electronics

Article ElectroPaper: Design and Fabrication of Paper-Based Electronic Interfaces for the Water Environment

Lijuan Liu 1,*, Jiahao Guo 1, Chao Zhang 2 , Zhangzhi Wang 2, Pinqi Zhu 3, Tuo Fang 4, Junwu Wang 3, Cheng Yao 1,* and Fangtian Ying 1

1 College of Computer Science and Technology, Zhejiang University, Hangzhou 310027, China; [email protected] (J.G.); [email protected] (F.Y.) 2 School of Software Technology, Zhejiang University, Hangzhou 310027, China; [email protected] (C.Z.); [email protected] (Z.W.) 3 School of Industrial Design, Hubei University of Technology, Xianning 430064, China; [email protected] (P.Z.); [email protected] (J.W.) 4 Department of Communication Design, Dankook University, Gyeonggido 16890, Korea; [email protected] * Correspondence: [email protected] (L.L.); [email protected] (C.Y.); Tel.: +86+18679429756 (L.L.)

Abstract: The fabrication of underwater devices is necessary for the exploration of water environ- ments and interactions in the Human–Computer Interaction (HCI) field. However, there are fewer approaches to support prototyping used in water environments. The existing prototype methods lack systematic waterproof treatments and provide insufficient software for balance and buoyancy analysis. To address these limitations, we present ElectroPaper, a new approach for the design



and fabrication of prototypes used in water environments (surface or beneath) with paper-based
 electronic interfaces with a crease layer, hardware distribution layer, and hollow-out layer to support

Citation: Liu, L.; Guo, J.; Zhang, C.; physical properties, such as waterproofing, foldability, and conformability. The approach includes a Wang, Z.; Zhu, P.; Fang, T.; Wang, J.; computational design tool for assisting in balance analysis, three-dimensional (3D) model unfolding, Yao, C.; Ying, F. ElectroPaper: Design and circuit drawing. We describe the design and fabrication process and provide several example ap- and Fabrication of Paper-Based plications to illustrate the feasibility and utility of our approach. ElectroPaper provides an inexpensive Electronic Interfaces for the Water and effective medium for the fabrication of customized digital prototypes for water environment use. Environment. Electronics 2021, 10, 604. https://doi.org/10.3390/ Keywords: paper electronics; water environment; prototyping; fabrication; design electronics10050604

Academic Editor: Han Eol Lee 1. Introduction Received: 6 February 2021 Recently, there is a focus on water interactions and underwater devices in the Human– Accepted: 3 March 2021 Published: 5 March 2021 Computer Interaction (HCI) field due to their extensive applications in the exploration of water environments, aquatic recreation, and installation art. These developments cannot be

Publisher’s Note: MDPI stays neutral separated from prototypes, which play an important role in concept design and system iter- with regard to jurisdictional claims in ation. However, most of the prototype approaches do not consider this special environment, published maps and institutional affil- and most electronic circuit fabrication is unsuitable for water environments. Methods, like iations. 3D printing, are time-consuming, involve many design iterations, and require considerable testing. Challenges, such as waterproofing, balance, buoyancy, and materials, make the design and fabrication complicated and expensive. Paper and papercraft methods are used in various scenarios, such as HCI, robotics, and scientific research. Researchers have explored the affordances of paper with Do It Yourself Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. (DIY) electronics to create interactive books [1,2], paper actuators [3], origami robots [4], and This article is an open access article tangible paper interfaces [5]. Paper is inexpensive, lightweight, environmentally friendly, distributed under the terms and and foldable, with myriad methods of creative expression through the medium itself [6]. conditions of the Creative Commons We rely on these advantages to fabricate the prototypes used in water environments, and Attribution (CC BY) license (https:// develop ElectroPaper, a low-cost and effective approach to implement them. creativecommons.org/licenses/by/ Several issues should be considered when making paper-based electronic prototypes 4.0/). for water environment use: (1) materials should be waterproof and able to protect the

Electronics 2021, 10, 604. https://doi.org/10.3390/electronics10050604 https://www.mdpi.com/journal/electronics Electronics 2021, 10, x FOR PEER REVIEW 2 of 19 Electronics 2021, 10, 604 2 of 19 Several issues should be considered when making paper‐based electronic prototypes for water environment use: (1) materials should be waterproof and able to protect the electronicselectronics inside; (2) materials should be foldable to fulfill fulfill the design requirement; (3) the prototype should be durable and last long enough to complete the test; and, (4) balance is essential,essential, and it must be evaluated during fabrication. ToTo address these issues, we designed a paper-basedpaper‐based electronic interface that can be usedused in in water water (Figure (Figure 1a).1a). The The multilayer multilayer structure structure includes includes a crease a crease layer, layer, hardware hardware dis‐ tributiondistribution layer, layer, and and hollowed hollowed-out‐out layer. layer. The Thecrease crease layer layer and andhollowed hollowed-out‐out layer layer are wa are‐ terproof.waterproof. Between Between them them is the is the hardware hardware distribution distribution layer, layer, where where electronic electronic components components areare arranged arranged and connected connected by copper copper wires. wires. This This structure structure ensures ensures the the high high functionality functionality ofof waterproofing, waterproofing, foldability, and durability of the interface. We created a design tool that was based on Rhinoceros 3D software for users to build prototypes. In the software, the useruser builds builds a 3D 3D model model and and then then places places electronic electronic components on the surface. The model’s balancebalance can can be evaluated, and the model adjusted based on the results. The 3D model is flattenedflattened using origami unfolding algorithms [7]. [7]. The user can then draw wires based on thethe circuit schematic,schematic, and and export export four four digital digital files files for for fabrication. fabrication. Our Our approach approach is low-cost, is low‐ cost,efficient, efficient, and functional.and functional. We believeWe believe that that it will it will spawn spawn new new designs designs and and applications applications of ofunderwater underwater devices. devices.

Figure 1.1. ((aa)) Waterproof,Waterproof, foldable foldable paper-based paper‐based electronic electronic interface interface of ElectroPaper of ElectroPaper system. system. Example Example applications: applications: (b) a paper (b) a paperdevice device to detect to detect water water quality; quality; (c) a decorative (c) a decorative device device that floats that floats according according to the to water the water temperature; temperature; (d) a camouflaged(d) a camou‐ flagedcamera camera to capture to capture underwater underwater scenes; scenes; and, (e )and, a deformable (e) a deformable robot for robot entertaining for entertaining interaction. interaction.

WeWe demonstrate our fabrication approach through a series series of of applications applications (Figure(Figure 11),) , includingincluding water water environment environment detection, detection, underwater underwater robots, robots, entertainment, entertainment, and and water water en‐ vironmentenvironment decoration, decoration, with with unique unique attributes, attributes, such suchas buoyancy, as buoyancy, water water quality, quality, and water and temperature.water temperature. Our main Our contributions main contributions are, as are,follows: as follows: 1. WeWe provide provide a low-costlow‐cost prototypeprototype approach approach for for paper-based paper‐based electronic electronic devices devices suitable suita‐ blefor for water water environments. environments. Tests Tests verify verify its its waterproofing, waterproofing, durability, durability, and and foldability. foldability. 2.2. WeWe present present a a design design tool tool for for balance balance analysis, analysis, 3D 3D model model unfolding, unfolding, and and circuit circuit draw draw-‐ ing,ing, which which simplifies simplifies the the design design and and fabrication fabrication process. process. 3.3. WeWe demonstrate demonstrate a a series series of of water water-related‐related applications applications created created using using our our method, highlightinghighlighting the the efficiency, efficiency, usability, and functionality of our approach.

2.2. Related Related Work Work 2.1. Underwater Devices and Interactions in HCI 2.1. Underwater Devices and Interactions in HCI Interactions in water environments remain largely unexplored, yet they hold great de- Interactions in water environments remain largely unexplored, yet they hold great velopment potential. With advances in technology, such as communication and waterproof- development potential. With advances in technology, such as communication and water‐ ing, underwater devices and interactions have seen increased focus. Oppermann et al. [8] proofing, underwater devices and interactions have seen increased focus. Oppermann et and Yamashita et al. [9] explored the use of Augmented Reality (AR) equipment in un- al. [8] and Yamashita et al. [9] explored the use of Augmented Reality (AR) equipment in derwater education and entertainment. Pell et al. [10] proposed a gravity well to support underwater education and entertainment. Pell et al. [10] proposed a gravity well to sup‐ underwater play through water-movement interaction. Choi et al. [11] designed a wearable port underwater play through water‐movement interaction. Choi et al. [11] designed a device for team underwater interaction. Many researchers have combined HCI and a wearable device for team underwater interaction. Many researchers have combined HCI water environment for novel user experiences [12–15], or focused on interactions between and a water environment for novel user experiences [12–15], or focused on interactions humans and underwater robots. Ukai et al. [16] made a buddy robot that could recognize, between humans and underwater robots. Ukai et al. [16] made a buddy robot that could follow, and present information to a swimmer. Chutia et al. [17] provided an overview of recognize,underwater follow, robots and and present their progress. information Raffe to et al.a swimmer. [1] proposed Chutia guidelines et al. [17] for theprovided design an of overviewunderwater of underwater equipment and robots player–computer and their progress. interaction Raffe et [18 al.]. [1] proposed guidelines for the designUnpredictable of underwater and hazardous equipment conditions and player–computer that make it interaction difficult to [18]. realize a general method to prototype equipment used in the water environment. Although digital manu- facturing tools, such as rapid prototyping printers, enable the production of underwater

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equipment, they still have difficulty integrating with the water environment for underwater interaction [18,19]. Moreover, the challenges of waterproofing, materials, and fabrication make it difficult for ordinary people to produce underwater devices. We facilitate the creation of low-cost, functional underwater devices from common paper materials.

