Researcher(s)
- David Kennedy, Electrical Engineering, University of Delaware
Faculty Mentor(s)
- Mark Mirotznik, Electrical and Computer Engineering, University of Delaware
- Michael Richards, Electrical and Computer Engineering, University of Delaware
Abstract
When fitting a prosthetic to a patient’s limb, the process is subjective, based on the patient’s reports of discomfort and visual inspection of pressure marks on the residual limb after fitting. Current methods rely on flexible sleeves that fit over the residual limb and tend to have significant measurement error. A rigid device created through thermo-forming has the potential to be more accurate by maintaining a constant reference of sensor locations.
This work aimed to identify an electrically conductive ink compatible with thermo-forming, identify a substrate compatible with both the process and the ink, and develop a scalable process for manufacturing functioning circuits on doubly curved surfaces.
To explore this potential, a polar grid pattern was printed onto a polymer substrate using an nScrypt 3Dn-300 and Smart Pump, then thermo-formed on a male hemispherical mold using a Formech 450DT vacuum former. The crack pattern showed that strain during forming was imparted radially rather than concentrically around the center. Initial trials using uncured ink on high-impact polystyrene (HIPS) caused significant cracking; curing beforehand reduced cracking but revealed the ink’s curing temperature (80°–140°C) overlapped with HIPS’s stress-relief threshold (85°–95°C), deforming the substrate. Polycarbonate, with a higher stress-relief threshold (~135°C), resolved this issue.
Using this process, a three-LED button-pair circuit, designed to mimic the function of a pressure-sensing sleeve, was thermoformed onto a female mold generated from the OpenLimbTT anatomical dataset to replicate the geometry of a residual limb’s tip. The final product demonstrates that thermo-forming is a viable process for manufacturing custom conformal circuits for prosthetic applications, successfully producing a circuit on a doubly curved surface capable of delivering enough current to power three LEDs simultaneously, directly addressing the low measurement accuracy and reliance on subjective observation that limit current prosthetic-fitting methods.