Organic paper, particularly cellulose-based paper, efficiently served in previous eras as an engineering material. In the Tang dynasty, soldier armors were mainly made of paper [57]. In the era of analog computing, paper volvelles [18] were used, in particular, in the calculation of physical phenomena. The E6-B flight computer is a paper volvelle that was used by pilots to determine, for instance, the effect of wind during flight. With the discovery of mineral resources and printing, paper became largely a platform for communication besides being used in banknotes, goods packaging, and hygienic products. With the recent developments in the digitization of information and the maker movement [14], paper is re-emerging as an engineering material serving in several high- and low-tech applications such as low-cost electronics, biomedical diagnostic platforms, robots design, civil construction and inner architecture connected to the internet.
Here, we list some of the arguments that motivate the reinvention of paper for electronics and 3D technology communities.
Argument 1: Solutions for a low-energy future and climate change
Certainly, paper has distinguished properties that are compatible with manufacturing processes as well as useful for the end engineering product such as mechanical bendability, porosity, dielectric properties, inertness and biocompatibility [29]. In a low energy future, the large surface area of a paper sheet and its light weight are compelling engineering properties that could be exploited in energy generation and energy saving. Considering the trend of increasing e-waste, biodegradability and renewability of paper may also represent an alleviation to the environment ([17], [24]).
Argument 2: Eliminating the digital divide
Mainly due to the low-cost of the raw material, its abundance, and compatibility with mass production techniques, using paper in electronic products is a solution to ensure that no one stays behind with technology. In October of 2015, the technology company, Google Inc., announced Google Cardboard as a low-cost alternative to Google Glass that allows the user of mobile devices to experience virtual reality (Figure 1 (a)). Using recycled corrugated paperboard, two companies based in Los Angeles, California, Signal Snowboards and Ernest Packaging, have designed and manufactured together the body of an electric guitar that sounds as good as a traditional instrument. Furthermore, paper as a widely accessible and low-cost material can be combined with simple techniques such as conductive ink pens and packaged off-the-shelf components to create first proof of concepts and rapid prototypes ([42], [16], [39], [41]) that will allow the amateur user to contribute to the invention of essential electronics.
In addition to the home-based early pregnancy test established in 1976, numerous point-of-care diagnostics and medical assays based on the principles of fluid filtration and propagation within paper ([27], [6], [51], [31], [7], [26]) are being developed to enable a wide coverage of medical diagnosis around the globe. In this case, the facile uptake of moisture by paper is a key technical property. Based on similar fluid dynamics in porous paper channels, laser emitters [48], batteries [47] and timers [32] are also being developed. To facilitate the optical diagnosis of biological substances for those who do not have easy access to diagnosis labs, an example of optical systems on paper are paper-based foldable microscopes ([9]), which were developed at a total cost of 50 cents per system (Figure 1 (b)).
Argument 3: Emerging paper engineering solutions
In order to serve future developments in the paper products, new paper types are being fabricated. For instance, a transparent smooth paper (> 90% optical transmission and 10 nm surface roughness) was developed and demonstrated in manufacturing optoelectronic devices [56]. By employing vulcanized paper fibers, strong paper was fabricated that was shown to work for mechanically stressed parts in the automotive industry [23]. Also, by multi-layering paper fibers with conductive polymers [53], growing conductive particles onto paper fibers [12] and filling the paper lumens with magnetic nanoparticles [49], paper was converted from an insulating carrier to a functional conductor. For electronics application, where the absorption of moisture may be harmful to the operation of the paper-based device, techniques were developed to modify the paper surface from being hydrophilic to being hydrophobic ([5], [2]). Furthermore, cellulose that is already fairly biocompatible can be converted to a bioactive material ([8], [55]) to serve paper-based medical assays.
Argument 4: Emerging paper-compatible system technology
For the success of paper-based engineering products, powering and connecting devices in a way that is as mobile, thin, lightweight, and mechanically bendable as paper itself are necessary. Hence, the components needed, such as batteries ([47], [55], [30]), capacitors ([43], [52]), antennas ([50], [46]) and photovoltaic cells ([4], [11]) should be manufactured directly onto the same paper that bears the devices. In this regard, thin film technology is to play an important role. As a matter of fact, companies such as Paper Battery Co., Power Paper, and Enfucell have launched ultra-thin batteries that are well compatible with paper. In [25], it was reported about an ionic-electronic conductive paper-based on nano-fibrillated cellulose that can be used for the fabrication of paper-thin batteries. In addition, energy harvesting from the mechanical pressure executed by the user on paper through electrets and piezoelectrics ([33], [19], [21]) are some first models of alternative mobile energy generation to be used with paper-based engineering devices. Beyond paper-based devices, batteries encapsulated in paperboard, called the Mini Power are a recent development of the designer Tsung Chih-Hsien that are a lightweight, low-cost, recyclable and mobile that allow users of personal devices, such as smartphones, a longer cable-free usage time. Together with the branding agency MultIAdaptor, Google demonstrated a notepad with an embedded circuit board connected to the pages through a conductive bookbinding glue allowing the interaction of the paper with the internet (Figure 2).
Argument 5: 3D and free-form paper products
The art and mathematics of origami and kirigami, contemporized with laser cutting ([45], [13], [44], [34]), are an inspiration to make non-planar paper products such as pop-ables [38], foldable circuit boards (Figure 3) and three-dimensional actuators. While foldability is self-evident for paper, non-planar and free-shaped platforms have proven to be difficult to realize with rigid materials such as silicon and ceramics. In the electronics industry, for instance, paper is widely used as an electrical insulator in cables, capacitors and transformer boards due to the electrical properties of cellulose and the ability of paper sheets to conform to free-shaped geometries. By making use of the light weight and anisotropy of paper, soft robots with a paper skeleton were developed [28]. Furthermore, an electro-active paper as a building block of micro-insect robots and micro-flying objects was demonstrated [20]. Also, by embedding shape memory alloys [22] or magnetic particles [46] within a paper structure, paper robots can be actuated.
Argument 6: Standards and engineering solutions for long-term reliability
If paper is to become a first-class citizen in the society of engineering materials, then the long-term reliability of paper-based products is definitely a concern. The passage of the public law 101-423 in the United States of America, a joint resolution to establish a national policy on permanent paper in 1990, among several other worldwide initiatives and standardizations, changed papermaking from a highly acidic process, mostly using rosin and alum, to a more neutral and alkaline process by using, for instance, calcium carbonate fillers [15]. With these improvements, it was achieved that paper and books last at least 500 years under ordinary storage conditions [40], in contrast to the conventional two-years life cycle of personal electronics.
In the last decade, several striking applications of paper have appeared that built upon a combination of the exclusive properties of paper and smart engineering. In civil engineering, for instance, heavy-duty paper tubes ([3],[37]) and fire resistant paper concrete [1] are used to construct temporary shelters as a low-cost remedy during natural catastrophes. Employing various types of engineered paperboard, a Chinese restaurant chain, called CartonKing, employs paper in the making of tables, chairs, walls and also the cooking pans used in the restaurant. Further efforts in employing paper and paperboards in the fabrication of lightweight and robust furniture exist [36]. Moreover, nano-cellulose is being used by American Process Inc. in collaboration with Futuris Automotive to reinforce the strength and decrease the weight of automotive components [35]. Hence, paper or, i.e., cellulose, if engineered the right way, is obviously able to serve under harsher environments than one could imagine.
Acknowledgments: The author acknowledges Daniel Johnson for his assistance with the literature review, and Autumn Pratt and Jennifer Blackburn for discussing the readability of this article.
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