Introduction

Traditional pursuits in organic electronics

Organic electronics

Organic electronics, plastic electronics or polymer electronics, is a branch of electronics dealing with conductive polymers, plastics, or small molecules. It is called 'organic' electronics because the polymers and small molecules are carbon-based...

 have demonstrated tremendous versatility in a wide range of applications including consumer electronics, photovoltaics, and biotechnology. However, the interface of biomolecules and organic semiconductors has recently explored the potential use of natural and synthetic polymers as structural components of electronic devices. The fabrication of electronically active system using biomaterials-based components has the potential to realize a large set of unique devices including environmentally biodegradable systems and bioresorbable temporary medical devices

Natural organic semiconductors

There are abundant opportunities in the convergence of biodegradable materials and organic semiconductors to produce electronic systems with unique overall material profiles. Melanins are a unique class of organic material that bridges organic semiconductors and biomaterials. Melanins have demonstrated unique switching properties as well as biocompatibility. Carotenoids, a class of naturally occurring small molecule pigments, have been studied as potential use in organic thin-film silicon transistors, OTFTs. Carotenoids are small molecule polyenes that serve as precursors for many biomolecules including Vitamin A. Bixin and beta-carotene, two specific types of carotenoids, were processed from solution to form OTFTs which exhibited hole mobilities on the order of 10⁻⁶ and 10⁻⁷ cm²-V⁻¹-sec⁻¹, respectively. While the electrical performance of these specific materials may not be yet suitable for particular device applications, this work demonstrates the wide range of electronic properties that biomaterials can possess.

Biomaterials-based electronic devices

One particularly intriguing concept is the notion of fabricating fully bioresorbable electronic devices for potential use in temporary electronically active medical devices. Towards this end, recent efforts have focused on the fabrication of OTFTs in a fully biodegradable platform as a proof of concept. Initial iterations of this concept utilize synthetic biodegradable polymers that are both ubiquitous in medical applications and exhibit appropriate electronic properties. Towards this end, Bettinger and Bao developed one of the first examples of a biodegradable biomaterials-based transistor. They focused on the fabrication of OTFTs in a fully biodegradable platform as a proof of concept. Initial iterations of this concept utilize synthetic biodegradable polymers that are both ubiquitous in medical applications and exhibit appropriate electronic properties. Poly(DL-lactide-co-glycolide) (PLGA) was melt processed to form the device substrate, which comprises over 99% of the device by mass. Solution-processed PVA was selected as the gate dielectric because of its demonstrated advantages in electronic and biomedical applications. The active layer consisted of hydrophobic small molecule semiconductor that has previously demonstrated stable operation in aqueous environments. While the biodegradation of the small molecule active layer has not been studied explicitly, it is hypothesized that the chemical breakdown of polyaromatic small molecules would commence via oxidative biodegradation processes within perioxisome organelles; processes that are similarly responsible for the free radical degradation of naturally occurring polyaromatic melanins. The top-contact OTFT device structure was completed by the use of silver and gold as the gate and source-drain contacts, respectively. These devices exhibited suitable device operation with hole mobilities up to 0.2 cm²-V⁻¹-sec⁻¹ and Ion-Ioff ratios on the order of 10³. OTFTs fabricated using UV-photocrosslinked PVA gate dielectrics maintained function after direct exposure to liquid water. In vitro biodegradation of these devices was demonstrated by incubation in citrate buffer

Device functionality was lost almost immediately while device disintegration occurred after approximately 50 days. This demonstration is only the first step toward the realization of fully biodegradable organic electronic systems. Ideal devices for these applications will be able to operate using low-voltages and be integrated with suitable packaging strategies to enable in vivo operation. The realization of biodegradable metal contacts is currently a complimentary research thrust that is being explored in the context of bioresorbable metals for orthopedic and cardiovascular applications. These materials including magnesium based alloys may also find significant utility in development of bioresorbable organic electronic devices.

Compostable electronics

The notion of fabricating organic electronic devices on environmentally compostable material platforms is an intriguing possibility for biodegradable electronic materials. Organic electronic components have been fabricated on substrate materials such as aluminum foil
and paper
to accommodate these expanded functionalities. In one embodiment of this idea, paper films were utilized as a combination substrate and gate dielectric for use with pentacene

Pentacene

Pentacene is a polycyclic aromatic hydrocarbon consisting of five linearly-fused benzene rings. This highly conjugated compound is an organic semiconductor. The compound generates excitons upon absorption of ultra-violet or visible light; this makes it very sensitive to oxidation...

-based active layers. This idea was expanded upon to create complete circuits using foldable paper-based substrates.

1. This demonstration was motivated by two primary factors:
   1. paper substrates are mechanically flexible and capable of small bending radii
   2. paper is ubiquitous in modern society

As such, utilizing paper-based substrates are a potentially very important strategy for widespread deployment of simple electronic devices. A wide range of functionality is demonstrated including the ability to fabricate circuits on rolls of paper that can be easily crafted into the final form by virtue of simple folding and cutting techniques.