Australasian Science Magazine

Conducting polymers provide several advantages over standard metal electrodes, and in the future they are likely to be integral in the development of the next generation of implants.

By Rylie Green

Plastics that conduct electricity could bring the bionic man from science fiction to reality.

Within the next decade the bionic man will make the leap from science fiction to reality. More than an exercise in science, advanced bionic devices restore quality of life to those who have lost functions through injury or disease.

Over the past 20 years cochlear implants have returned a sense of hearing to deaf patients, facilitating speech patterns in deaf children and improving the interactions of deaf adults both socially and in the workforce. Current bionic designs in development – including the bionic eye, robotic limb prostheses and brain–machine interfaces – have the ability to further improve the quality of life for millions of people globally.

The key to making these devices effective, and hence commercially viable, is the development of advanced materials that provide better functionality than materials conventionally used in medical implants. The most important factor here is the interface where a synthetic device meets the biological environment. My research aims to understand cell–material interactions and design material interfaces with bioactive components that promote the integration of living tissue and the device.

Conducting polymers are a type of plastic which is intrinsically conductive and recent research has shown that these materials are fundamental to achieving true integration between robotics and humans.

Conductive polymers can pass electronic charge due to their backbone structure (Fig. 1). These polymers were first discovered in the 1950s, but did not gain commercial significance until they found application in the electronics industry in the 1980s. Over the past decade, substantial efforts have been undertaken to establish biocompatible forms of conductive polymers that will allow them to be used in medical applications.

Conventional medical electrodes are fabricated from metals such as platinum, gold and iridium alloys, and have smooth surfaces that will not allow tissue integration (Fig. 2). As a result, the interface between a metal electrode and the neural tissue it stimulates is associated with a significant fibrous tissue gap through which the electrical signal must be passed. Additionally, due to their featureless surface, metal electrodes have a relatively small safe current injection limit when implanted.

My research is addressing the limitations of conventional metal electrodes by designing biologically active conducting polymers. My aim is to produce a surface that could not only be tolerated by the target tissue, but could promote intimate contact between both the electrode and nerve tissue.

Conducting polymers can pass charge in the form of both electrons and ions. This is important since metals pass electricity in the form of electrons but cells can only pass charge in form of ions. Since conducting polymers can transfer both electronic current from metals and directly produce ions to interact with neural cells they can communicate more efficiently than a metal electrode.

Conductive polymers also have a very different surface morphology compared with metals. Conventional metals are smooth but a typical conductive polymer has a nodular surface similar to cauliflower (Fig. 3).

This significantly rougher surface has a range of benefits. My research has shown that this increase in surface area produces safer electrodes since larger electrical currents can be passed through them without damaging the body tissues or the electrode. The conductive polymers I have fabricated can safely pass up to 30 times more electrical current than a metal electrode of the same size.

The surface roughness of conductive polymers can also be modified with proteins or peptides that have been specifically chosen to target the tissue that is being recorded from or stimulated. In most implants this will be either a type of nerve or muscle cell. For example, I have modified a conductive polymer with a molecule that attaches specifically to brain cells. It is designed for a brain–machine interface that allows quadriplegics who cannot speak to communicate via thoughts that produce a computer read-out.

The peptides are chosen based on their functionality in the healthy human body and the role it has in a specific organ. I have incorporated into conductive polymers some peptides of laminin (a matrix protein with a range of functions including cell attachment, neural cell growth and development), growth factors and polyamino acids (polymer chains comprised of usually a single type of amino acid). Many of these biological molecules produce distinct surface morphologies (Fig. 3, right).

Specifically I have designed conducting polymer electrodes that not only encourage cell attachment but can also promote cell growth by releasing growth factors. Figure 4 shows fluorescent nerve-like cells growing directly on platinum electrodes coated in a conducting polymer. The growth factor that was incorporated into the polymer diffuses out from the electrode and into the tissue, where it actively promotes the survival and regeneration of nerves.

Materials with this degree of functionality could be extremely beneficial in bionic devices such as robotic prosthetics where the patients’ limb has been damaged by injury or disease, helping the remaining cells to survive and possibly regenerate.

With this improvement in the performance and flexibility of electrode design, a new generation of implants is on the cusp of development. In the future this may include:

• cochlear implants with many more electrodes, allowing patients to listen to music and hear conversations in a crowded room;

• the realisation of a commercial bionic eye where users can recognise faces and read text;

• better control of robotic limbs; and

• a reduced need for battery changes in pacemaker devices.

Despite the potential applications of conducting polymers as biomaterials, they still require significant development prior to their use in human patients. As with all biomaterials, conducting polymers must be shown to be safe for permanent implantation and approved by appropriate regulatory bodies. In Australia this is the Therapeutic Goods Administration, but global acceptance and commercial viability of a material often requires approval by the Food and Drug Administration in the USA since its system is particularly stringent and recognised by most other countries.

It is clear that conducting polymers provide several advantages over standard metal electrodes, and in the future they are likely to be integral in the development of the next generation of implants. My future research aims to understand the mechanisms that influence cell–material interactions across a wider range of implant materials, and design artificial surfaces that more closely mimic the native biological environment.

It is important to understand that this includes not only the electrode interactions but the role and potential function of the entire implant device. Insulating materials such as silicone rubbers can be functionalised to support the stability of a device, and coatings can be applied to devices to release other drugs that reduce inflammation and the potential for implant rejection. These developing materials will allow implant devices to be successfully integrated within the human body.

With the advancement of these technologies, implants will be designed to have greater functionality and improved safety, bringing the bionic man out of science fiction and into reality.