The power of polymeric coating

Jeff Hendricks Biotectix outlines how polymeric coatings can help improve the performance of medical and consumer electronic devices

Today there exists a large and growing collection of medical, consumer, and industrial devices that harness the body’s electrical signals for applications. These applications range from the more obvious medical diagnostics, health and fitness trackers to entertainment, video games, and even workplace safety.  All of these devices function in one of two ways: either by sensing electrical signals such as heart rate or electrocardiogram (ECG) or by delivering electrical pulses to stimulate tissue, such as neural, cardiac or muscle tissue. For both sensing and stimulation applications, the ability to quickly and safely transfer electrical signals across the tissue-electrode interface is critical for device function. 

Because the characteristics of the tissue-electrode interface vary significantly across applications, the interface must be optimised to achieve device performance, longevity, and safety. One key way to optimise the interfacial properties is to select an appropriate electrode material.

Currently, different electrode materials are used to meet the requirements for a given device application. The physiological and mechanical environment, the duration of use, and the magnitude and characteristics of electrical communication all factor into electrode materials selection. For example, the rigorous environment and constant use over a period of years experienced by a cardiac pacing lead electrode is much more demanding than what a temporary ECG electrode worn on the skin for several hours must withstand. Furthermore, implantable electrodes must contend with the body’s chronic tissue response, which can often lead to fibrous encapsulation that can limit reliable long-term performance.

Traditionally most of these applications, including both implanted electrodes and short-term-use electrodes, have relied on some type of precious metal for electrodes. Implanted stimulation electrodes are usually platinum or platinum-iridium; catheter electrodes are typically platinum or gold; and ECG/EEG/EMG electrodes commonly use silver-silver chloride gels. While these materials are designed to satisfy the electrical and physiological requirements, they have limitations. They are generally expensive, have limited processing ability, and often have limited mechanical flexibility — which can be desirable for certain applications.

One of the primary challenges facing next-generation cardiac and neuromodulation devices is electrode miniaturisation. Smaller electrodes are desirable to communicate with single neurons or small groups of cells, and to provide highly targeted stimulation in procedures such as deep brain stimulation, for example, while avoiding the side effects of collateral stimulation. However, as the electrode size decreases its ability to transport charge also declines, leading to high interfacial impedance. This produces noisier recordings with lower signal amplitude for sensing applications. For stimulation, the amount of charge that can be safely delivered before damage occurs to the electrode or tissue depends on the electrode size and the electrode material’s charge injection density. Since the values of charge injection density are fixed for a given material, as electrode size shrinks, employing electrode coatings made of materials with higher charge injection densities can help solve this problem. The charge injection densities for conducting polymers are reported to be nearly 100X higher than for bare platinum [Cogan S. Neural Stimulation and Recording Electrodes. Annu. Rev. Biomed. Eng. 2008 10:275-309].

Conducting polymers are an excellent material choice for miniaturised electrodes because they can safely deliver the necessary charge without damaging the tissue for stimulation and reduce noise for higher quality recording.

Biotectix has developed a group of proprietary electrode materials based on conducting polymers that have been shown to facilitate electrical communication with the body. These are based on the polymer poly (3,4-ethylenedioxythiophene) or PEDOT, which has been used on large scales in the industrial and automotive industries for anti-static applications. Due to the chemistry of these polymeric electrode materials they are able to conduct charge via both electrons and ions, unlike traditional metals that conduct only via electrons. This makes them well-suited for transferring between electronic and ionic charge carriers at the tissue-electrode interface, leading to potentially superior recording and stimulation performance.

These materials were originally developed by Professor David C Martin and his group at the University of Michigan for brain-computer interfaces as part of strategies to address the deteriorating long-term performance of microelectrodes implanted in the brain. With the support of investment and innovation firm Allied Minds, the technology was spun-off from the university into a new company, Biotectix.  Since then the company has focused on the development, commercialisation, scale-up, and support of regulatory approval of these materials for medical devices. 

Biotectix’s Amplicoat is an electrode coating made from a proprietary blend of conducting polymers and biocompatible dopants that provide durability and increased conductivity. It is designed for use inside the body for cardiac and neural applications. Amplicoat is electrodeposited onto conductive components or sub-assemblies and can be applied at various stages during manufacturing. It is manufactured under an ISO 13485 certified quality system and is currently used clinically on CE-marked devices. Most customers ultimately incorporate the coating process into their own manufacturing process — whether in-house or at a contract manufacturer.

Combined with the coating’s morphology, which packs a very large effective surface area into a small footprint, the conducting polymer-based coatings exhibit very low interfacial impedance. Along with low impedance, Amplicoat’s higher charge injection density (compared with traditionally used platinum or platinum-iridium) is able to increase safe charge delivery, which may significantly increase patient safety. These properties of Biotectix’s coatings make them ideally suited for enabling electrode miniaturisation towards the scale of individual cells for both sensing and stimulation applications. This is particularly important for creating low-profile, minimally-invasive devices and for creating high density electrode arrays for applications such as neural communication. 

For sensing devices, the coatings are designed to provide low noise, high amplitude signals, from very small electrodes. For stimulating devices, the coatings can be used to help provide extended battery life, extend safe stimulation capabilities and reduce electrode size. Furthermore, the coatings can be used either in conjunction with precious metals or with lower cost alloys for non-implant applications such as cardio mapping catheters. This could reduce the cost of manufacturing catheters and other medical devices and also opens new manufacturing methods.  

Tecticoat was developed to create comfortable, customised, high signal quality skin-contacting sensors for a variety of medical and consumer devices. This conductive polymer coating can be easily applied or patterned onto a variety of textiles, foams and other insulating materials to create soft and flexible electrodes. These electrodes fall under the category of dry electrodes and can be used to collect ECG, EEG, or EMG signals for medical or health monitoring use, as well as for electrical stimulation applications such as Transcutaneous Electrical Nerve Stimulation (TENS) for pain relief, rehabilitation, or for neural training applications.  

One of the more widely used applications for Tecticoat is for creating non-invasive cardiac monitoring electrodes. Traditional ECG electrodes primarily use silver-silver chloride gel electrodes for their low contact resistance.  A major drawback of these non-polarisable ECG electrodes is that the signal quality deteriorates as the gel inside them dries out. As a result, they cannot be used for long-term use and are considered as disposable electrodes. Other downsides of these electrodes are that they can leave residue on the skin and can cause irritation after prolonged use for several hours. Biotectix’s Tecticoat-treated ECG electrodes are aimed at circumventing these drawbacks and can be made from foam or various textiles. Due to the composition of the electrodes, they are able to provide the same signal quality and amplitude as off-the-shelf ECG electrodes, but without the use of wet gel, reducing gel residue, skin irritation and making them more suitable for long-term monitoring.

Biotectix’s conducting polymer coatings offer the possibility to improve the safety and performance of existing medical devices and to enable electrode and device size reductions by reducing impedance. These materials can also be used to create custom textile-based electrodes for consumer wearable and medical monitoring devices. Furthermore, they can help reduce device cost, reduce device size, and have the potential to open up new capabilities for medical and consumer electronic devices.