2.2. Electronic Paper Prototypes Many researchers have recently focused on the use of paper in the HCI field, as paper is thin, lightweight, ubiquitous, and inexpensive [20]. Fuchs et al. [21] explored new types of wearable devices. Zheng et al. [5] uncovered the characteristics of paper as a tangible interface. Chang et al. [22] explored the potential of kirigami- and origami-based structures in sophisticated haptic feedback through simple cut-and-fold fabrication techniques. The integrated fabrication of body structures, actuators, sensors, and electronic circuits into the digital system is an open problem [23]. Advanced techniques make paper circuits available for creative interactions and applications [23,24]. There is already a focus on the design of electronic paper, which integrates electronic components to support digital interaction. Researchers have combined paper and electronics and presented toolkits for the design of interactive papercraft [6,19,25]. Oh et al. [20,26,27] presented paper mechatronics, which combined electronics and computation with traditional papercraft. Researchers have explored paper interaction with materials and technologies, such as shape memory alloy [28], electrical actuation [3], and retro-reflective material [29], which allow for flexible manufacture. Rus et al. [30] introduced paper-structured robotics, which combines scalable geometric structures, mechanical structures, and electronic components, and then discussed its use in different fields. Existing research has shown how to make paper-based prototypes by integrating electronic circuits with paper material, but scant attention has been paid to its applica- tion in a water environment. We use various paper materials, such as fiber-based copy paper, polymeric film-based synthetic paper with the properties of being waterproof, wear-resistant, foldable, and conformable, and then propose a multilayered, paper-based, electronic interface to create prototypes that are suitable for water environments.

2.3. Papercraft Fabrication Tools As the range of papercraft applications has expanded, the problem of folding has assumed greater importance [31]. Lang et al. [31,32] proposed a tree method to design a crease pattern folding flat into a base with any number of flaps. Mitani et al. [33] proposed a method for producing unfolded papercraft patterns from triangulated meshes by means of strip-based approximation. However, these methods might be too difficult for novices to design cutout sheets. Based on these unfolding algorithms, researchers have developed papercraft design tools that enable novices or experts to create paper-folding without technical restrictions [34–37]. Some design tools also provide papercraft design assistance, such as waste reduction [32], circuit construction [19], and mobility [6,38]. Being derived from origami art, origami mathematics proves that any geometry can be theoretically simulated by origami [36], and paper has become an excellent medium for prototyping and crafting with good foldability. Inspired by previous work, we present a design tool that leverages algorithms to flatten 3D models to two-dimensional (2D) representations, which is suitable for papercraft. Our design tool demonstrates customized functions for water environments, and allows users to analyze the balance state and working conditions of prototypes, such as floating, suspending, and sinking, to simplify testing and improve the usability.

3. ElectroPaper: Design and Fabrication In this section, we describe the ElectroPaper system, including materials, paper-based electronic interfaces, design software, and fabrication approach, so as to provide a low-cost and effective method to build electronic prototypes for use in water environments. Electronics 2021, 10, 604 4 of 19

3.1. Materials Introduction 3.1.1. Paper Materials We used several paper materials, including ordinary paper (copy paper) and synthetic paper (polymeric film with paper properties). Ordinary paper is not waterproof or durable in the water environment to guarantee the regular work of the electronic prototype. There- fore, we tried to find materials or ways with traditional papers’ advantages and the quality of waterproofing and durability. We tested several methods, such as waterproof synthetic papers, spraying waterproof chemicals, and waxing, and then compared their waterproofing, foldability, durability, price, and process. Table1 shows the results of pilot studies, evaluating features and ability on a scale of good, medium, or poor. We find that spraying waterproof chemicals is effective, but it is limited by foldability and durability. Waxing does not provide stable waterproofing, and waxed paper cannot be folded. We tested two types of waterproof paper. Polyethylene terephthalate (PET) paper (Hang Tang, PET synthetic paper) is adhesive and performs better than polyvinyl chloride (PVC) paper (Hang Tang, PVC synthetic paper). Waterproof synthetic papers require no chemical or physical processing. We decided to use PET paper as the substrate of our first layer, as it is adhesive, waterproof, tear-resistant, and foldable, and it supports laser printing.

Table 1. Comparison of waterproof materials and methods.

Methods Waterproofing Foldability Durability Price/A4 Sheet Process Others Spraying with waterproof paint Good (>10 h) Poor Poor $0.50 Poor - Waxing Poor (<2 h) Poor Poor $3.20 Poor - Waterproof paper (PVC) Medium (<5 h) Good Medium $0.10 Good Adhesive Waterproof paper (PET) Good (>10 h) Good Good $0.20 Good Adhesive Waterproof paper (OPP) Good (>10 h) Good Good $0.07 Good Translucent

We used a coated paper with one side that is covered with biaxially oriented polypropy- lene (BOPP) film (Han Tang, BOPP coated paper) to increase its toughness and wear- resistance. This material is not waterproof, and it formed the second layer, where the circuit was attached. The coated paper was stuck together with the first layer. It can be removed more easily than ordinary copy paper, which could be convenient for its adjustment or replacement. This commercial paper has a price of $0.10/A4 sheet. We used a waterproof oriented polypropylene (OPP) paper (Flower Season, OPP paper) (Table1), which is collapsible and translucent, as the third layer’s substrate. It allows users to directly view and check the hardware in the second layer. Unlike the first layer, this layer is not sticky, and it is easily removed for reuse without damaging the circuit.

3.1.2. Waterproof Shells Waterproofing is essential for electronic prototyping that is applied to water envi- ronments. Most of the electronic components that we use are standardized, small in size, and likely to be reused in the circuit. These features mean that it is complicated and time-consuming to provide waterproof protection for every component with paper folding. Therefore, we use reusable shells with stable waterproof effects to protect the electronic components. We measured the size of the needed electronic components and customized waterproof shells (Figure2a) using a common reverse mold method. The mold is made with silica gel, and we use it to make waterproof shells from epoxy resin. The shells have a standard size (5 mm edges, 1.5 mm thick, 1 mm gap between shell and electronics) and can be reused, as shown in Figure2b. Electronics 2021, 10, 604 5 of 19

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(a) (b)

Figure 2. (a) Waterproof shells. The soft silicon shell of the switch is different from the others, and supports its ups and downs; and, ( b) standard for making waterproof shells.

3.2.3.2. Structure Structure of of Paper Paper-based‐based Electronic Electronic Interface WeWe introduce the multilayer structure of the paper paper-based‐based electronic interface, which includesincludes three three layers layers of different paper materials (Figure3 3).). WaterproofWaterproof paper (PET) is the first first layer, which can be immersed in water for a long time.time. This This layer layer presents presents the the crease crease lines lines for for folding, folding, which which can can be laser printed on the paper. The hardware distribution layer is made of coated paper (BOPP). The copper wire andand electronic components are are fixed fixed on the non-coatednon‐coated side. The hollow hollow-out‐out layer, with translucenttranslucent paper paper (OPP), (OPP), can can seamlessly seamlessly cover cover the the second second layer, layer, except for the position of thethe electronic components. The The layer layer is is hollowed hollowed out out for for the electronic components, in orderorder to make the interface flat flat and easy to fold. WaterproofingWaterproofing isis essential essential in in electronic electronic prototyping prototyping that that is used is used in water in water environments. environ‐ ments.The first The and first third and layers, third whichlayers, contact which contact water, are water, waterproof, are waterproof, and they and are they glued are together glued togetherwith waterproof with waterproof adhesive. adhesive. The waterproof The waterproof shells are shells glued are toglued the thirdto the layer third tolayer cover to coverthe electronic the electronic components. components. For better For better waterproofing, waterproofing, the outer the outer contour contour sizes sizes of the of first the firstand and third third layers layers are are the the same, same, and and the the middle middle layer layer is is smaller. smaller. After After our our test, test, the outer contourcontour of of the the second second layer layer was was shifted shifted to to the the inside by at least 8 mm, which will have a stablestable waterproofing waterproofing effect. After stacking, stacking, the thickness thickness of the the three three-layer‐layer paper interface (excluding(excluding electronic electronic components components and and waterproof waterproof shells) shells) is is 0.3 0.3 mm, and it has excellent foldingfolding ability. ability. TheThe electronic electronic components components are commercially available. The wires are made from 0.05 mmmm copper copper tape and they can be obtained by simple processing. We We also tested other conductiveconductive methods. methods. For For example, example, screen screen printing printing is cumbersome, is cumbersome, high high cost, cost, and not and suit not‐ ablesuitable for fabricating for fabricating customized customized circuits. circuits. Moreover, Moreover, it is it not is not easy easy to control to control the the amount amount of inkof ink when when printing, printing, and and the the conductive conductive ink ink becomes becomes hard hard when when dry dry and and breaks breaks when folded,folded, so so its its conductivity is is unstable. Inkjet Inkjet printing printing is is a new and simple circuit circuit-making‐making method,method, but but it it is is expensive. expensive. Silver Silver foil foil is another is another method, method, which which is hard is hard to handle to handle and andcan causecan cause circuit circuit failure failure when when folded. folded. Overall, Overall, copper copper tape tapeconducts conducts electricity electricity well, well, folds folds eas‐ easily, and costs little. ily, and costs little.

