Compliant Electronics Enable Advanced Capabilities in Wearable Electronics and Prosthetic Skin Applications

As computers have become smaller and more portable, they have become increasingly integrated into many aspects of human life. With the invention of laptops and cell phones, computers could always be readily available. Recently, with the introduction of wearable fitness trackers and health monitors, we are wearing electronics on our body. The next evolution of this trend is to seamlessly integrate electronic devices with humans, which will require new strategies for making electronics that possess similar mechanical properties as the human body and that communicate in ways that are compatible with the human nervous system.

Wearable electronics have the potential to revolutionize healthcare by providing constant information about the state of the body, which gives information that cannot be obtained from visits to the doctor. For example, measuring blood pressure once a year or even once a day gives a snapshot of a health parameter, and large deviations from normal values could indicate a problem. However, by monitoring blood pressure continuously, much more complex and nuanced information can be obtained that could give more insight into disease conditions or earlier warnings about potential problems. As illustrated by the recent lawsuit against a major wearable technology company, the current generation of wearables are not sufficient to meet the needs for accurate and advanced diagnostics and evaluation. Largely, this is because they often rely on traditional rigid electronics that are housed in rubber in order to make them suitable to be worn. However, the lack of conformal contact between the electronics and the body can lead to artifacts and inaccuracy when the person is moving. Integrating the electronics with the body in an error-free manner requires the devices to conform to the wearer and mimic the mechanical properties of the body. Developing sensors and readout electronics that are flexible and stretchable is necessary to meet the needs of this new generation of electronic devices.

In order to measure the shape of the pulse waveform with good accuracy, it is important to have very high sensitivity sensors with fast response time. The Bao research group (https://baogroup.stanford.edu/) developed sensors based on microstructured rubber layers that allow the rubber to deform into free space as it is compressed, resulting in fast time response and high sensitivity. Using a thin, flexible substrate improves the ability of the sensor to conform to the body, and adding hair-like structures onto the surface of the device can further improve the sensitivity (Figure 1). In addition to cardiovascular characteristics such as pulse rate and blood pressure, measuring chemicals emitted through the skin could have important applications in detecting or diagnosing health problems. Measuring these characteristics using a small wearable and disposable device requires unique mechanical properties as well as low-cost devices. The Bao group works on solution-processed organic chemical sensors that are stable in a wide range of water-based solutions, potentially making them applicable for use on or in the human body.

Figure 1: Flexible pressure sensors with microhairs that improve contact with the skin and therefore increase the sensitivity of the devices. The devices were sensitive enough to measure the venous pulse, which is typically very challenging to measure. [Pang,…,Bao et al, Adv. Mater., 27:634, 2015]

In addition to health monitoring applications, skin-like electronics could be used to restore sensation to prosthetic limbs. Prosthetics users prioritize characteristics such as durability, life-like feel, low cost, and light weight. Sensor skins for prosthetic devices would also need to incorporate these properties. The durability of skin-like electronics can be improved by appropriate selection of materials such as carbon nanotubes and tough elastomers. The feel of skin-like electronics can be mimicked by using materials that have similar mechanical properties to human skin. Within the Bao group, electronic devices are being developed out of components that consist entirely of a new class of electronic materials that have skin-like mechanical properties (Figure 2). Fabrication processes based on solution deposition, such as spraycoating and inkjet printing, are used to minimize the cost.

Figure 2: A stretchable, skin-like device for converting pressure into signals that can be interpreted by the brain.

Prosthetic skin must be able to communicate the information from sensors into the nervous system of the user. Toward this goal, the Bao research group has collaborated with PARC to develop sensors for prosthetic skin that encode information as the time between short voltage pulses called action potentials, similar to how biological skin encodes information about sensory stimuli (Figure 3) (see a news release: https://abc7news.com/health/stanford-researchers-create-artificial-skin-that-senses-touch/1155174/). Like receptors in biological skin, these sensors only activate when a stimulus is applied, saving power in the absence of stimulation. Power savings could help to reduce the weight of the battery that is required in the prosthetic device. Furthermore, generating sensor signals in a biomimetic way could allow the pressure signal to be injected into the nervous system without the need for complicated external electronics. A simple proof of concept showed that signals from the biomimetic sensor could stimulate mouse brain slices in a petri dish through the use of an optogenetics technique developed in the Deisseroth lab at Stanford (https://web.stanford.edu/group/dlab/). The approaches being pursued in the collaboration between the Bao lab and PARC attempt to address the requirements for prosthetic electronic skin by implementing new materials to promote durability and life-like feel, new processes to enable low-cost fabrication, and new sensing paradigms to reduce power consumption and simplify the process of integrating with the nervous system.

Figure 3: A comparison between biological (A) and artificial (B) mechanoreceptors. Biological receptors produce action potentials with a frequency that depends on the pressure. Artificial mechanoreceptors consist of an organic oscillator circuit printed by PARC combined with a pressure sensor to create voltage pulses that mimic action potentials. An optical interface developed in the Deisseroth lab was used to convey the pressure information to mouse brain slices. [Tee, Chortos, Berndt,…Bao et al, Science, 350:313, 2015]

Alex Chortos, Stretchable Electronics and Electronic Skin researcher at the Stanford University Bao group. He will be speaking at the February 18, 2016 PARC Forum: Bio Inspired Sensors for Wearble Electronics and Prosthetics. Please register here to attend.