Monitor capsule formation around soft tissue

Piezoelectric sensors have been shown to respond reproducibly to changes in tissue mechanical properties surrounding an implant over a four-month period. A statistically significant change in vibrational amplitude at a frequency corresponding to the radial resonance is seen over time. The significant reduction in amplitude implies the initial period of inflammation, which indicates an increase in viscous dissipation of the tissue. As collagen effects the cellular response, the amplitude starts decreasing. Finally, as the tissue matures, the capsule becomes stiffer, and the viscous dissipation lessens. These results are consistent with qualitative assessments of explanted capsules.

Piezoelectric materials show piezoelectric effect. The piezoelectric effect is a reversible process in that materials exhibiting the direct piezoelectric effect (the internal generation of electrical charge resulting from an applied mechanical force) also exhibit the reverse piezoelectric effect (the internal generation of a mechanical strain resulting from an applied electrical field) [12]. When AC electric field is applied, devices such as the piezoelectric wafer active sensors (PWAS) are oscillated over a spectrum of frequencies[13]. The devices will resonate at a certain frequency and will have a high amplitude of motion. They can be made to possess characteristic motions (resonances) at specific frequencies, as there is a control over the structure of the piezomaterial. The sensor expands at a lower frequency resonance and contracts radially at a higher frequency thickness vibration. The amplitude of vibration changes over the frequency domain when the piezosensor is mechanically coupled with the medium, such as attachment to a rigid object or implantation in soft tissue, and the resonating frequencies of the sensor change as well. Thus both the phenomena constitute an electromechanical impedance shift, which is measured by an impedance analyzer. The impedance changes as the mechanical properties of the tissue surrounding the sensor change. Shear waves are generated when PWAS starts oscillating. And it propagates away from the surface in an exponentially decaying magnitude on the viscosity of the medium and the frequency of oscillation. The viscosity of the medium and the frequency of oscillation are the main factors affecting the rate of decay. For these sensors the interaction depth is calculated to be of order 50 pm at 100 kHz, assuming the viscosity of water. Thus the sensitivity of PWAS is much higher with respect to the variations in mechanical properties of materials to this distance. The amplitude of the radial resonance changes with the change in the viscoelasticity of the tissue surrounding the implant. As the tissue becomes more viscous, it dissipates more of the energy of the PWAS, leading to a damp PWAS amplitude of vibration. In this sense, the PWAS can quantify the relative viscous nature of the tissue during the process of wound healing.

Strain gauges

Strain gauges are complementary to the PWAS sensor, where the contractile force generated by the surrounding tissue is directly measured. The change in resistance of a thin metallic circuit is seen considerably as the circuit is compressed, expanded, or bent and is directly related to the compressive or bending force [14].

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