Microsensor for Intramuscular Pressure Measurement

Principal Investigator: Kenton R. Kaufman, Ph.D.
Project Coordinator: Duane Morrow — morrow.duane@mayo.edu

Currently, the integrated electromyogram (EMG) is the standard used as an indirect indicator of the timing and intensity of muscle contraction. However, the relationship between EMG and muscle tension is unclear. There is a need for a reliable measure of muscle tension under dynamic conditions. This need may be filled through the measurement of intramuscular pressure (IMP). The overall objective of this project is to provide a useful clinical tool for in-vivo quantification of muscle force. The specific aims of this proposed project will be to further develop a fiber optic microsensor for monitoring intramuscular pressure and validate this technology by animal testing, theoretical modeling, and in-vivo evaluation of research subjects and patients with muscle disorders.

Figure 1: Intramuscular pressure sensor.

The Mayo Clinic/Luna Innovations team has developed a prototype optical fiber micropressure sensor for in-vivo IMP measurement (Fig. 1). The diameter of the sensor is 360μm. A patent application has been filed regarding this design and has been evaluated and approved for biocompatibility, according to ISO Standard 10993-6.

Performance characteristics of the microsensors in response to a step input ranging between 0 and 250 mmHg have been previously evaluated in a calibration chamber. This study found the microsensors to have an accuracy, repeatability, and linearity better than 2% full-scale output (FSO) and hysteresis slightly higher than 4.5% FSO. Additional studies analyzing the dynamic performance of the sensors in response to harmonic inputs from 0.5 to 10 Hz have been conducted. The study revealed a flat frequency response to 6 Hz, with a slight but steady increase between 6 and 10 Hz. The study also noted temperature sensitivity.

Work continues in the enhancement of a mechanical muscle model for simulating muscle mechanics based on the finite element method. This model uses nonlinear continuum mechanics to study the contractile active and passive properties of skeletal muscle in the prediction of intramuscular pressure.

In the past year, progress has been made to further examine the performance characteristics of the microsensor. We have also continued modify components of the sensor design in order to perfect its use in in vivo studies.

Figure 2: Simultaneous recording of rabbit TA muscle force (magenta), intramuscular pressure (IMP, turquoise) and transducer movement (dark blue) during the shortening, lengthening shortening protocol.

While previous studies examining the relationship between intramuscular pressure and active and passive muscle tension in the isolated tibialis anterior (TA) of a New Zealand white rabbit indicate that IMP measurement provides a fairly accurate index of relative muscle tension. It was noted that transducer movement was witnessed during some experiments, causing discrepancies in the results. Transducer movement during the combined contractions was substantial, variable and repeatable. Real-time plots of tip movement relative to the muscle compared to both force and pressure data revealed an interesting pattern of movement that correlated with the type of contraction that the muscle experienced (Fig. 2). It can be seen in this example that the tip movement is very significant as the muscle is shortening and both force and pressure are dropping. Then, when force is redeveloped and the tip is still moving, pressure and force are uncorrelated.

Figure 3: Machined stainless steel tubes for improved mechanical attachment.

Figure 4: Average Representation of pull-out tests for Channel, V-cut, and Sawtooth micromachined housings.

Table 1: Results of Pull-Out Strength Tests.

A series of pull-out tests were performed to evaluate the mechanical holding strength of micromachined housings shown in Figure 3. Insertion was performed using a hypodermic needle to mimic sensor use in test situations. Representative averaged results are shown in Figure 4. Since we are interested in maintaining a constant sensor position in muscle tissue, failure points were defined as the first relative maxima reached in the tests. Using this criteria, we determined both the force (g) at failure, as well as the pull-out strength (g/sec), calculated from the slope of the force-time curve (Table 1). Based on these factors, it is clear that the channel-micromachined housings will provide the most secure anchoring. Sensors are now being manufactured according to this recommendation.