Mechanical Coupler for Reliable Muscle Contraction Sensing

Muscle contraction is associated with volumetric and stiffness changes, which exert radial forces (or pressures). A force-sensitive resistor (FSR) placed on a patient’s skin in correspondence with a muscle belly was used to sense contraction. Generally, FSRs consist of a conductive polymer, which changes its resistance when a force is applied to its surface. They can be made small and very thin (e.g., less than 0.5 mm), offer good shock resistance, can operate in moderately hostile environments, and are low-cost. However, there should only be concentrated and uniformly distributed force within the FSR active (or sensing) area for reliable use of the FSR. The assembling of the Interlink FSR [38 ] includes perimetral spacers that separate the two membranes holding the metallic contacts and the conductive polymer. A direct application of the FSR on skin to sense muscle contraction proved to be quite unsatisfactory. The mere sensor, without any mechanical coupler, provided uncertain and unreliable results. Contact with the patient’s skin was unstable and uncertain—the perimetral spacers of the FSR sensor transmits part of the applied force directly to the back of the sensor without involving the sensing area, and prevents the membrane with electrical contacts from properly flexing onto the resistive polymer layer. A specific mechanical coupler was designed in response to these drawbacks (see Figure 1). A rigid spherical cap, made of acrylic resin, provides advantageous force transmission to FSR. The spherical cap base was glued onto the FSR’s sensitive area (leaving out the perimetral spacers) and its convex part was made to face the patient’s skin. When the sensor is applied onto the patient, the dome creates a little subsidence that gently but firmly attaches to the skin. Furthermore, a flat, rigid sheet of plastic was attached to the back of the sensor to prevent improper bending. The elastic modulus of both the spherical cap and the back support were much higher than that of skin and muscle. The mechanical coupler provides a much more convenient and reliable muscle force transmission to FSR.
The assembly of the FSR and the mechanical coupler can be held in place onto a patient’s skin by a belt or other fastening methods (e.g., scotch tape). The increase of muscular transverse section during contraction, as well as the resultant skin stretching, impresses uniform pressure on the FSR active area via the rigid spherical cup. Furthermore, the small mechanical vibrations generated during muscle contraction (i.e., the mechanomyography MMG signal) are suitably transmitted to the FSR sensor.