Medical electronics inspired by our skin

Title Stretchable and Fully Degradable Semiconductors for Transient Electronics
Authors Helen Tran, Vivian Rachel Feig, Kathy Liu, Hung-Chin Wu, Ritchie Chen, Jie Xu, Karl Deisseroth, Zhenan Bao
Journal ACS Central Science
Year 2019

The process of implanting an electronic medical device, such as a pacemaker or glucose monitor, can be intense and painful. These devices, however, are incredibly useful and save lives daily. New implanted electronics could be of even greater benefit to medicine; for instance, devices could monitor healing progress after treatment or surgery. Such a device, however, would have to be accepted by the body and removed via surgery after it is no longer needed.

In creating this new generation of implanted electronics, scientists have turned to the body itself as inspiration. Skin, for instance, is a materials chemistry marvel: it’s strong, flexible, stretchable, and fully degradable by the body. If an implanted device behaved like skin, it would form and adapt to the organ on which it’s implanted and eventually degrade naturally without having to be surgically removed.

Researchers at Stanford University have recently created a semiconductor that is both stretchable and fully degradable – a major step in developing these “transient electronics.” The material is a polymer made by combining a well-characterized stretchy and biodegradable polymer with a semiconducting polymer.

 The material is successful thanks to a process called nanoconfinement, where tendrils of the semiconducting polymer are embedded into a more elastic polymer matrix (Figure 1). The elastic polymer matrix is a urethane-based polymer with biodegradable polycaprolactone. This matrix is not only fully degradable but extremely stretchable without cracking.

Figure 1. Structure of the stretchable, degradable semiconductor. (a) The material is stretchable and fully degradable. (b) The semiconducting polymer is nanoconfined (or embedded) within a biodegradable elastic polymer. (c) Chemical structure of the polymer. Inset: degradation process of the biodegradable elastic polymer. (d) Products resulting from degradation of the material. Adapted with permission from Tran, H., et. al.  Anal. Chem. 2019. Copyright 2019 American Chemical Society.

The more fragile semiconducting polymer is protected by the elastic polymer matrix. To make the semiconducting polymer degradable, the researchers used imine bonds in the backbone, which can be degraded in acidic solutions.

Together, these polymers form a material that was stretchable and performed well as a semiconductor. The efficacy of a semiconductor is determined by its charge mobility – faster charge mobility is a marker of better performance. The nanoconfined polymer performed equally as well as polymer that was not nanoconfined in the elastic polymer matrix, meaning nanoconfinement is a promising method to create these materials.

The polymer was degraded in acidic conditions – where the rate of degradation was proportional to the concentration of acid. Finally, the researchers tested whether the material is biocompatible, or whether cells can live in its presence. When human cells were cultured on glass coated in the polymer, over 99.5% of the cells lived, showing high viability.

This new stretchy, degradable semiconducting material will be essential as transient electronics are developed. Though inspired by the human body, such transient electronics will be useful for more than just medical implants. Transient electronics can be used for environmental monitoring and information storage – the possibilities are vast. However, electronics are complex devices, and their ongoing development will have to occur component by component. These materials are a major step forward in creating successful implantable electronics.

Feature image adapted from Wikimedia Commons.

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