Zinc oxide nanowires integrated with flexible polymer energy harvester devices for biomedical applications

Abstract

Flexible piezoelectric energy harvester devices for biomedical applications have been developed and characterized using different encapsulation polymers and ZnO NW growth patterns. A flexible piezoelectric energy harvester device must be biocompatible, cheap, low-temperature, and scalable. The device must also produce a minimum voltage output of 1 volt for pacemakers.

This work examines the growth of hydrothermal-grown ZnO NWs in terms of length, diameter, and density under different growth conditions. Precursor concentration, substrate type, and seed layer thickness were considered. Kapton film and silicon substrates were compared to understand how surface roughness affects ZnO NW growth. The best vertically aligned ZnO NW growth occurred at 200 nm silver seed layer thickness and 40 mM precursor concentration. Synthesis took 18 hours at 90 °C.

The devices were sandwich-like nanogenerators with ZnO nanostructures embedded in a dielectric polymer matrix between top and bottom electrodes. Testing showed that the best encapsulation was Polyimide (PI) with a thickness of 7 μm and a resistance of ~24 MΩ. Electrical leakage during polarization prevented PVDF-TrFE encapsulation from working.

For ZnO NW growth, a circular seed layer with varied diameter was designed. Circular area diameter increases piezoelectric response. As the seed layer diameter increased, ZnO NW growth morphology showed an increase in small empty patches. Thus a new design with several 1 mm by 1 mm square areas patterned within a larger 15 mm by 15 mm area with varying distances between the small squares. When the squares were 0.1 mm apart, piezoelectric response (1297 mV) and sensitivity (14.75 mV/kPa) were best.

The software Coventorware10 was used to run simulations using the finite element method (FEM). The ZnO NWs height, diameter, density, substrate area, applied pressure, encapsulating polymer, and design are some of the variables that will affect the piezoelectric response and sensitivity of the device. The FEM simulations that were run help in understanding these variables.

Using collected data, the optimised device was made. Substrate (Kapton film), seed layer (200 nm Ag), polymer encapsulating ZnO NWs (PI 7 μm), and design (distance between 1 mm by 1 mm ZnO NWs pads of 0.1mm). Finally, the fabricated flexible pressure sensor was compared to FEM simulations. Trends are similar.

The findings from this research can potentially be valuable in understanding the fabrication and design of a flexible MEMS pressure sensor in biomedical applications and potentially for other applications.