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Instant insight: Polymers in nanobionics
28 June 2007
Gordon Wallace and Geoffrey Spinks of the University of Wollongong, Australia, take a close look at the interface between electronics and biology.
Truly bionic limbs are still a long way from reality
Effectively bridging the interface between electronics and biology is critically dependent on advances in new electronically conducting materials. The discovery of inherently conducting polymers (ICPs) in the late 1970s revolutionised this field and organic electronic conductors are now at our disposal. The soft character of ICPs provides an extra dimension in designing interfaces between the hard, digital electronics world and the soft, amorphous world of biological systems. ICPs are unique with their potential to impact on bionic devices from the molecular, through the cellular, to the skeletal level.
This ability to control biomolecular interactions on conducting polymer surfaces provides a pathway for controllable interactions with whole cells. ICP platforms have been used to promote neuronal cell growth and the application of an electrical stimulus to the cell culture on the PPy film significantly increased the expression of neurites in the cells. These new materials might well eventually find use in medical implants that require electrical connection to nerve cells (for example, as bionic ears or eyes).
The popular concept of bionic devices is at the whole organ or the skeletal level. The ability to monitor and manipulate human movement and senses such as hearing and sight is physically demonstrable with devices such as the Cochlear Implant (the 'bionic ear'). Manipulating human movement is the most mammoth of tasks. It is amazing to admit that in this highly technological world we live in, we still do not have adequate light weight, low power consumption technologies that can be strapped on to assist in human movement.
While significant improvement in artificial muscle performance based on conducting polymers has occurred in recent years, there is still some way to go before we can match natural muscle in terms of speed, efficiency and control. The amount of movement that can be generated has increased dramatically in recent years to around 40% (comparable to natural muscle), although with conducting polymers this takes several minutes to occur. The speed of response has been increased by tuning the electronic control system and the conductivity of the structure used. The fastest response from an ICP actuator (15%/s) is still considerably slower than natural skeletal muscle (~80%/s). Next to large movements, speed is critically important in generating useful motion. The large, fast movements possible through musculo-skeletal systems in animals is the basis of running, flying, swimming - attributes that would be highly desirable in nimble, dexterous and possibly miniaturised robots.
Recent material developments move us closer to these possibilities. Like natural muscle, the formation of ICPs into fibres provides a better geometry for actuator performance. For example, polyaniline fibres are now readily available in high strength and conductivity. Furthermore, we have shown that the addition of small quantities of carbon nanotubes to polyaniline, followed by wet-spinning and drawing, produces superior actuation response under an external load. In fact, we measured an actuation response at more than 100 MPa applied stress, three times higher than previously reported for conducting polymer actuators.
This is just one example wherein advances in nanomaterials and nanofabrication have already impacted on the performance of organic conducting polymers as bionic devices. Nanostructuring provides dramatic improvements in the electronic properties of conducting polymers.
As material scientists delve into the nanodomain, the boundaries between electronics and biology become fuzzy. This is exactly what we want - a seamless transition between the hard world of electronics and the soft world of biology!
Read the full Opinion article 'Conducting polymers - bridging the bionic interface' in June's issue of Soft Matter.
References1 R Baughman et al., in Conjugated polymeric materials: opportunities in electronics, optoelectronics and molecular electronics, ed. J Bredas and R Chance, Kluwer, Dordrecht, 1990, p. 559.
Link to journal article
Conducting polymers – bridging the bionic interface
Gordon Wallace and Geoffrey Spinks, Soft Matter, 2007, 3, 665