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FigureFigure 3. 3. MultilayerMultilayer structure structure of of the the paper paper-based‐based electronic electronic interface. interface. 3.3. Design Software 3.3. Design Software The design software is implemented in Rhinoceros 3D with Grasshopper and Human The design software is implemented in Rhinoceros 3D with Grasshopper and Human UI plugins. Different from other design software, our tool adds electronic components UI plugins. Different from other design software, our tool adds electronic components and circuits to the model design phase, provides buoyancy and balance analysis of the 3D and circuits to the model design phase, provides buoyancy and balance analysis of the 3D models, achieves the unfolding of 3D models into 2D planar sheets, and generates folding models, achieves the unfolding of 3D models into 2D planar sheets, and generates folding creases to simplify manual fabrication. The software effectively improves the feasibility of creases to simplify manual fabrication. The software effectively improves the feasibility of prototypes. prototypes. We built the design tool’s fundamental framework based on the functions of Launch We built the design tool’s fundamental framework based on the functions of Launch Window, Add Elements, and Merge of Human UI (Figure4a). It includes five parts: Window,component Add library Elements, module, and balance Merge analysisof Human module, UI (Figure 3D unfolding 4a). It includes module, five circuit parts: drawing com‐ ponentmodule, library and filemodule, export balance module. analysis The component module, 3D library unfolding module module, is created circuit with drawing Create module,Button, and Import file export 3DM, etc.module. (Figure The4 b).component When the library user module clicks on is acreated component with Create in the Button,library, Import its 3D 3DM, model etc. can (Figure be imported. 4b). When The the balance user clicks analysis on a component module runs in the code library, in the itsGrasshopper 3D model can Python be imported. Script Editor The balance to analyze analysis the workingmodule runs state code and in balance the Grasshopper of modules Python(Figure Script4c). In Editor order toto buildanalyze the the 3D unfoldingworking state module, and webalance use the of Commandmodules (Figure Line function 4c). In orderof Grasshopper to build the to 3D call unfolding the unfolding module, algorithm we use provided the Command in ref. [7 Line], achieving function the of target Grass 3D‐ hopperobject flattenedto call the to unfolding 2D patterns algorithm that will provided be imported in ref. back [7], achieving into Rhinoceros the target 3D. 3D As object for the flattenedcircuit drawing to 2D patterns module, that the will Create be imported Rhino Command back into Button Rhinoceros is used 3D. to call As Drawfor the Multiple circuit drawingCurves module, functions the in Create Rhinoceros Rhino 3D.Command It then Button combines is used Offset to call Curve, Draw Boundary Multiple Curves Surface, functionsetc., to achieve in Rhinoceros the circuit 3D. drawing It then functioncombines (Figure Offset4 Curve,d). Finally, Boundary the file Surface, export moduleetc., to achievewas built the through circuit drawing the Export function Objects (Figure Component 4d). Finally, in Human the file UI export to export module 2D patterns was built into throughprintable the files. Export Objects Component in Human UI to export 2D patterns into printable files.

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Figure 4. Software building: (a) fundamental framework built with Human UI; (b) grasshopper wiring diagram of the component library module; (c) code of the balance analysis module; and, (d) grasshopper wiring diagram of the circuit FigureFigure 4. 4.Software Software building: building: ( a(a)) fundamental fundamental frameworkframework built with with Human Human UI; UI; (b (b) )grasshopper grasshopper wiring wiring diagram diagram of ofthe the drawing module. componentcomponent library library module; module; ( c()c) code code of of the the balance balance analysisanalysis module; and, and, ( (dd)) grasshopper grasshopper wiring wiring diagram diagram of of the the circuit circuit drawing module. drawing module. 3.3.1. Component Library 3.3.1. Component Library 3.3.1.We Component built a component Library library in the software, including electronic components, wa‐ terproofWe shells,built a papercomponent structures, library and in the counterweight software, including modules, electronic and users components, can add wa other‐ We built a component library in the software, including electronic components, wa- components.terproof shells, Data, paper such structures, as the weight and counterweightand dimensions modules, of electronic and users components can add andother wa ‐ terproof shells, paper structures, and counterweight modules, and users can add other terproofcomponents. shells, Data, are suchpredefined. as the weight The software and dimensions can read of the electronic model’s components volume, volume and wa cen‐ ‐ components. Data, such as the weight and dimensions of electronic components and water- troid,terproof hardware shells, centerare predefined. of mass, andThe othersoftware parameters can read to the analyze model’s its volume,buoyancy volume and balance. cen‐ prooftroid, shells, hardware are predefined.center of mass, The and software other parameters can read the to analyze model’s its volume, buoyancy volume and balance. centroid, hardware center of mass, and other parameters to analyze its buoyancy and balance. 3.3.2. Primitive Structure 3.3.2. Primitive Structure 3.3.2.We Primitive provide Structure multiple groups of paper structures in the software library based on pre‐ We provide multiple groups of paper structures in the software library based on pre‐ viousWe origami provide research. multiple These groups can ofserve paper as structures3D model references in the software for prototype library based designs. on vious origami research. These can serve as 3D model references for prototype designs. previousThe deployable origami structure research. supports These can the serve transformation as 3D model of the references prototype. for The prototype rigid structure designs. The deployable structure supports the transformation of the prototype. The rigid structure can be used as a supporting or stationary form in the prototype. Figure 5 shows the 3D Thecan deployable be used as structurea supporting supports or stationary the transformation form in the ofprototype. the prototype. Figure The 5 shows rigid structurethe 3D models of the paper structures, which are developed into a 2D crease pattern, and the canmodels be used of the as apaper supporting structures, or stationary which are formdeveloped in the into prototype. a 2D crease Figure pattern,5 shows and the the 3D paper prototypes can be folded by referring to the 2D patterns. Below, we show water modelspaper prototypes of the paper can structures, be folded which by referring are developed to the 2D into patterns. a 2D crease Below, pattern, we showand the water paper prototypestemperaturetemperature can sensingsensing be folded and by starfish referring robotrobot to the designsdesigns 2D patterns. based based on on Below, the the structures structures we show that water that are are temperature shown shown in in FiguresensingFigure 4. 4. and starfish robot designs based on the structures that are shown in Figure4.

FigureFigure 5. 5. PaperPaper structures in in component component library. library.

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Electronics 2021, 10, x FOR PEER REVIEW3.3.3. Software Workflow 8 of 19 We show the software workflow in Figure 6. 3.3.3. Software WorkflowWorkflow We showshow thethe softwaresoftware workflowworkflow inin Figure Figure6 .6.

.

Figure 6. Software workflow. . Figure 6. Software workflow. Figure 6. Software workflow. (1)(1) Model Building. In In this this step, step, the the prototype prototype model model is is built built and and the the relevant relevant com com-‐ ponents are placed on it. The The Rhino Rhino modeling software software can can be used to build the initial (1) Model Building. In this step, the prototype model is built and the relevant com‐ modelmodel (Figure7 7b),b), and and the the paper paper structure structure model model in in the the component component library library can can be used be used for ponents are placed on it. The Rhino modeling software can be used to build the initial forassistance. assistance. The The user user selects selects the the components components from from the the library library and and then then places places them them on theon model (Figure 7b), and the paper structure model in the component library can be used thesurfaces surfaces of the of the model model (Figure (Figure7c). We7c). alsoWe also provide provide waterproof waterproof shell shell models. models. The userThe user can for assistance. The user selects the components from the library and then places them on canmanually manually adjust adjust the positionsthe positions of the of components.the components. the surfaces of the model (Figure 7c). We also provide waterproof shell models. The user (2)(2) Balance Analysis.Analysis. Integrating Integrating balance balance analysis analysis into into the the model model design design can effectivelycan effec‐ tivelyimprovecan manually improve the effectiveness adjust the effectiveness the positions of the of prototype of the the prototype components. and simplify and simplify the iterative the iterative process. process. The userThe userselects selects(2) the Balance model’s the model’s Analysis. working working Integrating conditions conditions balance from from “float analysis “float on water”,into on water”,the “suspend model “suspend design in water”, in can water”, effec and‐ “sinkandtively “sink under improve under water” the water” effectiveness (Figure (Figure7d). The 7d).of the modelsThe prototype models and componentsand and components simplify are the selected are iterative selected for process. buoyancy for buoy The‐ ancyanduser balance andselects balance the analysis. model’s analysis. The working softwareThe software conditions shows shows the from results the results“float and on suggestsand water”, suggests adjustments. “suspend adjustments. in The water”, userThe usercanand change“sinkcan change under the 3D the water” model’s 3D model’s (Figure shape, shape,7d). rearrange The rearrange models the positions theand positions components of electronic of electronic are selected components, components, for buoy and‐ andchangeancy change and the balance numbersthe numbers analysis. of counterweights of Thecounterweights software to shows meet to meet designthe resultsdesign goals. andgoals. suggests adjustments. The user can change the 3D model’s shape, rearrange the positions of electronic components, and change the numbers of counterweights to meet design goals.

