Fully Degradable, Soft and Biocompatible Tungsten/Beeswax Conductive Interconnection for Implantable Bioelectronics

In biomedical engineering, the need for biodegradable electronic devices that dissolve in the body has emerged to solve the problems of risk-taking removal surgery. Recent advances in biomaterials enable the devices to operate and dissolve in the body within a designed period. In addition, different applications of transient electronics, such as biodegradable sensors and actuators, can also decrease electronic wastes.<br/>The main circuit components of transient electronics are biodegradable metal films, inorganic materials, and conductive interconnection. Also, the stable interconnection of each part is crucial for the desired lifespan of devices. However, the unstable connection due to a mismatch of mechanical properties between the soft components and rigid interconnection is the main reason for shortening the lifetime of the devices.<br/>Thus, there are several approaches to fabricating soft and conductive interface materials. So far, candelilla wax (CW) has been used as the matrix for conductive composite. Although this composite has excellent electrical properties, high stiffness causes breakage of connection even under weak impact. Other studies using carbon nanotube (CNT) or metal particles mixed with soft polymer matrix report good flexibility. However, the conductivity is still low compared to commercial solder or silver paste for the actual application. Additionally, the use of organic solvent in the manufacturing process has the risk of toxicity. It is required for better mechanical properties, conductivity, biocompatibility, and convenience of use than existing ones to overcome these problems.<br/>Therefore, this study introduces the (1) fully biodegradable, biocompatible, and soft interconnection composed of tungsten (particle size: 0.5~3 μm) and refined beeswax with a lubricant (tetraglycol) additive. This interconnection also secures (2) high electrical performance and convenience as commercial silver paste and lead solder.<br/>This soft interconnection has a conductivity of up to 7 [kS/m] and resistivity of about 2 [μΩ cm] at the optimized volume fraction. Depending on the particle size, we can tailor the electrical performance. In addition, the conductivity can increase even more by 10-20% with the addition of tetraglycol due to the effect of homogeneous dispersion. The results of the three-point bending stress test represent 50% reduced flexural modulus compared to candelilla wax. With tetraglycol, it is reduced by 90%, which can be applied to flexible devices and shape memory devices. During two weeks of degradation test in phosphate-buffered saline (PBS) at physiological temperature (37°C), the interconnection maintained stable conductivity and dissolved entirely after one month.<br/>Therefore, this bio-interconnection is entirely biodegradable, soft, and has excellent electrical properties, so it can be used in various implantable medical devices to be developed in the future. It will ensure their electrical, physical safety, and performance. It can also reduce electronic waste and address environmental issues.