialization. Such smart systems are not restricted to detecting

diseases in humans but can also be used to identify illnesses in livestock for early control over diseases or

limiting spread of viruses in the livestock. POC devices have also been utilized for white blood cell (WBC)

counting using a smartphone based paper electrochemical sensor155. The system diagram of the paper-based, portable, sensing platform is illustrated in Figure 13d. A sample is trapped in the paper electrode and the sensing signal is generated from the portable potentiostat which is transmitted through Bluetooth to a smartphone. The smartphone app then analyzes the collected data and gives concentration values of WBC with internal calibration155.

5.4. POC Devices for Food Safety Detection

A lack of quality control tools limits the enforcement of fortification policies. A portable and low- cost device that can aid in identifying toxins in food is highly desirable, providing wide-spread and on- demand analysis for chemosensing and biosensing applications. These devices need to be generally aligned with the World Health Organization’s criteria of assuring affordability, sensitivity, specificity, user- friendliness, rapidity, robustness, and equipment-free156. Accordingly, several smartphone-interfaced POC

devices have been reported for the development of paper-based colorimetric sensors for on-spot quantitative detection of analytes in drinking water, such as ﬂuoride (F−), lead (Pb2+), and pH157 as well as the

quantification of iron fortificants (iron concentration from ferrous sulfate and ferrous fumarate) in fortified

foods156. Similarly, regular on-spot monitoring of drinking water quality has become mandatory wherein diverse classes of materials, such as organic, inorganic, heavy metals, or biological wastes, are detected employing standard protocols. A study by Demir et al., reports the quantification of cyanimide in drinking water158, enabling instrument-free paper-based detectors for rapid water quality control and safety in off- field conditions. Figure 14a illustrates the steps for cyanide detection assay and the apparatus for quantitative analysis using simple image processing. The apparatus shown is made of black cardboard equipped with UV-LED source fitted into a smartphone camera. Three spots on the test strips are used to perform the analysis of the color development. The quantitative analysis of the color changing is performed

by a color calibrated algorithm and the results are displayed in the smartphone application (Figures 14b and 14c).

Furthermore, in analytical science, it is of great interest to explore convenient, rapid, and reliable methods to trace H2O2 levels in various environments. Mesoporous carbon-dispersed Pd nanoparticles (Pd

NPs/meso-C) based paper sensors are reported as a promising peroxidase to mimic the visual determination

159 of H2O2 target in milk matrices . Combining paper-based test strips, a smartphone and an easy-to-access color scanning app, the integrated platform can be used for quantitative analysis of H2O2 with good selectivity and repeatability. To meet the increasing need for on-demand analysis and in-field detection, the presented technique offers a practical, connected, rapid and convenient device. Upon exposure to different concentrations of H2O2, the colorimetric paper strip immediately undergoes a color reaction, visible to the

naked eye. The color is scanned using the smartphone camera, and using the corresponding app, the color

parameters can be obtained by analyzing the intensities of Red (R), Blue (B), Green (G), Hue (H), and

Saturation (S). By correlating the relationship between color parameters and the substrate concentration,

quantitative determination of H2O2 can be realized. This development suggests the promise of integrated

platform use in practical applications such as food safety where H2O2 is crucial in biological metabolisms

and in industrial processes. The analysis and detection of bacteria in contaminated drinking water is a

matter of critical interest. Several advancements have been made in the development of analytical biosensor

chips for detecting bacterial strains such as E. coli, S. Mutans and B. Subtilis. For a more cost-effective, on- the-spot, rapid detection, an electrically-receptive and thermally-responsive (ER-TR) sensor chip comprised of simple ﬁlter paper was reported for the detection of both Gram-positive (S. Mutans and B. subtilis) and Gram-negative (E. coli) bacterial cells in real-time160. Tap water, lake water and milk samples were tested with bacterial strains at varying concentration ranges, 101–105 cells/mL. The interaction of E. coli, S. mutans and B. subtilis cells with the functionalized and treated paper biosensors resulted in a change

of electrical resistance, and the readout was monitored in real-time using a MATLAB algorithm. The paper-

based POC device demonstrated reproducibility of 85-97% with a shelf-life of up to 4 weeks for lake water

testing.