Figure 7. (a) Component library; ( b) build initial model; ( c) place components on model surface; and, ( d) analyze balance

and buoyancy. Figure 7. (a) Component library; (b) build initial model; (c) place components on model surface; and, (d) analyze balance and buoyancy. (3)(3) 2D 2D Patterns Patterns Generation. Generation. Origami Origami-based‐based flattening flattening and and paper paper model model unfolding unfolding al‐ gorithmsalgorithms [7] [7 are] are integrated integrated into into our our design design tool tool in in Rhinoceros Rhinoceros 3D, 3D, enabling enabling 3D 3D objects objects to bebe flattened flattened(3) 2D Patterns to 2D patterns. Generation. Model Origami surfaces ‐based should flattening be intact, and without paper modelhollowing unfolding out, and al‐ thethegorithms user user must must [7] are patch patch integrated the the hollow hollow into surface surface our design before before tool flattening flattening in Rhinoceros (Figure (Figure 3D, 88a).a). enabling Subsequently, 3D objects the the 3D3D to be flattened to 2D patterns. Model surfaces should be intact, without hollowing out, and the user must patch the hollow surface before flattening (Figure 8a). Subsequently, the 3D

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model can be flattened flattened to generate crease, hardware distribution, and hollow-outhollow‐out patterns (Figure 88b,c).b,c). TheThe creasecrease patternpattern showsshows thethe folding folding trace, trace, cutout cutout sheets, sheets, and and hollow-out hollow‐out trace for for guiding guiding manual manual fabrication. fabrication. The The hardware hardware distribution distribution pattern pattern presents presents the out the‐ lineoutline paths paths of the of theelectronic electronic components components and andcounterweights. counterweights. The Thehollow hollow-out‐out pattern pattern pre‐ presentssents the the outline outline path path of ofthe the electronic electronic components, components, which which must must be be cut. cut. Our Our tool allows for customization, so that users can edit and parameterize the patterns, such as the size and shape of the paper flapsflaps (Figure8 8d).d).

Figure 8.8. (a) Patching hollow surfaces; (b) select and unfold three-dimensionalthree‐dimensional (3D) model; (c) generate three kinds of patterns; and,and, ((dd)) usersusers cancan editedit two-dimensionaltwo‐dimensional (2D)(2D) patterns.patterns.

(4) Circuit Drawing. To To provide a variety of functions,functions, our paper circuitscircuits containcontain components, such as batteries, sensors, actuators, controllers, and communication mod mod-‐ ules. The outline of each electronic component and the locations of connection points are displayed inin thethe 2D2D hardware hardware distribution distribution pattern, pattern, allowing allowing for for the the user user to directlyto directly connect con‐ pointsnect points to draw to draw circuits circuits (Figure (Figure9a). The 9a). connection The connection lines are lines non-overlapping, are non‐overlapping, which allows which allowsfor direct for processing direct processing of the wiring of the wiring path by path the laserby the cutter. laser cutter. The parametric The parametric design design of our oftool our also tool enables also enables the user the to user adjust to adjust the width the width of the of connection the connection wire wire (Figure (Figure9a). In 9a). this In thisstep, step, the software the software will generatewill generate a new a hardwarenew hardware distribution distribution pattern pattern that shows that shows the paths the pathsof electronic of electronic components, components, counterweights, counterweights, and copper and copper wires. wires. Finally, Finally, the user the can user export can exportfabrication fabrication files, including files, including the crease the pattern,crease pattern, hardware hardware distribution distribution pattern, pattern, hollow-out hol‐ pattern,low‐out andpattern, wire and path wire (Figure path9 b).(Figure 9b). Our software software is is a a design design tool tool for for creating creating underwater underwater devices devices that that are arebased based on pa on‐ papercraft.percraft. It combines It combines the the functions functions of of modeling, modeling, balance balance analysis, analysis, 3D 3D model model unfolding, and circuit circuit drawing, drawing, which which can can simplify simplify the the design design and and fabrication fabrication and andimprove improve the usa the‐ bilityusability of the of theprototype. prototype.

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ElectronicsElectronics 2021 2021, ,10 10, ,x x FOR FOR PEER PEER REVIEWREVIEW 1010 ofof 1919

FigureFigure 9.9. ((aa)) CircuitsCircuits drawing.drawing. drawing. DrawDraw Draw coppercopper copper wireswires wires onon on thethe the hardwarehardware hardware distributiondistribution distribution patternpattern pattern andand and generategenerate generate aa newnew a new pattern;pattern; pattern; ((bb)) (ExportExportb) Export files.files. files.

3.4.3.4. Fabrication Fabrication Approach Approach AllAll of of the the materials materials are are commercially commercially available available and and they they can can be be processed processed by by common common tools,tools, such such as as a a laser laser cuttingcutting machinemachine and and printer.printer. UsersUsers cancan effectivelyeffectively makemake waterproofwaterproof paperpaper-basedpaper‐‐basedbased electronic electronic interfacesinterfaces and andand prototypes prototypesprototypes of ofof underwater underwaterunderwater devices devicesdevices at atat a aa low lowlow cost. cost. This This approachapproach has has six six steps.steps. DesignDesign 3D 3D models models in in the the Software. Software. TheThe user user designs designs and and builds builds the the 3D 3D model model inin thethe software,software, arrangesarranges the thethe components, components,components, and andand tests teststests its itsits balance. balance.balance. The TheThe 3D 3D3D model modelmodel is isis unfolded unfoldedunfolded into intointo a a2Da 2D2D patterns, patterns,patterns, on onon which whichwhich the thethe user useruser draws drawsdraws wires. wires.wires. The TheThe software softwaresoftware generates generatesgenerates four fourfour digital digitaldigital files. files.files. PrintPrint and and Cut. Cut. FourFour materialsmaterials must must be be processed. processed. First First is is the the crease crease layer layer (Figure (Figure(Figure 10a). 1010a).a). TheThe user user can can print print patternspatterns onon thethe the waterproofwaterproof waterproof sideside side ofof of thethe the PETPET PET waterproofwaterproof waterproof paperpaper paper withwith with aa laseralaser laser printer.printer. printer. TheThe The paperpaper paper isis cutcut is cut alongalong along thethe theoutlineoutline outline pathpath path withwith with aa laserlaser a laser cuttercutter cutter (Figure(Figure (Figure 11a).11a). Sub11Suba).‐‐ sequently,Subsequently,sequently, thethe the useruser user cutscuts cuts thethe the secondsecond second layer’slayer’s layer’s contourcontour contour onon on thethe the oppositeopposite opposite sideside side ofof of thethe first firstfirst layer layer (Figure(Figure 10b).1010b).b). Only Only the the protected protected side side of of the the paperpaper isis toto bebe pierced.pierced. TheThe innerinner partpart ofof thethe protectedprotected paper paper isis removedremoved (Figure(Figure 1111b).11b).b). The The second second part part isis thethe hardwarehardware distributiondistributiondistribution layerlayer (Figure (Figure 10c).1010c).c). The The user user prints prints the the patterns patterns on on coated coated paper, paper, cutscuts alongalong thethe contourcontour withwith aaa laser laserlaser cutter, cutter,cutter, and andand then thenthen pastes pastespastes it onitit onon the thethe sticky stickysticky side sideside of the ofof the waterproofthe waterproofwaterproof layer layerlayer (Figure (Figure(Figure 11c). 11c).Third11c). ThirdThird is the isis hollow-out thethe hollowhollow layer‐‐outout layerlayer (Figure (Figure(Figure 10d). 10d).10d). The userTheThe useruser cuts cuts thecuts translucent thethe translucenttranslucent paper paperpaper along alongalong the outline path and the electronic components’ hollow-out position. We use copper tape for thethe outlineoutline pathpath andand thethe electronicelectronic components’components’ hollowhollow‐‐outout position.position. WeWe useuse coppercopper tapetape the electric wires (Figure 10e), which is sticky on one side and can be cut with a laser cutter forfor thethe electricelectric wireswires (Figure(Figure 10e),10e), whichwhich isis stickysticky onon oneone sideside andand cancan bebe cutcut withwith aa laserlaser (Protolaser U3, LPKF) [39] or a carving machine (Silhouette America SILH-CAMEO-4, cuttercutter (Protolaser(Protolaser U3,U3, LPKF)LPKF) [39][39] oror aa carvingcarving machinemachine (Silhouette(Silhouette AmericaAmerica SILHSILH‐‐ CrafterCuts). CAMEOCAMEO‐‐4,4, CrafterCuts).CrafterCuts).

