Soft electronics are gathering a lot of interest in the medical sector. The inherent ‘soft’ nature of these electronic systems means that they can often withstand various mechanical deformations, making them an interesting prospect for a number of medical applications where flexibility and mechanical strength are key properties of an electronic system—such as in healthcare monitoring and medical implant applications.

Despite the potential for soft electronics in both monitoring and in-vivo applications, many of the systems have so far had some reliability issues—some of which have caused system failure. For many commercial and/or industrial applications, device failure is always something of concern, but ensuring that an electronic device is running optimally and safely is of utmost importance in the medical space—especially in applications with the potential for the device to be inserted inside a patient.

Therefore, ensuring soft electronic devices can be made more reliable for medical applications is one of the key focus areas in the soft electronics space and progress is being made to improve these systems.

The Failure of Many Soft Electronic Systems

The effective function and reliability of soft electronic systems rely heavily on the electrodes and interconnects to connect the many components together and interface the different components to the biological tissues. The different interconnects are expected to interface with, and conformally attach to, a range of irregular and non-flat surfaces and maintain a high level of stability when these interfaces undergo some level of mechanical deformation. However, it is in these interfacial areas where there are currently some issues.

As it stands, a number of different soft materials and flexible structures have been used to create reliable electrodes that can measure physiological signals from the body in real-time. These materials include hydrogels, carbon composites, liquid metal composites, metal composites, and conductive polymers. However, despite being able to make effective components, the natural biological environment, with its soft biological tissues (such as skin and muscles), presents a challenge to many of these established systems.

The core of the issues at the device-tissue interface comes down to a poor adhesion strength and mechanical mismatch between the device components and the biological tissue. This mechanical mismatch arises because many of the interfaces are irregular. This mismatch leads to increased noise in the detection signal and an overall decrease in the sensitivity when measuring physiological signals. In some cases, beyond measurement impairment, the mechanical mismatch at the sensing/monitoring interfaces can also cause complete device failure, so more work needs to be done in this area before the conversation of using these types of soft electronics in widespread clinical applications can take place.

The Potential for PEDOT:PSS in Soft Electronics

Poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate), commonly known and referred to as PEDOT:PSS, is a widely used and versatile polymer material that has already found use in different medical applications. PEDOT:PSS is a commercially available conductive polymer with a number of beneficial properties that make it suitable for clinical applications—such as a good solution processability, tuneable electronic properties, and most importantly, a good biocompatibility.

However, on their own, PEDOT:PSS films tend to be highly rigid and very inflexible, as well as possess a poor adhesion strength—something that is not particularly ideal when it comes to creating soft and flexible electronic systems. Because PEDOT:PSS has a range of other properties that could make it suitable for the functioning on the measurement side of these devices, several routes have been attempted to make PEDOT:PSS more flexible and stretchy.

These routes include doping approaches, polymer blending methods, the creation of hydrogel systems, and the creation of polymer composites. While hydrogels are the most flexible, PEDOT:PSS composites currently show the most promise because of their conductivity properties. While progress has been made, the rigidity and mechanical modulus of these composites are still higher than human tissues, so their conformability is still limited. Another aspect that needs to be considered when using some polymer systems is the potential for plastic deformation, as it’s possible that these types of devices will not return to their natural state if they are stretched too much. Researchers continue to look into these systems to see how they can be improved and made more suitable for clinical applications.

Creating a Polymer Composite for Soft Electronics

A new approach has been recently realised to tackle the inflexibility and rigidity in many PEDOT:PSS composites by doping the composites with biocompatible supramolecular solvents. This has led to new composites being created, which have been referred to as self-adhesive conductive polymer (SACP) composites.

The researchers created these new PEDOT:PSS-based composites by taking a mix of supramolecular solvents (such as citric acid and cyclodextrin) and mixing them with PEDOT:PSS and an elastic polymer network (made of PVA and glutaraldehyde). Compared to predecessor PEDOT:PSS composites, this SACP was much more flexible and less rigid—with a low mechanical modulus, low residual strain and a high stretchability (up to 700%). These flexibility and mechanical properties also made the composite have a much higher interface adhesion strength compared to its predecessors while keeping the high conductivity properties that made PEDOT:PSS become a material of interest for soft electronics.

After understanding the improved potential for the SACP over other composites, SACP electrodes were created for a range of soft electronic devices, including alternating current electroluminescent devices (ACEL) and electromyography (EMG) monitoring, and an integrated bioelectronic system (using EMG sensors and an array of ACEL devices) for the visualisation of electromyography signals during muscle training.

Progress is currently being made in this area to make soft electronic devices more compatible and suitable for medical monitoring applications, especially over long periods. Several solutions are being sought, and recent research has made a breakthrough into making PEDOT:PSS composites more suitable from a mechanical perspective and offers a way to combat the interfacial issues of many soft electronic systems. While it is still in the early stages, the SACP-based electronics show some promising features for wearable and comfortable bioelectronic devices that can measure the physiological electric signals of the human body during daily activities.

Reference:

Zhou X. et al, Solution-processable, soft, self-adhesive, and conductive polymer composites for soft electronics, Nature Communications¸ 13, (2022), 358.