6. Conclusion and Future Outlook

Paper has proved to be a viable alternative to conventional materials used in electronic devices for various applications. Figure 15 provides an overview of the whole discussion in the article. Vertical axis contains the applications that we have discussed while horizontal axis identifies the desirable properties for these applications. In the third diagonal axis, paper is compared with the other common materials used in electronics. Paper as a material is widely available and developing countries deprived of recent technological marvels will benefit the most from the proliferation of low-cost paper-based devices. Paper

processing techniques at all stages can be performed at larger scale to allow for mass production of paper-

based devices. Even though paper is biodegradable, it can also be easily disintegrated by burning it. Many

healthcare applications have been shown using paper as an active material or a substrate. Being thin, light,

and easy to handle, paper can be used to make portable point-of-care devices. Such devices can potentially

match the performance of laboratory analytical techniques. Taking advantage of its porous nature, paper is

being widely used in paper-based energy devices, besides being utilized in sensing applications. In MEMS,

paper has presented itself as a material which can be cut into complex shapes with relatively simpler

techniques in comparison to highly complex and expensive processes that are needed to make conventional

MEMS devices on silicon substrate. While paper is a low-cost and environmentally friendly alternative to

silicon devices, paper-based devices cannot match the performance of silicon-based devices. Most of the

research has been confined within the walls of research laboratories. The proof-of-concept paper-based devices fail to match the performance of commercial devices in terms of reliability and repeatability.

Although beneficial in some applications, the porous and rough surface of paper produces low performing transistors. In addition, paper is more susceptible to environmental changes, potentially causing instabilities in electrical conductance of printed inks on paper. Paper is incompatible with high temperature processes which limits the devices that can be made on paper. Although the cellulose-based mesh network gives paper

unique properties that, for example, make it a better humidity sensor. However, being an insulator, the

potential use-cased for paper remain limited. For fluidic applications, evaporation of liquid analyte from

the surface of paper remains a common issue. Mathematical modelling of paper-based devices has been

largely neglected due to the randomness and inconsistencies in its porous structure. One area which every

application area needs is high performance, an area that paper lags in comparison to other materials. In

addition, paper-based electronics suffer from low spatial resolution in comparison to Silicon. Paper cannot

replace any of the other materials, however, it has proven itself to be a viable alternative for variety of

applications.

Exploiting the existing mobile phone infrastructure to monitor health conditions and the environment will accelerate efforts towards diagnostics as well as low-cost healthcare for existing and emerging diseases. Rapid advances in smartphones (both hardware and software) are enabling researchers to explore their usage as stand-alone platforms for POC devices especially in resource-limited areas. With the help of paper biosensors, lab-on-chip systems, individual genetics, and smartphone monitoring parameters, we aim to democratize personalized medicine and advance primary healthcare systems. The next decade will bring breakthroughs in terms of precision, efficiency, and portability, enabling accuracy, consistency, and rapidity in capturing data in real-time, and streamlined workflow. Current smartphone- based POC combine numerous promising technologies such as paper-based sensors, flexible sensors, and microfluidics. The problem of insufficient accuracy for smartphone-based paper sensors is being solved by improving algorithms161 and materials155. There are several challenges that still need to be addressed for better development of smartphone-integrated paper-based POC devices: (1) Cost of smartphone-enabled

POC devices is an important factor. Reducing the cost of materials and consumption of reagents is the key to realize democratization of health care. The integration between paper-based platforms and 3D-printer technology can potentially greatly reduce processing and manufacturing costs. (2) The developed POC system should be able to detect multiple analytes simultaneously. This is crucial for diagnosing serious diseases, where several analytes need to be measured selectively and simultaneously in order to provide an effective comprehensive diagnosis. (3) The quality of the custom-built app will determine the functionality and performance of the POC system. For users, the app should be simple and easy to operate, and for developers, the app should meet commercial and technological standards, and be easily expandable in the future. When uploading patient or user information into the app and/or the cloud, security also becomes a

29 major concern. (4) Sample testing techniques of smartphone based POC devices should be more diversiﬁed.

Presently, colorimetry is still the dominating technique due to its ease of usage and reduced cost, but data interpretation and analysis needs to be further improved for accuracy and precision. (5) Looking further into the future, integrating power methods, such as wireless energy transfer, biofuel cells, and body energy harvesters, should be included to overcome rapid energy consumption.