FigureFigure 10. 10. MaterialsMaterialsMaterials thatthat that havehave have beenbeen been processed.processed. processed. ((aa)) ( aCreaseCrease) Crease layer;layer; layer; ((bb)) ( oppositebopposite) opposite ofof thethe of thecreasecrease crease layer;layer; layer; ((cc)) hardware(hardwarec) hardware distributiondistribution distribution layer;layer; layer; ((dd)) ( hollowdhollow) hollow-out‐‐outout layer;layer; layer; ((ee)) ( copperecopper) copper wire.wire. wire.

CircuitCircuit Fabrication. Fabrication. TheThe previously previously cut cut copper copper wire wire can can be be transferred transferred to to the the hard hard-hard‐‐ wareware distributiondistribution layer layer through through the the transfertransfer paperpaper (Aihua,(Aihua, PVC) PVC) (Figure (Figure 11d).1111d).d). The The elec elec-elec‐‐ tronictronic components components are are taped taped to to thethe markedmarked positionposition inin thethe hardwarehardware distributiondistribution layerlayer (Figure(Figure 1111e)11e)e) and andand then thenthen bondedbonded toto thethe coppercopper wirewire byby thethe conductiveconductive adhesiveadhesive (Ausbond, (Ausbond, 3811).3811). SuperposeSuperpose Layers.Layers. TheThe protectedprotected paperpaper isis removedremoved fromfrom thethe waterproofwaterproof paper,paper, andand thethe transparenttransparent hollowhollow‐‐outout paperpaper isis gluedglued toto thethe waterproofwaterproof paperpaper (Figure(Figure 11g).11g). CustomCustom

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Superpose Layers. The protected paper is removed from the waterproof paper, and Electronics 2021, 10, x FOR PEER REVIEW 11 of 19 the transparent hollow-out paper is glued to the waterproof paper (Figure 11g). Custom shells are placed on the electronic components (Figure 11h) and then glued to the hollow- shells are placed on the electronic components (Figure 11h) and then glued to the hollow‐ out layer with waterproof glue (V-705, silicon rubber) (Guangdong, China). The user sticks out layer with waterproof glue (V‐705, silicon rubber) (Guangdong, China). The user thesticks counterweight the counterweight to the to specified the specified position position if necessary. if necessary. At least At least 30 minutes 30 minutes are requiredare re‐ forquired the glue for the to dry.glue to dry. FoldFold and and Assembly. Assembly.The The user user folds folds the the interface interface that that is basedis based on on the the creases creases and and sticks papersticks interfacespaper interfaces together together using waterproofusing waterproof tape (Darit tape (Darit Tape, Tape, PVC) PVC) (Guangdong, (Guangdong, China) (FigureChina) 11(Figurej–l). 11j–l). PatchPatch andand Decorate. CheckCheck the the condition condition of the of prototype. the prototype. The user The can user apply can deco apply‐ decorationsrations or make or make repairs repairs on paper on paper interfaces. interfaces.

FigureFigure 11. 11.Fabrication: Fabrication: (a ()a print) print and and cut; cut; (b ()b remove) remove the the inner inner part part of of the the protected protected paper; paper; ( c()c) paste paste the the hardware hardware distribution distribu‐ layertion on layer the on sticky the sidesticky of side the waterproofof the waterproof layer; (layer;d–f) circuit(d–f) circuit fabrication; fabrication; (d) copper (d) copper wire transfer;wire transfer; (g–i) ( superposeg–i) superpose layers; layers; and, (j–l) fold and assembly. and, (j–l) fold and assembly. 4. System Evaluation 4. System Evaluation 4.1.4.1. Testing Testing Waterproofing Waterproofing WaterproofingWaterproofing is is crucial crucial to to underwater underwater equipment. equipment. TheThe internationalinternational industrial wa water-‐ terproof grade standard IPX [40,41] requires that equipment applied in water can still be proof grade standard IPX [40,41] requires that equipment applied in water can still be used used after immersion at a specific pressure for a specified period of time. ElectroPaper is after immersion at a specific pressure for a specified period of time. ElectroPaper is the first the first paper‐based electronic interface to examine the waterproof ability. The interface paper-based electronic interface to examine the waterproof ability. The interface that we that we tested includes a battery, switch, controller, and RGB LED, which can shine blue, tested includes a battery, switch, controller, and RGB LED, which can shine blue, green, green, and yellow, in that order (Figure 12a). We added counterweights on the interface and yellow, in that order (Figure 12a). We added counterweights on the interface to make to make it sink to the target bottom. If the paper interface is flooded, the circuit will be it sink to the target bottom. If the paper interface is flooded, the circuit will be damaged, damaged, the program cannot run normally, and the lights fail to shine or they shine in‐ the program cannot run normally, and the lights fail to shine or they shine incorrectly. correctly. Additionally, there would be water stains between the paper layers. The equip‐ Additionally, there would be water stains between the paper layers. The equipment must ment must withstand water immersion for at least 30 minutes at a depth of 1 m, according withstand water immersion for at least 30 minutes at a depth of 1 m, according to the IPX7 to the IPX7 waterproof test [40]. We put the paper interfaces in the water environment at waterproofdepths of 0.4 test m [and40]. 1 We m putto test the its paper waterproof interfaces ability. in the The water 0.4 m environment test environment at depths was a of 0.4glass m andtank 1 in m the to testlab (Figure its waterproof 12b), and ability. the 1‐ Them water0.4 m environmenttest environment was a was swimming a glass tankpool in the(Figure lab (Figure 12c). The 12 b),water and environments the 1-m water were environment calm during was the a tests, swimming and the pool water (Figure was ap 12‐ c). The water environments were calm during the tests, and the water was approximately proximately◦ 26 °C. The two tests were repeated 30 times, each time with a new paper 26interface.C. The Each two teststest took were five repeated hours, and 30 times, was recorded each time on withvideo. a new paper interface. Each test tookIn the five 30 hours, tests at and a 0.4 was m depth, recorded all of on the video. circuits continued to work after half an hour and worked fine during the next 4.5 h. At a 1 m depth, 29 circuits continued to work after half an hour and they were not flooded for the next 4.5 h, with the circuits working nor‐ mally. One interface failed, and the LED went out nearly six minutes after the test began.

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In the 30 tests at a 0.4 m depth, all of the circuits continued to work after half an hour Electronics 2021, 10, x FOR PEER REVIEWand worked fine during the next 4.5 h. At a 1 m depth, 29 circuits continued to12 work of 19 Electronics 2021, 10, x FOR PEER REVIEWafter half an hour and they were not flooded for the next 4.5 h, with the circuits working12 of 19 Afternormally. removing One interfaceit, we found failed, a broken and the slit LED in the went crease out layer, nearly which six minutes was perhaps after thecaused test Afterduebegan. to removing the After quality removing it, ofwe the found it, waterproof we a found broken paper a brokenslit initself the slit orcrease in a fabrication the layer, crease which layer, error. was which Overall, perhaps was the perhaps causedresults dueshowedcaused to the due that quality to ElectroPaper the qualityof the waterproof ofhas the a stable waterproof paper waterproofing itself paper or itself a fabrication ability. or a fabrication error. Overall, error. Overall, the results the showedresults showed that ElectroPaper that ElectroPaper has a stable has a waterproofing stable waterproofing ability. ability.

