Flexure Design for Muscle Tissue Measurement
My MS thesis: a variable-stiffness flexure with custom electrolessly plated strain gauges for measuring the contraction force of engineered muscle tissue.
Motivation
Muscle tissue diseases affect millions, but drug trials are costly, invasive, and slow. Current methods also can't directly measure the force a muscle produces, even though impaired force output is the central symptom of these diseases. Lab-grown (hiPSC-derived) muscle tissues small enough to fit in a standard 24-well plate offer a scalable alternative for preclinical testing, and are also being explored as biological actuators for soft robotics. Both applications need the same missing tool: a precise, scalable way to measure muscle contraction directly.
For my Master's thesis in the Culpepper Lab (co-advised by the Raman Lab), I designed, built, and tested a compact device that does exactly that.
The Device
The heart of the device is a compliant flexure that fits inside a single well of a standard 24-well plate. It combines three functions:
- Variable stiffness — a latching variable boundary condition lets one flexure switch between a soft state for muscle growth and a stiff state for contraction testing, shifting the effective stiffness from 0.38 ± 0.04 N/m to 3.85 ± 0.11 N/m without ever moving the muscle.
- Motorized tensioning — a stepper-driven tensioning arm applies controlled pre-strain from 0–20%, removing slack and standardizing the muscle's starting condition.
- Integrated sensing — custom strain gauges plated directly onto the flexure measure contractions as small as 9.4 µm, an analog alternative to camera-based tracking.
I used a pseudo-rigid-body model (PRBM) to guide the beam dimensions, boundary conditions, and latch geometry.
Plating Strain Gauges onto Plastic
The most novel (and most stubborn) part of this work was fabricating strain gauges by electrolessly plating nickel directly onto a soft LDPE flexure. Metal and soft plastic don't like each other: mismatched thermal expansion during the hot plating bath causes residual stress and cracking. Through fabrication studies, I mapped how temperature gradients, part size, metal thickness, and process sequence govern plating quality — practical design rules for putting metal sensors on soft substrates. Stress-relief conditioning and smaller part dimensions dramatically improved results, and early tests suggest lower-temperature copper plating is a promising, more ductile alternative to nickel.
Preparation & Plating of an LDPE part
Manufacturing of LDPE Sensing Flexure
Results
The assembled device successfully tested live engineered muscle: it held two reliable stiffness states, applied controlled tension, and the plated gauges resolved micron-scale dynamic contractions during optical stimulation at 2 Hz. Together, this establishes a foundation for scalable, automated platforms for drug screening and biohybrid actuator research and a set of design principles for anyone plating sensors onto compliant plastic parts.