Figure 1. (a) Paper based sensor in a bent form. (b) Tensile strain in the silver conductive ink when paper is bent in concave form. (c) Compressive strain in the silver conductive ink when paper is bent in convex form. Figures 1(a-c) are reproduced from [45]. Copyright 2017 IEEE. (d) SEM image of torn paper showing the cellulose fiber mesh. Reproduced from [162]. Permission under Creative Commons License (CC by 3.0). (e) The load-strain curve of paper highlighting the strength of paper to endure large amount of strain past the elastic limit. Reproduced from [163]. (f) Stress-strain profile of brittle, strong (non-ductile), ductile, and plastic materials. Reproduced from [164]. Copyright 2019 Cambridge University Press.

Figure 2. (a) Illustration showing the process of creating different shapes in hydrogel films using paper as a template. (b) The process for creating complex shapes in hydrogel films using paper. Figures 2(a,b) are reproduced from [55]. Copyright 2009 Wiley.

Figure 3. (a) The output response of the ILPL-based paper chip for different levels of input irradiation power. (b) The actual image of the sensor (left photograph) along with the output current response against time as the bending angles are changed (right photograph). Figures 3(a,b) are reproduced from [56]. Copyright 2017 American Chemical Society. (c) The fabrication process of the tactile sensor using plastic-laminated graphite-on-paper. Reproduced from [58]. Copyright 2015 IEEE. (d) Illustration (left photograph) and actual sample (right photograph) of a capacitive-based disposable pH sensor. Reproduced from [29]. Copyright 2016 Wiley. (e) Interdigitated sensor on a paper substrate with ZnO droplet on the electrodes (left) and a pencil-drawn sensor on Ag IDEs (right). Reproduced from [61]. Copyright 2015 AIP.

Figure 4. (a) An actual image along with labelled schematic of the paper sensor. Reproduced from [69]. Copyright 2017 Elsevier. (b) A visualization of the color changes observed in the paper sensor as the Fe(II) concentration is increased. (c) Fe(II) R, G, B calibration curves. Figures 4(b,c) are reproduced from [74]. Copyright 2014 IEEE. (d) Color changes observed in the DA 2-derived PDAs as the temperature is increased or decreased (left photograph) and the same ink used on parking ticket to hide QR codes (right photograph). Reproduced from [75]. Copyright 2013 American Chemical Society.

Figure 5. (a) The CAD layout of different parts of Foldscope that can be cut in a single A4 sized paper sheet. (b) Top view of the fully assemble Foldoscope with sample insertion place labelled. (c) Cross-sectional view demonstrating the procedure of changing the focus by pulling on the paper ends as indicated in the image. Figures 5(a-c) are reproduced from [77]. Copyright 2014 Cybulski et al.

Figure 6. Image outputs from the Foldoscope using a (a) Brightfield (at 1,450X zoom), (b) Fluorescent (1,140X zoom), (c) 2X2 lens-array Brightfield (1,450X zoom), (d) Darkfield (140X zoom) lens setup. Reproduced from [77]. Copyright 2014 Cybulski et al.

Figure 7. The process of; (a) Folding four MFCs stacked using the origami folding techniques. The folding process algins the carbon electrodes such that the MFCs are connected in series. Reproduced from [78]. Copyright 2016 Elsevier. (b) Two paper-based biofuel cells in a series using origami. Reproduced from [81]. Copyright 2017 The Chemical Society of Japan. (c) Creating TEG pairs on paper by cutting and folding. Reproduced from [84]. Copyright 2017 Elsevier.

Figure 8. (a). Layer by layer breakdown of the layers in a conventional lithium ion battery. (b) The 45° Miura-Ori folding technique to completely compress paper-based battery in one direction (left two photographs) and the 45° Miura-Ori folding technique to collapse paper-based battery in biaxial direction (right two photographs). (c) The output power of the origami paper-based batteries for 50 cycles of folding and unfolding. Figures 8(a-c) are reproduced from [85]. Copyright 2014 The Royal Society of Chemistry. (d) A four-cell paper based fluidic battery (left photograph) powering up an LED (right photograph). (e) 4 galvanic cells-based paper battery with Y-shaped right photograph channels. Figures 8(d,e) are reproduced from [86]. Copyright 2014 Springer Nature.

Figure 9. (a) Schematic of two different ways to create a paper humidity sensor using paper itself or a microfiber wipe as the dielectric (humidity sensitive material). (b) Response of paper humidity sensor tested by blowing breaths on the sensor. (c) Illustration of the structure of paper-based pressure sensors using microfiber wipes and sponge as the pressure sensitive layer. (d) SEM images of foam (left photograph) and microfiber wipe (right photograph) highlighting the porosity of the materials. Figures 9(a-d) are reproduced from [29]. Copyright 2016 Wiley. (e) An actual image of the force sensor with bent paper that will act as the input point of force. (f) The change in output resistance in response to application of force as it bends the cantilever. Figures 9(e,f) are reproduced from [132]. Copyright 2018 Elsevier.