Figure 12. (a) Paper interface for waterproofing test; (b) 0.4 m depth water environment; and, (c) 1 m depth water envi‐ Figureronment. 12. ((a)) PaperPaper interface interface for for waterproofing waterproofing test; test; (b) ( 0.4b) m0.4 depth m depth water water environment; environment; and, ( cand,) 1 m (c depth) 1 m water depth environment water envi‐. ronment. 4.2. Foldability Foldability Test Test 4.2. Foldability Test Foldability is an advantage of paper paper materials materials and and the the premise premise of of papermaking papermaking tech tech-‐ nology.Foldability We tested is an ElectroPaper’s advantage of foldability paper materials by folding and thea paper-basedpaper premise‐based of electronicpapermaking interface tech‐ nology.with a single We tested electrical ElectroPaper’s connection, foldability which included by folding a button button a paper battery battery‐based and and electronic LED LED (Figure (Figure interface 13a).13a). withThe paper papera single interface interface electrical was was connection, 42 42 cm cm × 5× cm, which5 cm,the batteryincluded the battery and a button LED and were LED battery at were either and at LED eitherend of(Figure endthe circuit, of 13a). the Theandcircuit, paperthe and copper interface the copperwire was in wirethe 42 cmmiddle in × the 5 cm, middle provided the battery provided space and for space LED folding. for were folding. We at either folded We end folded the of paper the the circuit, paperinter‐ andfaceinterface thein two copper in ways: two wire ways: vertically in the vertically middle (Figure (Figureprovided 13b) and 13 spaceb)at a and certain for at folding. a angle certain (FigureWe angle folded 13c). (Figure the Each paper 13 time,c). inter Each we‐ facefoldedtime, in we twothe folded paper ways: the interfacevertically paper until interface(Figure it lacked 13b) until and folding it at lacked a certainspace. folding Weangle folded space. (Figure the We 13c).circuit folded Each again the time, circuit along we foldedtheagain previous along the paper the crease previous interface on the crease until opposite onit lacked the side opposite foldingto ensure side space. that to ensure We the folded circuits that thethe in circuitscircuit the same again in theposition along same thewereposition previous folded were multiple crease folded on multipletimes. the opposite times. side to ensure that the circuits in the same position wereWe folded carried multiple out 30 times. tests each of vertical folding and tilted folding, and all of the folded circuitsWe worked carried outnormally. 30 tests We each then of verticalplaced the folding folded and paper tilted interfaces folding, and in in a a all water water of the environ environ- folded‐ circuitsment with worked 1 1-m‐m depth, normally. and Wethe thencircuits placed worked the folded underwater paper for interfaces more than in a 30 water minutes. environ The‐ mentexperimental with 1‐m results depth, showed and the that circuits the paper paper-based worked‐based underwater electronic for interface more than has good30 minutes. foldability The experimentaland it can work results stably showed after multiple that the paperfolds. ‐based electronic interface has good foldability and it can work stably after multiple folds.

Figure 13. (a) Paper interface for foldability test; (b) vertical folding; and, (c) tilted folding. Figure 13. (a) Paper interface for foldability test; (b) vertical folding; and, (c) tilted folding. 5. Example Example Application Application 5. ExampleInteractionInteraction Application occurring inin aa waterwater environmentenvironment is is mainly mainly influenced influenced by by the the water water envi- en‐ vironment,ronment,Interaction human, human, occurring and and device device in a [ 42water[42],], presenting presenting environment challenges challenges is mainly to to the influencedthe designdesign ofofby prototypesprototypes the water en for‐ vironment,water environmentenvironment human, use. anduse. ElectroPaperdevice ElectroPaper [42], presenting provides provides an challenges an integrated, integrated, to highlythe highly design functional functional of prototypes design design tool for watertooland fabricationand environment fabrication method use.method forElectroPaper electronic for electronic prototypesprovides prototypes an that integrated, werethat were used highly used in water infunctional water environments. environ design‐ toolments.The systemand The fabrication system has a multilayer has method a multilayer structurefor electronic structure to ensure prototypes to goodensure folding thatgood were folding capability used capability in and water a waterproof and environ a wa‐ ments.terproofeffect. The In effect. order system In to order has evaluate a tomultilayer evaluate the feasibility structurethe feasibility of to this ensure of approach, this good approach, folding we provide we capability provide five and electronic five a elec wa‐ terprooftronicapplications applications effect. in aIn water order in a environment,water to evaluate environment, the combining feasibility combining physical of this physical properties,approach, properties, we such provide as such water five as quality, water elec‐ tronicquality,temperature, applications temperature, depth, in anddepth,a water buoyancy, and environment, buoyancy, to illustrate tocombining illustrate different physicaldifferent water-based properties, water‐based interactions such interactions as water and quality,andfunctions functions temperature, (e.g., (e.g., environmental environmental depth, and monitoring, buoyancy, monitoring, to underwater illustrate underwater different robots, robots, entertainment, water entertainment,‐based interactions and deco- and anddecoration).ration). functions They They illustrate(e.g., illustrate environmental the advantagesthe advantages monitoring, of paper of paper underwater as the as the medium medium robots, for for prototyping entertainment, prototyping used used and in decoration).in water environments, They illustrate such the as foldabilityadvantages (applications of paper as the 5.1, medium 5.4), flexibility for prototyping (5.3), and usedto be indeployable water environments, (5.2, 5.5). The such applications as foldability were (applications created using 5.1, our 5.4), approach flexibility in (5.3),shallow and water to be deployable(a depth less (5.2, than 5.5). 1 m). The applications were created using our approach in shallow water (a depth less than 1 m).

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water environments, such as foldability (applications 5.1, 5.4), flexibility (5.3), and to be deployable (5.2, 5.5). The applications were created using our approach in shallow water (a Electronics 2021, 10, x FOR PEER REVIEW 13 of 19 depth less than 1 m).

5.1. Water Quality Detection The total total dissolved dissolved solids solids (TDS) (TDS) measures measures the the dissolved dissolved combined combined content content of all of in all‐ organicinorganic and and organic organic substances substances that that are present are present in a liquid in a liquid in molecular, in molecular, ionized, ionized, or micro or‐ granularmicro-granular (colloidal (colloidal sol) suspended sol) suspended form [43]. form The [43 first]. The application first application is a water is a quality water quality detec‐ tiondetection device device with with a TDS a TDS sensor sensor (Meter (Meter V1.0) V1.0) (Sichuan, (Sichuan, China) China) and and two two LEDs LEDs to to monitor water qualityquality in in real real time time (Figure (Figure 14a). 14a). A higher A higher value value indicates indicates more more dissolved dissolved substances, sub‐ stances,i.e., poor i.e., water poor quality water (Figure quality 14 (Figuree). The LEDs14e). The will presentLEDs will a red present light when a red thelight data when exceed the dataa set valueexceed (Figure a set value 14c). (Figure In this application, 14c). In this all application, of the electronic all of the components electronic are components placed on arethe bottomplaced on paper the interface.bottom paper Specifically, interface. the Specifically, probe of the the TDS probe sensor of penetratesthe TDS sensor the bottom pene‐ papertrates the interface bottom to contactpaper interface the water to to contact detect the water water quality to detect (Figure water1b). The quality paper (Figure structure 1b). Theat the paper top is structure hollow (Figure at the top 14d), is hollow which requires (Figure 14d), the user which to draw requires the outline the user of to the draw hollow the positionoutline of on the the hollow surface position when on building the surface the initial when model. building The the 2D initial patterns model. also The show 2D pat the‐ ternshollow-out also show traces. the hollow‐out traces.

Figure 14. Total dissolveddissolved solidssolids (TDS) (TDS) detection detection device: device: ( a()a) Paper Paper circuit; circuit; (b ()b placing) placing device device in in water; (water;c) red ( lightc) red illuminates light illuminates to alert to when alert detectedwhen detected TDS values TDS values exceed exceed a set value;a set value; (d) folded (d) folded shape of shapedevice; of (e device;) values (e detected) values bydetected device. by device.

5.2. Water Temperature SensingSensing An interactive interactive decoration decoration is is designed designed for for an anoutdoor outdoor shallow shallow water water environment, environment, and itand can it react can to react a water to a temperature water temperature change. change.It comprises It comprises a microcontroller a microcontroller (ATmega32U4) (AT- (Guangdong,mega32U4) (Guangdong, China) , three China), DC motors three DC (N20) motors (Guangdong, (N20) (Guangdong, China), a China), battery a (3.7 battery V, 1800 (3.7 V,mAh) 1800 (Guangdong, mAh) (Guangdong, China), China), a temperature a temperature sensor sensor (DS18B20) (DS18B20) (Guangdong, (Guangdong, China), China), and twoand twoDC motor DC motor driver driver modules modules (L298N) (L298N) (Guangdong, (Guangdong, China), China), as shown as shown in Figure in Figure 15a. 15Wea. Weselected selected deployable deployable structures structures from from the thecomponent component library library and and then then connected connected them them to threeto three motors motors with with three three polyethylene polyethylene (PE) (PE) threads threads (Figure (Figure 15c). 15c). A A PE PE thread thread is woundwound around each motor’s drive shaft and then connected connected to the the top top of of the the deployable deployable structure. structure. With buoyancy, threethree starstar structuresstructures provideprovide thethe power for the device to rise (Figure 15e).15e). This application can can efficiently efficiently switch switch between between the the tightened tightened and and released released state. state. When When the ◦ temperaturethe temperature sensor sensor indicates indicates that that the thewater water temperature temperature is over is over 34 °C, 34 theC, theDC DC motors motors ro‐ taterotate clockwise clockwise to to release release the the threads, threads, and and the the star star structures structures float float to to the the surface surface as a result ◦ of their buoyancy (Figure 1515e).e). In contrast, when the temperature falls below 3434 °C,C, thethe DC motors rotate counterclockwise and the star structures start to drop under thethe strainstrain of the threads (Figure 1515d).d). In order to make the entire applicationapplication sink, we defineddefined thethe model’s working conditions in the software tool as “sink under water”, which let us know how many counterweights must be added to the paper interface (Figure(Figure 1515b).b).

