Physically transient medical devices are temporary devices that are designed to be functional for a set period and later dissolve in vivo, yielding biocompatible end-products. Heart strain sensors are one technology that can particularly benefit from a physical transit embodiment. These sensors are often used during surgeries to help doctors monitor the heart, but they can also be used on an outpatient basis to monitor the heart over a longer period, allowing doctors to detect any potential problems early on. Traditionally, the microelectromechanical systems (MEMS) used for heart strain sensing are implanted permanently, as removing them is not widely recommended due to the high risk of infection this carries.

A physically transient heart strain sensor would offer patients the flexibility of an impermanent device and physicians the dependability of a precise monitoring system. During the summer of 2020, I worked with Quansan Yang under Prof. John Rogers at the Querrey Simpson Institute for Bioelectronics to realize such a device. I assisted with the design and fabrication of parts of our system, in addition to characterizing a novel fabrication strategy for making MEMS.

I started by using eco/bioresorbable materials to construct insulated induction coils. I tested various types of metal wire, made of tungsten, magnesium, and zinc, to see which were the most conducive for forming induction coils. I ended up finding that zinc wire was the best for making induction coils due to its low yield strength since I could easily make it plastically deform by hand. I deformed the wire by hand onto a glass slide with double-sided Scotch Tape on it, which kept it in place and allowed me to form a spiral shape.

Once coils were constructed, their signal quality factors (Q-factors) were characterized with a network analyzer. If the measured Q factors, were over 50, I deemed the coils adequate, and if not, I would adjust their shape to better approximate a spiral using tweezers and try again. Through this iterative process, I was able to make coils with differing numbers of turns, corresponding to unique resonant frequencies.

I then needed to encapsulate the coils with a bioresorbable polymer, polylactic acid (PLA). To accomplish this, I used small amounts of chloroform applied with an absorbent applicator to locally dissolve the surface of PLA film, which was applied atop the coil on the glass slide, and heat pressed to evaporate all of the chloroform. This causes the PLA to form and adhere to the metal wire. After this, the PLA can be pulled from the glass slide, and, with some luck, take the metal coil with it. Finally, this process can be repeated with another piece of PLA to create a fully encapsulated coil.

Three coils, with 4, 6, and 9 turns respectively, were placed in a triangular configuration, and their resonant frequencies were measured with a network analyzer.

Sharp peaks when sweeping AC frequencies indicate good signal quality factors (Q-factors).

Next, the coils were encapsulated all together with PLA using the same solvent-bonding/ heat pressing method as before.