When we think of polymers, the very first images to pop into our heads are most likely of the water bottles currently polluting our oceans, or the insulating layer of plastic covering our electrical chargers and devices.
It can be very difficult, especially in our day and age to still be able to view polymeric materials as being an advantage to our societies and not just a liability we inevitably have to succumb to. The purpose of this article is to shed light upon a class of polymers, which differentiate themselves from the rest due to their conductive properties.
Electrically conductive polymers show great potential and are currently employed in various applications such as supercapacitors, light emitting diodes (LED), artificial muscles and biosensors .
You may now be wondering how and why a polymer material is used, and the answer to these questions are hopefully found in the following passages.
A brief history of polymers and conductive polymers
Natural polymers have been around us since the beginning of time, surrounding us in various forms and shapes. The earliest production of synthetic polymers in industry, on the other hand, dates back to 1907, in which Baekeland made the first synthetic thermoset polymer, phenol-formaldehyde also commercially known as Bakelite.
The commercialisation of Bakelite sparked the industry of synthesised polymers and continued in a manner such that engineering plastics we encounter daily such as polyamide (PA) and polyethylene (PE) were later developed .
Although engineering polymers such as PA are known for their great mechanical strength, stiffness and chemical stability, it is also intuitively assumed that they are great electrical insulators. Even though this is true for PA, it is also very possible to stumble upon electrically conductive polymers as well.
It was not until the 1970s that the development of conducting polymers gained traction and attention throughout the world of scientists and engineers. In 1975 polysulfur nitride was discovered to be superconducting at low temperatures, and this discovery paved the way for intensive research in the field of conductive polymers .
One of the most well-studied conducting polymers, polyaniline, has found its way into various applications in some of the most research-heavy industries of recent times. Polyaniline is, for instance, used as a sensor in the detection of ammonia. This is an area subject to a lot of research regarding how to improve the environmental stability and repeatability of the sensors by addition of different fillers .
Ultimately, the possibility of having organic materials with the conductive properties of non-organic materials along with the intrinsic polymeric features of mechanical flexibility and relatively low-cost production gives rise to endless opportunities .
How can a polymer conduct electricity?
Although the mechanism behind the conductivity of polymers still is not fully understood by engineers and scientists, some structural properties are known to influence the conductivity of said polymers.
One common trait of all the conductive polymers is that they are conjugated polymers. Conjugated polymers are a class of organic polymers, which are intrinsically semiconducting and can, in some cases, exhibit quasi-metallic properties with regards to conductivity .
For us to fully understand the physics behind the conductivity of this class of polymer, we must look at the molecular structure of a conjugated polymer. Let us take a closer look at the molecular structure of polyaniline.
Figure 2 gives a schematic of the molecular structure of different oxidation states of polyaniline. In terms of the conductivity of the polymer, we can observe the interchanging single and double bond occurring throughout the backbone of the polymer. This is also known as the conjugated backbone, but what does it really mean when a polymer is conjugated?
Conjugation in terms of linguistics refers to a link between two separate things. Essentially what is obtained in a conjugated backbone of a polymer is an overlapping of p-orbitals, which allows for the delocalisation of electrons along those orbitals.
The delocalised electrons thus may act as charge carriers as they are free to move throughout the whole system. Electron delocalisation is also the mechanism responsible for the electrical conductivity of metals.
To induce or enhance the electrical conductivity of the conjugated polymers, a process known as doping is carried out. The doping process is essentially like that of silicon semiconductors, in which you can have a p-doping (oxidation, or addition of a delocalised electron hole) and n-doping (reduction, or addition of a delocalised electron).
Following the doping process, the electrons are now free to move around, resulting in an electrical current as they pass along the conjugated polymer backbone.
Application in the medical industry
The medical industry is one of the industries that would be revolutionised by the introduction of conductive polymers, specifically with regards to tissue engineering and biosensors.
In terms of tissue engineering, 3D scaffolds utilising conductive polymers can be obtained by means of electrospinning. The use of conductive polymers is of special interest due to them being a suitable biocompatible matrix for biomolecules. One of the major companies which currently has a patent filed for a biodegradable electrically conductive polymer for tissue engineering is Polymer Chemistry innovations Inc.
The conductive polymer is believed to also be applicable in areas such as wound healing, bone repair and spinal cord regeneration .
An interesting application of conductive polymer in the medical industry is the development of artificial muscles by utilising their response to an electrical field. Electroactive polymers (EAP) are polymers which observe dimensional changes in response to electrical stimuli.
Several groups have managed to produce an actuator device that can transform electrical pulses into mechanical movement, which is a process which mimics that of our own natural muscles. 
It is merely recently that the field of EAP has progressed from research to commercialization. In 2010 ‘European Scientific Network for Artificial Muscles’ (ESNAM) was founded to gather some of the most active research institutions and relevant industrial interest groups. The network was established to push forward the scientific progress and bridge the gap between the industry and research institutions .
The use of conductive polymers in medical application is a field of on-going research, due to the inherently interesting properties of said polymer group.
What does the future hold?
The industrial application of electrically conductive polymers is an area which is still subject to active research. As previously mentioned, the medical industry would observe a revolutionary change with the introduction of conductive polymers.
The global market for EAP was 484.9 million GBP in 2016 and is expected to increase to 725.6 million GBP in 2021. This corresponds to an increase in global compounds annual growth rate (CAGR) of 8,4% in the span of those years .
So, what does the future hold for conductive polymers? As mentioned throughout the article, the promises of said polymers are many, and potential application in biosensors, supercapacitors and corrosion protection are merely some of the places in which they can be utilised by substituting metallic components.
The future is looking very promising, which can be deduced from the forecasted global market size in 2021. We are moving towards a greater industrial commercialisation of conductive polymers, and as a consequence, we should not be surprised if our future medical treatments or electrical devices are based upon the intriguing conductive properties of an inherently simple polymeric material.
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