Figure 10. (a) Sticker containing paper-based temperature and humidity sensor. Reproduced from [133]. Copyright 2018 IEEE. (b) Flexible electronic interface decal attached to a paper sensor decal in bent form, displayed in hand (left photograph) and inside a prescription bottle (right photograph). Reproduced from [134]. Copyright 2019 IOP. (c) Aluminum foil-based asthma sensor in bent form; the inset highlights the small gap between the two sheets in a negative and zoomed image. (d) The wheezing sensor attached to the chest of a human subject with real time data transmission to a computer. Figures 10(c, d) are reproduced from [137]. Copyright 2019 IEEE. (e) The security tag (inclinometer) formed using a metal piece and triangular copper foil piece with a 3D printed housing. Reproduced from [139]. Copyright 2019 AIP. (f) The security tag attached to a household object.

Figure 11. (a) A schematic of the paper-based force sensor where carbon resistor acts as the sensing material printed on the paper cantilever. (b) An image of the paper-based force sensors fabricated in series that can be later cut into singular sensing devices. (c) Schematic of paper-based force sensor highlighting the point of fold to increase the strength of the paper cantilever. (d) An image of the paper-based force sensor displaying the folded portions that provide the mechanical strength to the sensor. Figures 11(a-d) are reproduced from [140]. Copyright 2011 The Royal Society of Chemistry. (e) Schematic of the kirigami split-ring resonator highlighting the points where kirigami cuts are made and the inkjet printed sensing inks, (f) and the working principle showing that a stress causes an increase in the kirigami cuts leading to a change in resonance frequency. Figures 11(e,f) are reproduced from [142]. Copyright 2019 IOP.

Figure 12. Diagram showcasing sampling point on the human body for different disease identification, and the correlation between sample type, disease detection, and principle of detection used via smartphone enabled POC devices. Reproduced from [145]. Copyright 2019 Elsevier.

Figure 13. (a) Schematic displaying the components of a “LFM-POCT” unit consisting of a mouthpiece, paper-based humidity sensor, and micro-heater assembly. User blows into the LFM-POCT device; Data is wirelessly transmitted to a smartphone via Bluetooth and analyzed for lung health function through a custom-built APP for lung function assessment. Reproduced from [151]. Copyright 2017 Elsevier. (b) Smart “Paper Barcode” platform for blood type detection. (i) Blood typing test result “B+” shown visually via 3 strips of sensing channels (barcode-like paper-based blood typing sensor); (ii) Digital photo is captured via smartphone camera to analyze data using colorimetry technique; and finally (iii) the APP displays to the user the blood type result via a text on the screen. Reproduced from [153]. Copyright 2014 American Chemical Society. (c) In-vivo one-touch-activated blood multi-diagnostic system (OBMS). Paper-based POC device applied on a rabbit ear artery for glucose and cholesterol detection. Blood sample is collected through a one-finger press and separated within the sensor’s chamber, and finally transported to the reaction zones. After 5 mins, the final image is captured via a smartphone camera and color change is analyzed through the custom APP for results. Reproduced from [154]. Copyright 2015 The Royal Society of Chemistry. (d) White Blood Cell (WBC) counting Device. Principle of detection based on an electrochemical sensor output, where the sample interacts with the sensing material of the paper sensor, and the output signal is generated from the portable potentiostat and transmitted to a smartphone for analysis. Reproduced from [155]. Copyright 2017 Elsevier. 41

Figure 14. (a) Sequential schematics displaying fabrication steps of paper-based sensor, proposed colorimetric detection, and analysis methodology through a test setup and a smartphone apparatus. (b) The corresponding quantitative analysis is computed by image processing in the built-in app (left photograph) where the algorithm correlates the sensor’s “Red” color intensity to Cyanide concentrations (right photograph). (c) H2O2 Detection in milk: screenshots depicting sequential steps for operating the POC devices through an easy-to-use smartphone app, where RGB analysis is performed and tabulated to generate a “Non-drinkable” or “Drinkable” message. Figures 14(a-c) are reproduced from [158]. Copyright 2017 Elsevier.

Figure 15. Comparison of different types of materials used in electronics (diagonal axis), their inherent properties (horizontal axis) and the suitability of each property in different areas (vertical axis).

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