Flexible future for electronics

Ben Smye, Head of Growth at materials resource and search engine Matmatch, explores how a new paradigm shift in stretchable electronics could herald further innovation.

The days of the hard computer chip may be numbered. Covered in transistors and other semiconducting elements, these rigid devices likewise render the items in which they are found – our televisions, laptops and smartphones – similarly inflexible.

Despite many false starts, stretchable electronics have been developing, and being commercialised, for about a decade. According to a recent report by IDTechEX Research – Stretchable Electronics 2017-2027 – the market for stretchable electronics could grow to at least US$600mln by 2027.

Stretchable electronics are required to conform to a desired shape and survive the environment where they operate. Wearable technology has been the main application behind the growth potential. Wearables, whether relied upon in military, medical or sports applications, demand 1D electronic devices that are light, flexible and adaptable to frequent deformations.

Other applications include soft robots, stretchable sensors, circuits, displays, batteries, energy harvesters, transistors and photovoltaics. Stretch sensors are finding use in a variety of applications from electronic textiles to robotic arms, and the industry is considering new applications, including the notion that stretchable electronics could one day offer biomimicry of human skin. This idea of replicating human skin with stretchable electronics has attracted several researchers over the years. Achieving an artificial skin would allow for more tactile feedback in soft robotics and prosthetics, with sensors detecting external stimuli and triggering a response. Nevertheless, many of these ideas are still in the early proof-of-concept stages due to cost and production challenges.

Beyond skin deep
Smart, artificial skin is one of the most intriguing fields of development in stretchable electronics, but the fabrication remains time-consuming and complex. Engineers are trying to integrate multiple stretchable components, such as temperature, pressure and electrochemical sensors, to create a material that, like human skin, can use signals from sweat, tears or saliva for real-time, non-invasive healthcare monitoring. 

There are two main ways to create this. The first method is based on the use of intrinsically stretchable materials like rubber, which can be deformed in the long term without losing properties. In addition to its ability to deform, it has a very high electrical resistance. The other option is by using innovative design, including brittle semiconductor materials like silicon. This substance, for example, can be grown on a pre-stretched surface, which means that it can be compressed to create buckling waves. 

Another alternative is mixing rigid conductive materials using flexible interconnections, such as soft and liquid metals. This is something that the McCormick School of Engineering at Northwestern University, USA, tested and commercialised. It can help people who suffer from chronic medical conditions. 

The researchers, working with a team of American and international scientists, have recently developed a design that allows electronics to bend and stretch to more than 200% of their original size. This is four times greater than currently possible with commercialised technology. The scientists have achieved this goal by combining a liquid metal in a porous polymer, which delivers impressive stretchability from a material that does not normally stretch.

For all stretchable electronics, the fundamental challenge is stretching without impacting system performance. Researchers have been tackling this by developing stretchable conductors, which are either made of inherently flexible materials or able to stretch due to design techniques. Either way, to achieve stretchable conductors, conductive components, such as metal nanowires, carbon nanomaterials and polymers, are often used as fillers and arranged in an elastomer matrix, whereby materials are cross-linked. This matrix is arranged to a desired structural design. This may be a wavy configuration, a fractal design, or a horseshoe-shaped planar structure.However, the precise challenges depend on the designed electronic device. Stretchable heaters, for example, need to achieve a uniform and stable temperature distribution safely.

Improving stretchability
Several methods for producing stretchable materials have emerged in recent years. One is remote epitaxy, or the ‘peel and stack’ method. Developed by engineers at the Massachusetts Institute of Technology (MIT), USA, remote epitaxy involves growing thin films of semiconducting material on a large, thick wafer of the same material. This is covered in an intermediate layer of graphene. Any number of thin, flexible semiconducting films can then be peeled away from the graphene-covered wafer. Crucially, it is possible to stack films made from different materials to produce flexible, multifunctional electronic devices. The essence of the technology depends on polarity, defined by the charges between the atoms flowing over graphene and the atoms in the underlying wafer.

The study conducted at MIT has had positive results, as the team has shown that they can use remote epitaxy to make semiconducting materials from metals in groups three and five of the periodic table, but not from group four. The reason, they find, boils down to polarity.

Researchers in China have developed an alternative to MIT’s peel and stack method. They have fashioned a metal-polymer conductor (MPC) material. However, the metals are not typical conductive solids, but are, instead, gallium and indium that exist as a syrupy liquid through which electricity can flow. In simple terms, the MPC has been described as islands of round liquid metal floating in a sea of silicone-based polymer. 

The polymer network itself yields mechanically resilient materials with enough conductivity to support functioning circuits. Different MPC formulations have been tested in a range of applications, including wearable keyboard gloves.

While these methods work well for polymer-based stretchable electronics, there are other types of materials, such as textiles, that show potential in the field. Textiles such as yarn and fabric can be interwoven with sensors, electronic circuits and conductive fibres so that signals are transmitted across the material, creating an e-textile.

This field of stretchable and wearable e-textiles dates to around the turn of the century when the European Commission’s sixth and seventh framework programmes (EU FP6 and FP7) focused on funding research in this area. Most looked at incorporating health monitoring into clothing, allowing for applications like wearable electrocardiograms (ECGs) that conform to the body. 

Electrical stimulation
More recently, textiles are one of the potential materials for creating more flexible artificial muscles, alongside electroactive polymers, to produce textile actuators, or ‘textuators’. 

These textuators respond to electrical stimulation due to the use of polypyrrole (PPy), which is a conducting polymer that deforms in response to electrical signals. The researchers coated yarns in PPy and wove them into various yarn constructions, which improved mechanical stability and brought about a 53-fold increase in strain compared to current motor and pneumatically driven artificial muscles.

This idea of experimenting with constructions and designs is the foundation of many recent studies into stretchable electronics, as researchers move away from relying solely on intrinsically stretchable materials. Instead, the aim is to develop structural designs and configurations that allow greater flexibility and capacity for stretching. Several structural design techniques are growing in popularity, such as origami configurations, mesh structures and fractal design.

Origami configurations require very little work and are useful in a wide range of applications. Origami is the traditional art of paper folding where the folds follow pre-defined crease patterns. The same mechanism can be applied to stretchable electronics as crease patterns can be used in thick panels or engineering materials, which cause the structure to deform and stretch.
 
The stretchable reactions can also come from nanoelectronics using a reaction and response to a magnetic field. Layered magnetic structures revealing a giant magnetoresistance effect are crucial components of highly sensitive magnetic sensor devices. Stretchable and highly sensitive elements that respond to a magnetic field are in demand for novel applications such as smart skin and stretchable consumer electronics equipped with magnetic functionalities.

Despite the positive benefits of stretchable technology, barriers remain. Cost is one of the main issues. It is important to find cheaper available materials and faster, more scalable, manufacturing methods. While the days of the rigid computer chip are not over, we can look forward to the expanded possibilities of a more flexible future.