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FigureFigure 15. WaterWater temperature temperature sensing. sensing. (a ()a )Paper Paper circuit; circuit; (b ()b inner) inner part part of offolded folded Stone Stone structure; structure; (c) deployable paper structure; (d) the device sits at the bottom when the water temperature drops (deployablec) deployable paper paper structure; structure; (d) ( thed) the device device sits sits at the at the bottom bottom when when the the water water temperature temperature drops drops below 34 °C; and, (e) the device rises to the surface when the water temperature exceeds 34 °C. belowbelow 3434 ◦°C;C; and, (ee)) thethe devicedevice risesrises toto thethe surfacesurface whenwhen thethe waterwater temperaturetemperature exceedsexceeds 3434 ◦°C.C.

5.3.5.3. WaterWater WaveWave DetectionDetection InspiredInspired by by the the flying flying fish, fish, we we used used ElectroPaper ElectroPaper to to design design a Flya Fly Fish Fish robot robot supporting support‐ aning entertaining an entertaining experience. experience. The The robot robot floats floats on on still still water water in in a a dormant dormant state; state; when when the the useruser slapsslaps oror disturbsdisturbs thethe waterwater (Figure(Figure 1616c),c), itit willwill vibratevibrate thethe wingswings toto createcreate ripplesripples (Figure(Figure 1616e).e). InIn orderorder toto achieveachieve thisthis interaction,interaction, thethe FlyFly FishFish robotrobot containscontains aa gyroscopegyroscope (JY60)(JY60) (Guangdong,(Guangdong, China),China), aa microcontrollermicrocontroller (ATmega32U4)(ATmega32U4) (Guangdong,(Guangdong, China),China), andand twotwo suction-cupsuction‐cup electromagnetselectromagnets (Guangdong,(Guangdong, China)China) (Figure(Figure 1616a).a). AA thinthin piecepiece ofof metalmetal isis gluedglued onon eacheach wing.wing. ThisThis metalmetal piecepiece cancan bebe attachedattached byby electromagnetselectromagnets (Figure(Figure 1616b).b). TheThe robot’s position changes changes as as the the user user flaps flaps the the water water surface. surface. When When the the gyroscope gyroscope de‐ detectstects a 15 a 15-degree‐degree angle angle change, change, the the suction suction-cup‐cup electromagnet electromagnet will continuously changechange itsits magnetismmagnetism (Figure(Figure 1616d),d), whichwhich willwill drivedrive thethe flexibleflexible paperpaper wingswings toto vibratevibrate andand taptap thethe waterwater surface.surface. The prototype must be balanced to floatfloat onon thethe water,water, whichwhich cancan bebe analyzedanalyzed byby ourour designdesign tool.tool. TheThe prototypeprototype hashas aa sealedsealed spacespace toto provideprovide buoyancy,buoyancy, soso itit mustmust bebe watertight.watertight.

FigureFigure 16.16. Fly Fish robot: ( (aa)) paper paper circuit circuit of of robot; robot; (b (b) )thin thin pieces pieces of of metal metal are are stuck stuck to to the the wings wings andand cancan bebe attracted attracted by by electromagnets; electromagnets; (c ()c the) the user user flaps flaps the the water water to to interact interact with with the the robot; robot; (d )( datad) captureddata captured by gyroscope; by gyroscope; and, (and,e) the (e robot) the robot vibrates vibrates its wings its wings to create to create ripples. ripples.

5.4. Camouflaged Underwater Camera Underwater cameras help people to observe and understand the underwater world through photos or videos. The Octopus waterproof underwater camera shares the

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5.4. Camouflaged Underwater Camera Electronics 2021, 10, x FOR PEER REVIEW Underwater cameras help people to observe and understand the underwater15 world of 19 through photos or videos. The Octopus waterproof underwater camera shares the octopus’s octopus’scolor-changing color‐changing and camouflage and camouflage abilities. abilities. We used We this used device this to device explore to theexplore interaction the in‐ teractionbetween underwaterbetween underwater devices and devices their and surroundings. their surroundings. It can camouflage It can camouflage by emitting by emit light‐ tingof a recognizedlight of a recognized color (Figure color 17d–f). (Figure The 17d–f). camera The (Raspberry camera Pi(Raspberry Camera V2) Pi (Cambridge,Camera V2) (Cambridge,UK) is rotated UK) by is steering rotated gearby steering (SPT5425LV-W) gear (SPT5425LV (Guangdong,‐W) (Guangdong, China) and China) it randomly and it randomlycaptures anything captures in anything front of in it, front such asof underwaterit, such as underwater topography topography and aquatic and organisms aquatic organisms(Figure 17 h,j,l)).(Figure The 17h,j,l)). program The (Raspberryprogram (Raspberry Pi), which Pi), is which based onis based OpenCV, on OpenCV, analyzes ana the‐ lyzesdata andthe data determines and determines the dominant the dominant color of the color surroundings of the surroundings to adjust to the adjust RGB the value RGB of valuethe LED. of the During LED. fabrication,During fabrication, the users the can users fold can the fold multilayer the multilayer interface interface to shape to or shape save orspace. save The space. foldability The foldability of our paper-basedof our paper‐ electronicbased electronic interface interface can ensure can ensure the correct the correct signal signaltransmission transmission (Figure (Figure 17a). We 17a). selected We selected a rigid a structure rigid structure (Figure (Figure 17b) from 17b) the from component the com‐ ponentlibrary library to connect to connect the robot’s the robot’s head to head the to steering the steering gear togear rotate to rotate it. Like it. Like application application 5.1, 5.1,above, above, the robotthe robot needs needs to be to hollowed be hollowed out. The out. plasticity The plasticity of paper of allowspaper allows users to users design to designand cut and different cut different patterns patterns into its into surface, its surface, through through which which light canlight leak can outleak (Figure out (Figure 17c). 17c).This prototypeThis prototype works works underwater, underwater, and counterweights and counterweights are glued are glued to the to paper the paper interface inter to‐ faceincrease to increase gravity gravity (Figure (Figure 17b). 17b).

Figure 17. UnderwaterUnderwater camera: camera: (a) ( apaper) paper circuit circuit of Octopus; of Octopus; (b) folded (b) folded paper paper prototype prototype with witha rigid a structure rigid structure and coun and‐ counterweights;terweights; (c) appearance (c) appearance of Octopus; of Octopus; (d–f (d): –Octopusf): Octopus can can recognize recognize the the color color of of its its surroundings surroundings to to change change its its body’s color; (g,i,k) are posters with different colors,colors, placedplaced onon thethe outsideoutside ofof thethe tank;tank; and,and, ((hh,,jj,,ll)) areare photosphotos takentaken by by the the robot. robot.

5.5. Deformation Deformation in in Water Water InspiredInspired by soft underwater robots, Starfish Starfish was designed as a deformation robot. It floatsfloats and detects its surroundingssurroundings inin realreal time. time. When When people people touch touch it, it, it it retracts retracts five five tenta- ten‐ tacles,cles, darkens darkens its its body, body, and and changes changes to to the the self-protection self‐protection statestate to show fear fear (Figure(Figure 18d).18d). When untouched, it will stretch out its tentacles to feel thethe environment.environment. If safe, it will stretchstretch out allall ofof its its tentacles tentacles and and make make its its body body glow glow to turnto turn on anon active an active state state(Figure (Figure 18e). 18e).To help To Starfishhelp Starfish detect detect its surroundings, its surroundings, we folded we folded the paper the interfacepaper interface (Figure (Figure 18a), so 18a), that sothe that infrared the infrared pyroelectric pyroelectric sensor (AM412) sensor (AM412) (Guangdong, (Guangdong, China) faces China) the outside.faces the Two outside. RGB TwoLED lightsRGB LED are used lights for are lighting, used for and lighting, the battery, and controllerthe battery, (ATmega32U4), controller (ATmega32U4), communica- communicationtion module (NRF24L01, module (NRF24L01, 2.4 GHz) (Guangdong, 2.4 GHz) (Guangdong, China), and China), voltage and regulator voltage regulator module module(AMS1117) (AMS1117) (Guangdong, (Guangdong, China) China) are arranged are arranged on the on paper the paper interface. interface. Starfish Starfish can also can alsointeract interact with with people people through through a wristband a wristband (Figure (Figure 18f) 18f) [42]. [42]. The The wristband wristband can can capture capture the theuser’s user handʹs hand movements movements through through the the internal internal battery battery (9 (9 V), V), Bluetooth Bluetooth module,module, Arduino Pro Mini, and MPU MPU-6050‐6050 (Guangdong, China) (Figure 18c)18c) [42]. [42]. After After receiving receiving the the data, StarfishStarfish “dances” “dances” in in the the water water according according to to the the user user’sʹs hand hand movements. movements. It Itstretches stretches tenta ten-‐ cles when the user raises the arm and then retracts tentacles when the arm is dropped. Five sets of actuators, air pumps (kjh‐370‐br) (Guangdong, China), and airbags are placed out of the water. The stretchable tentacles can expand and contract through built‐in air‐ bags (Figure 18b). The initial models of the tentacles came from the component library of our design tool. We used this paper deployable model and parameterized it to simplify

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tacles when the user raises the arm and then retracts tentacles when the arm is dropped. Five sets of actuators, air pumps (kjh-370-br) (Guangdong, China), and airbags are placed Electronics 2021, 10, x FOR PEER REVIEWout of the water. The stretchable tentacles can expand and contract through built-in airbags16 of 19 (Figure 18b). The initial models of the tentacles came from the component library of our designthe modeling tool. We process. used this The paper middle deployable part of the model prototype and parameterizedmust be waterproof it to simplifyto allow it the to modelingfloat. process. The middle part of the prototype must be waterproof to allow it to float.

