Investigating the influence of the viscoelastic material as a heart muscle simulator on the powering leadless pacemaker from heartbeats by using a piezoelectric beam

This paper studies experimentally and analytically the influence of the viscoelastic cardiac muscle on the energy harvesting from heartbeats for powering the leadless pacemakers by using a piezoelectric beam. An appropriate representative gel-type viscoelastic material that mimics the heart tissue is used in the tests. The piezoelectric beam coupled with a gel-type material is analytically modeled, and experimentally tested. By considering a combination of the translational standard linear solid model and a rotational spring component for the gel-type material, the analytical model of the coupled system is developed utilizing the generalized Hamilton’s principle. The system is attached on top of a shaker and excited harmonically, and the time history of output voltage and accelerations are measured. The analytical model is verified by experimental results for the tip displacement, voltage, generated power, phase-portraits histories, and voltage FRF with harmonic base excitations. Transmissibility analysis by the analytical model shows that for excitation frequencies beyond a specific frequency, the viscoelastic material can magnify the amplitude of the excitation and incredibly improve the power generation. Experimental results demonstrate that by coupling the gel into the harvester, oscillations of the tip are increased into high energy orbits and large tip deflections around the resonance frequency. The significantly widened frequency bandwidth and the increased power output at specific input frequencies are the other results of considering the viscoelastic characteristics of the heart wall in the dynamic investigations. By simulating the response of the energy harvesting system to the heartbeat impulsive rhythm, when the energy harvesting system is attached to the viscoelastic material, the output power is increased from 18 to 55 µW. The obtained results reveal that influence of the viscoelastic properties of the heart muscle is crucial in the accurate design of the energy harvesting system for the self-powered medical leadless pacemaker.

Identifying the parameters of viscoelastic model for a gel-type material as representative of cardiac muscle in dynamic tests

In this paper, mechanical parameters of a calf heart muscle are identified and a gel-type material as the representative of the cardiac muscle in dynamic tests is introduced. The motivation of this study is to introduce a replacement material of the heart muscle to use in experimental studies of the leadless pacemaker. A particular test setup is developed to capture the experimental data based on the stress relaxation test method where its outputs are time histories of the force and displacement. The standard linear solid model is used for mathematical modeling of the heart muscle sample and a gel-type material specimen namely α-gel. Five tests with different strain history (13.6%,17.1%,20.6%22.4%and,23.8%) are performed by regarding and disregarding the influence of the initial ramp of the loading. The mechanical parameters of the standard linear solid model were identified with precise curve fitting. Consideration of the initial ramp significantly influences the consequences and they are so close to their experimental counterparts. The identified parameters of the standard linear solid model by regarding the influence of the initial ramp for the gel-type material are within an acceptable range for the viscoelastic properties of the calf heart tissue. These results show that the gel-type material has the potential to represent the cardiac muscle in the leadless pacemaker experimental studies. Dynamic mechanical analysis is used to characterize the dynamic viscoelastic properties for the gel by utilizing the identified parameters with taking into account the initial ramp in the frequency domain. Results show that Storage modulus, Loss modulus, and Loss tangent are strongly frequency-dependent especially at low-frequency around the heartbeat frequency range (0-2 Hz).

Experimental and numerical study on the performance of printed alginate/hyaluronic acid/halloysite nanotube/polyvinylidene fluoride bio-scaffolds

The growing usage of printed bio scaffolds in the field of regenerative medicine has made this field very important in biomedical engineering. In this regard, three-dimensional printing (3D) technique needs bio-materials with higher mechanical and biological performance. The biomaterials with high mechanical performance beside its bio compatibility are limited. A novel bio-material made of Alginate, Hyaluronic acid, Halloysite Nanotube and Polyvinylidene Fluoride was used and characterized for printing cartilage bio scaffolds through numerical studies. CaCl2 was used for crosslinking of biomaterial. Scanning Electron Microscopy, mechanical tests (tensile and compressive test), MTT assay were conducted for evaluating this novel biomaterial. Different structures of bio material were simulated for numerical studies. The numerical study was performed in ANSYS 18 using three parameter Mooney-Rivlin model. According to experimental and numerical results, Halloysite Nanotube increases the tensile and compressive strength of biomaterial up to 47%. Results show that biomaterial have good mechanical performance due to mechanical forces required for cartilage bio scaffolds besides its high biological performance. Polyvinylidene fluoride reduces the mechanical performance while increasing the cell viability. MTT assay results performed on day 0, day 2 and day 6 show increase in cell number to be about twice for biomaterial containing 40 mg/ml alginate, 40 mg/ml halloysite nanotube, 10 mg/ml hyaluronic acid and 1 w/v Polyvinylidene fluoride. Numerical simulation shows high mechanical performance of bio material in different scaffolds structure. The best structure of bio scaffolds was achieved with 0.4 mm nozzle diameter and 0.4 space between rows.