Progress toward stretchable electronics

Xueying Zhao & Ching-Ping Wong, PhD

Stretchable electronics can potentially alter our conception of electronics, from rigid and planar wafer-based ones to soft and curvilinear stretchable ones. Stretchable electronics are able to be achieved in two approaches. One relies on the use of novel materials in conventional layouts, while the other relies on new structural layouts of conventional materials. Stretchable electronics have promising applications in the biomedical sciences as well as consumer communications. Although stretchable electronics may not be able to immediately eliminate their wafer-based counterparts that have dominated the industry for several decades, they warrant further investigation into their potential applications to fill the gaps that are beyond the limits of wafer-based technologies.

Introduction

At the heart of current electronics are wafers -- thin slices of semiconductor material on which electronic devices such as resistors, capacitors, and transistors are mounted (Laplante, 2005). Wafer-based electronics utilize single crystal inorganic materials (Kim et al. 2012) such as silicon, which has been the mainstay of the industry for many years (Osborne, Lavine, & Coontz, 2010). According to Moore's law, the number of transistors on integrated circuits doubles every couple of years; this exponential growth and ever-shrinking transistor size has led to faster and cheaper devices. In addition to this continuous effort in bringing about quantitative improvement in today's electronic devices, researchers in recent decades have been exploring a qualitative transformation of electronics.

Wafer-based electronics are rigid and planar; the human body is, by contrast, soft and curvilinear. This mismatch impedes the development of devices capable of conformal integration with biological tissues. In search of solution, researchers have focused on the development of circuits on bendable substrates; one example is the pen-on-paper electronics (Russo et al., 2011). Such bendable electronics are softer than their wafer-based counterparts, but offer only simple bendability. More recent efforts look for circuits that withstand extreme mechanics, including stretching, compressing, bending, rolling, and twisting, which gives rise to a brand new evolution of electronics -- stretchable electronics (Kim & Rogers, 2008).

Stretchable electronics require the circuits to be able to absorb large levels of strain (>>1%) without fracture or significant degradation in their electronic properties (Kim & Rogers, 2008). Unlike wafer-based electronics that are rigid and planar, stretchable electronics must be soft and curvilinear. Stretchable electronics not only deserve extensive research in the development of devices capable of conformal integration with biological tissues, which is beyond the limit of their wafer-based counterparts; but they also inspire a new way of thinking in the discipline of materials science through the exploitation of new material structures.

Novel materials and new structures

Stretchable electronics can be realized in two approaches. One relies on the use of novel materials in conventional layouts, while the other relies on new structural layouts of conventional materials (Rogers, Someya, & Huang, 2010). In the context of novel materials, the polymer/inorganic nanocomposite that displays a blend of properties of both polymer matrix and nano filler, is a novel class of hybrid material that has been widely researched in applications of stretchable electronics. This approach requires use of materials that are themselves elastic (such as polydimethylsiloxane and polyurethane) to enable stretchability, as well as inorganic nano filler (such as silver nanowires and single-walled carbon nanotubes) to allow electrical conductivity. The elastic polymer and conductive dopants are combined to form elastic conductors that are then used as electrical interconnects between the devices (Rogers et al., 2010). However, all known stretchable conductors have relatively high resistivities and properties that are highly temperature and strain sensitive, limiting their applicability (Kim & Rogers, 2008).

A distinct approach to achieve stretchability is not through novel materials, but through new structural configurations (Kim & Rogers, 2008). The underlying idea proposed by Rogers et al. is very straightforward: any material that is sufficiently thin is flexible, by virtue of bending strains that decrease linearly with thickness. For example, a silicon wafer (750-800 microns thick) is rigid and planar, but nanoscale wires, ribbons, or membranes of silicon (100 nm - 1 micron thick) are flexible (Rogers et al., 2010). Therefore, stretchability can be realized in inherently rigid inorganic materials. Compared with the novel materials approach discussed above, an appealing advantage of this strategy is that it decreases the reliance on elastic polymers to accomplish stretchability, giving rise to systems with electrical performance and reliability comparable to those of wafer-based electronics (Kim & Rogers, 2008).

Potential applications

The most prominent applications of stretchable electronics lie in biomedical science and engineering, since softness and curvedness offer matching of both shapes and mechanical properties to biological tissues and organs (Rogers et al., 2010). Examples include ultrathin health-monitoring tapes that seamlessly conform to the skin, as well as imaging devices that utilize hemispherical detector layouts to resemble an eyeball (Rogers & Huang, 2009). The applications of stretchable electronics have the potential to revolutionize the field of medicine. These bio-inspired devices, if implemented with biodegradable materials, might produce far less electronic waste than would traditional wafer-based electronics.

In addition to the field of biomedicine, a non-negligible profit-driven market lies in consumer electronics, as suggested by Samsung's release of a phone with a bendable screen. The commercial reality of phones with bendable screens may bring in an unprecedented revolution in everyday electronics leading to enormous profits for the companies involved. Despite the attractiveness and novelty of bendable screen as shown in Fig. 1, there are those who doubt the necessity of this feature by pointing out the concern that it could be annoying to use a touch screen if it bends away from our finger ("Samsung unveils new phone with flexible screen," 2013). However, just as we have adapted to the touch screen, a bendable screen might just be another element that most people can adjust to and may not be able to live without.

Taking into consideration their benefits and drawbacks, stretchable electronics may not be able to immediately eliminate their wafer-based counterparts that are thicker, more rigid, and have dominated the industry for several decades. Nevertheless, they warrant research to further investigate their potential applications to fill the gaps that are beyond the reach of wafer-based technologies.

Conclusions

In addition to their various promising applications, stretchable electronics have the potential to fundamentally alter our conception of electronics, from rigid and planar to soft and curvilinear (Rogers et al., 2010). Stretchable electronics not only have the potential to change how we use electronics, but also have the potential to revolutionize the world we live in, as evidenced by the realization of skin-mounted health-monitoring tape as well as electronic eyeball camera. Finally, the approach that exploits new material structures rather than novel materials is at the forefront of this brand new evolution of electronics, as the world is facing an impending crisis in material supply (Osborne et al., 2010). This enlightening "new structure approach" is expected to be applied in other materials-related research as well. The stretchable electronics is no longer a miracle too good to be true.

References

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