FigureFigure 18.18. Starfish:Starfish: ( (aa)) folded folded paper paper circuit; circuit; (b ()b deployable) deployable structure structure with with airbag; airbag; (c) ( internalc) internal structure struc‐ ofture the of wristband; the wristband; (d) in (d the) in self-protection the self‐protection state, state, the robot the robot retracts retracts its tentacles its tentacles and darkens and darkens its body; its (body;e) in the (e) activein the active state, thestate, robot the stretchesrobot stretches its tentacles its tentacles and lightens and lightens its body; its body; and, (and,f) wristband (f) wrist‐ for band for users to interact with the Starfish. users to interact with the Starfish.

6.6. DiscussionDiscussion andand LimitationsLimitations ElectroPaperElectroPaper simplifiessimplifies the fabrication of of prototypes prototypes for for water water environment environment use use in in a alow low-cost‐cost way. way. This This process process was was previously previously expensive, expensive, and and prototypes prototypes were were complex complex to tofabricate. fabricate. When When compared compared to traditional to traditional methods, methods, this this approach approach can cansystematically systematically and andstably stably ensure ensure the regular the regular operation operation of electronic of electronic components components in water, in water, and and it allows it allows the theconsideration consideration of electronic of electronic design design in the in modeling the modeling stage, stage, such such as the as use the of use electronic of electronic com‐ componentsponents and and circuit circuit drawing. drawing. Balance Balance assessment assessment and andbuoyancy buoyancy analysis analysis improve improve the use the‐ usefulnessfulness of the of theprototype prototype and andhelp help users users to reduce to reduce iteration iteration times. times. However, However, this approach this ap- proachalso has also disadvantages. has disadvantages. When When compared compared with with 3D 3D printing, printing, a a common common prototypingprototyping methodmethod containscontains moremore manualmanual work,work, suchsuch asas folding,folding, waterproofwaterproof shellshell making, making, andand each each stepstep needsneeds to to be be carefully carefully made. made. Moreover, Moreover, it it needs needs more more auxiliary auxiliary materials, materials, such such as paper,as pa‐ waterproofper, waterproof glue, glue, and conductive and conductive glue, glue, which which are low-cost are low but‐cost essential. but essential. WeWe use use syntheticsynthetic materialsmaterials (PET,(PET, BOPP,BOPP, OPP) OPP) as as substratessubstrates andand definedefine them as paper. AllAll ofof thesethese materialsmaterials areare low-costlow‐cost commercialcommercial productsproducts thatthat areare easyeasy toto fold.fold. Specifically,Specifically, thethe paperspapers havehave aa goodgood waterproofwaterproof effect.effect. However,However, thethe materialsmaterials areare offoff thethe shelf,shelf, andand theirtheir qualityquality mightmight injectinject uncertaintyuncertainty intointo thethe fabrication fabrication effect. effect. TheThe adhesionadhesion betweenbetween layerslayers mainlymainly dependsdepends on on thethe glueglue onon thethe firstfirst layer.layer. WeWe apply apply waterproof waterproof glue glue to to the the edge edge ofof thethe paperpaper interfaceinterface inin orderorder toto increaseincrease the the waterproof waterproof effect. effect. WeWe customizedcustomized waterproofwaterproof shellsshells forfor electronic electronic components. components. TheThe waterproofwaterproof epoxyepoxy shellshell cancan bebe easilyeasily installed,installed, usedused repeatedly,repeatedly, and and simply simply processed, processed, and and the the materials materials areare low-costlow‐cost andand easilyeasily available,available, butbut itit stillstill hashas deficiencies.deficiencies. TheThe component’scomponent’s shapeshape and and size size determinedetermine thethe designdesign ofof thethe waterproofwaterproof shell.shell. ThatThat meansmeans thatthat thethe shellsshells areare notnot universaluniversal forfor allall components.components. TheThe shellshell thatthat isis pastedpasted onon thethe paperpaper interfaceinterface is is aa solid,solid, rigidrigid geometricgeometric structure,structure, andand itit cannotcannot bebe arrangedarranged onon thethe crease.crease. AccordingAccording to to our our test, test, the the crease’s creaseʹs limitlimit positionposition is is the the waterproof waterproof shell’s shell edge.ʹs edge. In theIn the future, future, we will we improvewill improve the foldability the foldability of paper of circuitspaper circuits by adding by foldable,adding foldable, flexible printed flexible circuit printed boards, circuit and boards, using and soft waterproofusing soft water shells‐. proofWe shells. refer to origami-related algorithms [7] to expand 3D objects into 2D planes and generateWe creasesrefer to and origami flaps.‐related The current algorithms algorithm [7] to has expand limitations. 3D objects It requires into 2D the planes 3D model and generate creases and flaps. The current algorithm has limitations. It requires the 3D model to be intact, without hollow‐out surfaces, somewhat limiting the user’s modeling. For this reason, the user needs to patch the hollow‐out surfaces before unfolding the 3D model. Additionally, the initial model requires a flat surface if the user intends to place electronics on it. The placement of the electronic components requires manual adjustment. The size of the model is influenced by the size of the components. The user must consider all of

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to be intact, without hollow-out surfaces, somewhat limiting the user’s modeling. For this reason, the user needs to patch the hollow-out surfaces before unfolding the 3D model. Additionally, the initial model requires a flat surface if the user intends to place electronics on it. The placement of the electronic components requires manual adjustment. The size of the model is influenced by the size of the components. The user must consider all of these things when building a model, so as to expedite the fabrication process. Although there are geometric limitations, these do not affect the fabrication process or the prototype’s presentation. We set the waterproof test time of five hours based on the waterproof effect of the material itself, the working time of electronic components (such as batteries), and the prototype test time. We believe that five hours of underwater testing time is sufficient for most of the prototypes. Our goal is to provide an approach for underwater prototyping that may not have as long an underwater duration as a mature product. For some special applications that require a long testing time, we think that this can be achieved by successive use of multiple identical prototypes. Based on our experience testing waterproofing materials, if a system is not waterproof, then water will enter it in a short time, and much faster in 1 m of water. In particular, there is little randomness in the waterproofing test, so we set up 30 groups, and we believe that this is sufficient to show that our fabrication method has a stable waterproofing effect.

7. Conclusions We presented ElectroPaper, a low-cost and effective fabrication approach for simpli- fying the creation of electronic prototypes that are used in water environments, with a paper-based electronic interface to achieve high functionality. The structure integrates a crease layer, hardware distribution layer, and hollow-out layer to enhance the water- proofing and foldability. ElectroPaper includes a design tool with the functions of balance analysis, 3D model unfolding, and circuit drawing, which empowers individuals to create prototypes of devices for water environment use. This approach embraces affordable paper circuits for water environments and offers new possibilities for papercraft techniques. We designed five devices using this approach, demonstrating the integration of different electronics in the paper-based interface and their use in water. Application examples and evaluations verified the effectiveness and universality of this approach, as well as its water- proofing and foldability. Our work allows researchers to focus on the water environment, which supports the creation of prototypes to explore it.

Author Contributions: Conceptualization, methodology, formal analysis, validation, and writing— original draft preparation, L.L.; resources and writing—review and editing, J.G.; software, C.Z.; Z.W.; P.Z.; hardware, T.F.; J.W.; supervision, C.Y.; F.Y. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the Engineering Research Center of Computer Aided Product Innovation Design, Ministry of Education, Fundamental Research Funds for the Central Universities, National Natural Science Foundation of China, grant number 52075478, Major Project of Zhejiang Social Science Foundation, grant number 21XXJC01ZD. Data Availability Statement: No new data were created or analyzed in this study. Data sharing is not applicable to this article. Conflicts of Interest: The authors declare no conflict of interest. Electronics 2021, 10, 604 18 of 19

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