Additive Manufacturing of Transient Metal for Bioresorbable Sensing Implants

Transient electronics have increasingly elicited interest in recent years, due to their potential to reduce electronic waste by enabling devices to degrade in the environment, and the possibility for novel biomedical implants which eliminate the need for re-operation. In this work, we focus on the latter, also called “bioresorbable electronics”, devices fabricated with materials that are naturally degraded into benign byproducts in the body. However, due to their reactivity, these materials are often not compatible with processes that involve high temperature or harsh chemicals, making them challenging to pattern with conventional semi-conductor fabrication methods. Printing techniques show great promise to address this challenge and could pave the road to personalized implants based on digital additive manufacturing. However, they require joint optimization of the ink formulation, the printing conditions and the post-treatment. In this work, we present the development of technological building blocks that enable the facile manufacturing of personalized transient electronic implants, which allow to record physiological signals and degrade in bodily fluids. To this aim, methods based on solution processes were developed in order to manufacture and integrate substrate, dielectric and conductive layers into functional devices. In particular, a bioresorbable metal ink based on zinc microparticles was formulated. To create a conductive metal layer, a hybrid sintering approach that is compatible with degradable substrates is introduced. The metal layer is combined with solution processible bioresorbable polymers to obtain various device architectures.<br/> <br/>A range of substrates was assessed for the fabrication of transient devices, and while the aforementioned process was shown to be versatile, polylactic acid (PLA) was chosen for further experiments, as its lifetime is compatible with envisioned clinical applications of bioresorbable electronics. Various additive manufacturing methods (stencil printing, screen printing and direct ink writing) were assessed for the patterning of the zinc metal layer on the substrate. After deposition, the metal microparticles layer does not conduct electricity due to the native oxide layer that is present. The zinc layer was thus chemically reduced with an acidic solution to temporarily remove the oxide layer, and subsequently treated with a low-temperature process that ensures particle sintering and conductivity in the metallic layer. The electrical conductivity, morphology and stability of the metal traces were characterized as a function of the process parameters. This hybrid method was found to yield higher conductivity than state of the art approaches for printed transient metal, while offering advantages in terms of scalability and compatibility with temperature-sensitive substrates. The physical and electrical degradation dynamics of the conductive traces were assessed in phosphate-buffered solution at 37°C. This fabrication method was leveraged to fabricate flexible piezoresistive temperature and strain sensors, based on zinc resistors patterned on PLA. Wireless capacitive pressure sensors based on a RLC circuit were also fabricated. For the dielectric layer of the parallel plate capacitor, a soft photocurable bioresorbable elastomer, poly(octamethylene maleate (anhydride) citrate) (POMaC), was used. It was synthesized as previously described and polymerized by exposure to UV light and subsequent thermal curing. The sensors were characterized within the relevant physiological ranges. These results demonstrate the development of a technological platform allowing the additive manufacturing of fully bioresorbable, functional and flexible bioelectronics. This represents a promising step towards personalized 3D transient implants for biomedical applications.