3164 IEEE SENSORS JOURNAL, VOL. 15, NO. 6, JUNE 2015 Technologies for Printing Sensors and Electronics Over Large Flexible Substrates: A Review Saleem Khan, Leandro Lorenzelli, Member, IEEE, and Ravinder S. Dahiya, Senior Member, IEEE Abstract— Printing sensors and electronics over flexible substrates are an area of significant interest due to low-cost fab- rication and possibility of obtaining multifunctional electronics over large areas. Over the years, a number of printing technolo- gies have been developed to pattern a wide range of electronic materials on diverse substrates. As further expansion of printed technologies is expected in future for sensors and electronics, it is opportune to review the common features, the complementarities, and the challenges associated with various printing technologies. This paper presents a comprehensive review of various printing technologies, commonly used substrates and electronic materials. Various solution/dry printing and contact/noncontact printing technologies have been assessed on the basis of technological, materials, and process-related developments in the field. Fig. 1. The classification of common printing technologies. Critical challenges in various printing techniques and potential research directions have been highlighted. Possibilities of merging various printing methodologies have been explored to extend are difficult to realize with the conventional wafer-based the lab developed standalone systems to high-speed roll-to-roll fabrication techniques. The printed electronics on flexible production lines for system level integration. substrates will enable conformable sensitive electronic systems Index Terms— Printed sensors, printed electronics, flexible such as electronic skin that can be wrapped around the body electronics, large area electronics, roll-to-roll, dispersion solution. of a robot or prosthetic hands [20]–[25]. Printed electronics on polymer substrates has also opened new avenues for low-cost fabrication of electronics on areas larger than I. INTRODUCTION the standard wafers available commercially. In accordance RINTING technologies are aiding and revolutionizing with the electronics industry roadmap, the research in this Pthe burgeoning field of flexible/bendable sensors and field is slowly inching towards a merge of well-established electronics by providing cost-effective routes for processing microelectronics and the age-old printing technologies [26]. diverse electronic materials at temperatures that are compatible This is evidenced by development of devices such as, large with plastic substrates. Simplified processing steps, reduced area printed pressure sensors [5], [27]–[29], radio frequency materials wastage, low fabrication costs and simple patterning identification tags (RFID) [11], [12], solar cells [30], light techniques make printing technologies very attractive for emitting diodes (LED) [13] and transistors [14]. the cost-effective manufacturing [16]–[18]. These features Traditional approaches for printing electronics and sensors of printed electronics have allowed researchers to explore involve bringing pre-patterned parts of a module in contact new avenues for materials processing and to develop sensors with the flexible (or non-flexible) substrates and transferring and systems on even non-planar surfaces, which otherwise the functional inks or solutions onto them [6]–[9], [11], [13]. The two major approaches usually followed for development Manuscript received June 16, 2014; revised November 10, 2014; accepted November 11, 2014. Date of publication December 4, 2014; date of current of printing/coating system are contact and non-contact version April 16, 2015. This work was supported in part by the Engineering printing, as shown in Fig. 1 and described later in Section IV. and Physical Sciences Research Council (EPSRC) through the Fellowship In contact printing process, the patterned structures with inked for Growth-Printable Tactile Skin under Grant EP/M002527/1, in part by the European Commission under Grant PITN-GA-2012-317488-CONTEST, surfaces are brought in physical contact with the substrate. and in part by the EPSRC-Flexible Electronic Device Modelling under In a non-contact process, the solution is dispensed through Grant EP/M002519/1. The associate editor coordinating the review of this via openings or nozzles and structures are defined by moving paper and approving it for publication was Prof. Zheng Cui. S. Khan and L. Lorenzelli are with the University of Trento, Trento 38100, the stage (substrate holder) in a pre-programmed pattern. Italy, and also with the Centre of Materials and Microsystems, Microsystems The contact-based printing technologies comprise of gravure Technology Research Unit, Fondazione Bruno Kessler, Trento 38122, Italy printing, gravure-offset printing, flexographic printing and (e-mail: [email protected]; [email protected]). R. S. Dahiya is with the Electronics and Nanoscale Engineering Research R2R printing. The prominent non-contact printing techniques Division, School of Engineering, University of Glasgow, Glasgow G12 8QQ, include screen-printing, slot-die coating and inkjet printing. U.K. (e-mail: [email protected]). The non-contact printing techniques have received greater Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. attractions due to their distinct capabilities such as simplicity, Digital Object Identifier 10.1109/JSEN.2014.2375203 affordability, speed, adaptability to the fabrication process, This work is licensed under a Creative Commons Attribution 3.0 License. For more information, see http://creativecommons.org/licenses/by/3.0/ KHAN et al.: TECHNOLOGIES FOR PRINTING SENSORS AND ELECTRONICS: A REVIEW 3165 reduced material wastage, high resolution of patterns and three categories: (a) conductors; (b) semiconductors; and easy control by adjusting few process parameters [17], [18], (c) dielectrics [19], [31], [35]–[37]. Beside these, some [31]–[34]. Recently, the newly emerging polymeric stamp composite materials, having dual nature as insulator, based printing methods such as nanoimprint, micro-contact ferroelectric, piezoelectric, piezoresistive, and photosensitive printing and transfer printing have also attracted properties are also used in thin film printed devices. Hybrid significant interest, especially for inorganic monocrystalline organic/inorganic materials have also been used to compensate semiconductors based flexible electronics [19], [30], [33]–[37]. for the slow speed organic based electronic devices [34], [37]. This paper presents a survey of various printed electronics Majority of printable materials are in the form of solutions, technologies. A few review articles reported in this field which require specific properties to allow proper printing on previously have reviewed some of the printing technologies variety of plastic or paper substrates. For example, proper individually [17]–[19], [30]–[37]. For example, the review dispersion of nanoparticles is essential to avoid agglomeration. paper by Singh et al. [38] focusses on inkjet printing, Further, an acceptable level of chemical and physical stabil- the one Schift et al. [39] focusses on nanoimprinting, itiesis needed to maintain a balance between Brownian and Carlson et al. [40] have discussed transfer printing, gravitational motion of the particles. In the following sections, Perl et al. [2] have presented review on microcontact we discuss the common organic/inorganic materials that are printing and Søndergaard et al. [41] have discussed R2R suitable and easily processable through printing technologies. fabrication. Differently from previous reviews, this survey paper brings together various printing techniques and provides a detailed discussion by also involving the key electronic A. Conducting Materials and substrate materials, and the systems. Critical limitations Conducting materials are the main structural blocks of all of each technology have been highlighted and potential electronic devices as they form the fundamental part of the solutions or alternatives have been explored. This paper device layers or interconnections. Deposition techniques for also evaluates printed electronics on the basis of electrical patterned metal structures and interconnects are now at mature characteristics of the resulting devices or sensors and materials stage with possibility of obtaining structures with controlled (organic/inorganic) they are made of. Often, the advantages of thickness and resolution. Various printing technologies require printing technologies eclipse the challenges associated with a different set of parameters such as viscosity, surface tension, them. As most of the printing technologies share common conductivity and compatibility of the solvents with the under- processing techniques, the development of a common platform lying materials (in multilayer structures) (see Table II) [30]. is also explored assuming that the limitations of one method Therefore, a careful selection of appropriate conductive could be overcome by the advantages of the others. material is needed by also taking into account the work func- This paper is organized as follows: The Section II presents tion of the neighboring materials. Some of these metals have various printable electronic materials and gives an overview of already secured their place in printing technologies by showing solution processable conductors, semiconductors and dielectric good dispersing properties in the form of colloidal solutions.