# Cure State Sensing of Polymethylmethacrylate Using a Vibrating Axial Probe

## TL;DR

A new vibrating sensor measures the curing progress of PMMA, a material used in bone cement, by tracking changes in its acoustic properties.

## Contribution

A novel axial probe using a voice coil transducer and PZT microphone for real-time PMMA curing state sensing is introduced.

## Key findings

- Resonant frequency shifts up to 14 Hz indicate PMMA curing progress.
- A 5 kHz high-Q ring-down response unambiguously signals the dough-to-hard transition in PMMA.
- The sensor's frequency shift rate is 0.0462 ± 0.00624 Hz·s−1 during the dough state.

## Abstract

A new axially vibrating sensor based on an audio voice coil transducer and a lead zirconate titanate (PZT) piezoelectric disc microphone was developed as a probe for the measurement of in vitro rheological fluid properties, including curing progress for polymethylmethacrylate (PMMA) mixtures with important uses as bone cement in the field of orthopedics. The measurement of the vibrating axial sensor’s acoustic spectra in PMMA undergoing curing can be described by a damped harmonic oscillator formalism and resonant frequency (ca. 180 Hz) shift can be used as an indicator of curing progress, with shifts to the blue by as much as 14 Hz. The resonant frequency peak was measured in 19 different 4.0 g PMMA samples to have a rate of shift of 0.0462 ± 0.00624 Hz·s−1 over a period of 400 s while the PMMA was in a dough state and before the PMMA transitioned to a hard-setting phase. This transition is unambiguously indicated by this sensor technology through the generation of a distinct circa 5 kHz high-Q under-damped ring-down response.