University of Cambridge
The effect of nanostructuring and crystallinity on triboelectric energy harvesting devices for biomedical applications
Abstract
dc:description.abstractTriboelectric energy harvesting devices are an excellent choice for removing the reliance on batteries in the medical space. This is due to the wide range in possible material choice, conformability, and excellent ability to harvest energy at the low frequencies (0–10 Hz) present from human motion. For internally implanted devices, such as pacemakers, surgery is required to replace a depleted battery. This is a significant undertaking for a population that is likely already at a higher risk level. In the case of externally powered devices, the batteries must be removed, recharged, and then replaced. Triboelectric devices can be used to harvest energy from body movement to power wearable medical devices, or alter-natively, high frequency ultrasound vibrations can be used to wirelessly transmit energy to implanted devices. Triboelectric generators rely on contact-generated surface charge transfer between materials with different electron affinities, converting mechanical energy into useful electricity. Two of the main hurdles to consider before replacing batteries with triboelectric generators (TEGs) are power output and biocompatibility. The performance of a TEG is dependent on the maximum surface charge of the material pair used. Nanostructuring the material surfaces increases the surface area and therefore charge density, increasing the power output. Taking biocompatibility and relative charge affinity into consideration, poly-L-lactic acid (PLLA) was chosen in this work as the tribopositive material, and polytetrafluroethylene (PTFE) and poly(vinylidene fluoride-co-trifluoroethylene) (P(VDF-TrFE)) as tribonegative materials. These materials were all nanostructured using a meltpress template-wetting method, using an anodised aluminium oxide (AAO) membrane as the template. This method was carried out on a hot plate which allowed fine control of heating times and temperatures to control the nanostructure and crystallinity of the resultant material. Devices using combinations of PLLA vs. PTFE and PLLA vs. P(VDF-TrFE) were investigated as biocompatible materials to use in TEGs to power medical devices. Characterisation of the materials used was carried out through atomic force microscopy (AFM), differential scanning calorimetry (DSC), and X-ray diffraction (XRD) techniques to determine how material properties relate to the output of the TEGs in different modes and at different frequencies presented in this work. PLLA and P(VDF-TrFE) are both piezoelectric materials which experience the added effect of molecular alignment during nanostructuring, improving their piezoelectric properties and surface polarisation from additional dipole alignment. Different crystallinities of PLLA nanostructures and films were tested against nanostructures and films of PTFE in vertical contact modes at 2 Hz to simulate common body motion frequencies. It was found that the highest crystallinities of PLLA structures produced the highest output, likely due to a combination of the triboelectric and piezoelectric effect. The highest crystallinities of PLLA were tested in vertical contact mode and horizontal sliding mode against nanostructures and films of PTFE and P(VDF-TrFE). Horizontal sliding mode simulates the shear forces that are common in body motion such as clothes rubbing against skin. It was found for both material combinations that film-based devices showed a decrease in output when actuated in sliding mode compared to tapping mode. On the other hand, fully nanostructured devices showed an increase in output power for the same change, showing their suitability for this mode. The nanostructured and film-based PTFE with PLLA devices were also tested at ultrasound (US) frequencies in vertical contact mode to characterise the power output that could be transferred using US to an implanted device. The change in output at different frequencies demonstrates how important finding the resonant frequency of the device is to achieving the highest output. This work opens the avenue for the use of triboelectrics as battery replacements for powering implanted and externally worn medical devices. It also shows how critical crystallinity and topology are for overall device performance, especially when considering high compliance triboelectric surfaces such as polymers.
Degree
thesis:*- Name dc:type.qualificationname
- Doctor of Philosophy (PhD)
- Level dc:type.qualificationlevel
- Doctoral
- Grantor dc:publisher.institution
- University of Cambridge
- Year dc:date.issued
- 2024
Author and committee
dc:creator, dc:contributor.*- Author dc:creator
-
- Margaronis, Kalliope
- Advisor dc:contributor.advisor
-
- Kar-Narayan, Sohini
Subjects
dc:subject × 9Rights
dc:rightsIdentifiers
dc:identifier.*- DOI dc:identifier.doi
- https://doi.org/10.17863/CAM.118149
- OAI identifier oai:identifier
- oai:www.repository.cam.ac.uk:1810/383986