Making conductive gels stick for biomedical devices

Materials World magazine
24 Apr 2020

Idha Valeur explores how conductive gels can be made to stick to biomedical sensors or microchips even when wet. 

Making conductive polymer gels stick to surfaces while in the human body is the focus of research at the Massachusetts Institute of Technology (MIT), USA. 

Conductive polymer gels improve the electrical and mechanical properties of biomedical devices, enhancing the interface with human tissues. However, a sticking problem is that when they are exposed to moisture from the body, they do not adhere well, limiting the device’s stability in the long term. Improved adhesion would lead to fewer replacement surgeries and subsequently less trauma for patients. 

MIT researchers solved the conundrum by creating a coating between the polymer and the substrate material, whether that be a sensor or other biomedical device. This enables strong adhesion ‘by allowing diffusion and dense interpenetration of the conducting polymer into the nanoscale coating layer while tightly bonded to the device substrate – forming a sort of bridging interfacial layer’, explains MIT PhD student Hyunwoo Yuk.

The adhesive coating layer is made up of a hydrophilic polyurethane – a block copolymer of hydrophilic and hydrophobic segment, which offers several benefits. ‘First, its ability to imbibe water allows efficient diffusion and interpenetration of conducting polymers into the adhesive layer to give strong mechanical integration, by polymer network-level interlocking, as well as good electrical properties after forming interpenetration,’ Yuk says. 

‘Second, it can be dissolved in a benign ethanol-based solution, which can be applied into various device substrates based on common manufacturing techniques such as spin-coating and dip-coating
‘Third, polyurethanes are physically crosslinked elastomers that can readily restore stable and robust mechanical property after coating and evaporation of a solvent. Also, polyurethane has excellent adhesive properties to most device materials,’ he adds.

The adhesion strength is promising. Yuk states that even when the conducting polymer undergoes bulk failure the adhesive does not peel off the substrate ‘which indicates very strong adhesion beyond the strength of the polymer itself.
‘To quantify the adhesion strength, we adopt the standard lap shear test method to measure shear strength. We found that our adhesion method provides over 120kPa shear strength between the wet conducting polymer, PEDOT:PSS as representative example, and various substrates [including] gold, glass, indium tin oxide, polyimide, etc., while wet, conducting polymers without adhesion layer showed very low shear strength below 5kPa,’ Yuk says.  

The bonding’s stability has been tested in a simulated physiological environment for a year, using a phosphate-buffered saline as it is the most common cell culture media that has similar salinity to body fluids. The researchers believe that their approach removes the need for heavy modification of either the device or the polymer to function in wet conditions. According to Yuk, doing this would be expensive, difficult, slow, challenging to scale up and only applicable for very specific materials, ‘significantly limiting their utility and translation into practical applications and industries’. 

The team is now moving onto further testing in realistic conditions with a longer time scale, including more electrical inputs similar to current device operations.