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Title: Electro-mechanically interfacing with biology using piezoelectric polymer nanostructures
Author: Smith, Michael
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 2019
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Biological cells are naturally exposed to a wealth of stimuli that influence their function and behaviour. In fields where the focus is on artificially growing biological material, such as tissue engineering and regenerative medicine, it is important to consider this array of senses to ensure correct cell signalling and tissue development. For decades, however, research has targeted only the chemical aspects of these stimuli. Mechanical and electrical signals are also fundamental in the development of our biology. Piezoelectric materials offer a promising solution to the electrical stimulation issue, and have drawn much attention recently as `active' cell culture scaffolds. However, little thought has been given to the mechanical properties of these materials and how they align with the requirements of cellular systems. Furthermore, the composition of many piezoelectric materials raises questions about biocompatibility. In this thesis, nanostructures of the piezoelectric bio-polymer poly-\textsc{l}-lactic acid (PLLA) have been fabricated, characterised and implemented in cell culture devices to investigate their potential for electromechanical stimulation of living tissue. Novel variations on the template-wetting method have been developed to create the nanostructures. PLLA nanowires were fabricated for the first time using temperature-controlled solution template-wetting. The nanowires were characterised using Differential Scanning Calorimetry (DSC), X-ray Diffraction (XRD) and Piezo-response Force Microscopy (PFM) combined with Finite Element Analysis (FEA). The results indicated that the nanowires were highly crystalline (up to 45 %) with a degree of molecular alignment, and that the nanowires displayed shear piezoelectricity with an estimated piezoelectric coefficient d₁₄ = 8 pC/N. This was the first observation and quantification of shear piezoelectricity in PLLA at the nanoscale. FEA was used to show that hollow nanotube structures would more closely align with the requirements for an active cell culture platform - namely, a greatly enhanced direct piezoelectric response compared to an equivalent wire. Therefore, melt template wetting was subsequently used to create PLLA nanotubes. Crystallisation induced by heat treatment was investigated using XRD, DSC and polarised light optical microscopy (POM). The results indicated that crystallisation in the confined nanotube environment leads to molecular alignment with the polymer chain parallel to the nanotube axis. No significant changes in crystal structure were observed between bulk PLLA and PLLA nanotubes. Various Scanning Probe Microscopy (SPM) modes were used to characterise the PLLA nanotubes at the nanoscale. PeakForce Quantitative Nanomechanical Mapping (PF-QNM) revealed the mechanical properties and lamellar structure of the polycrystalline polymer, although rigorous quantitative analysis proved challenging, as verified by FEA simulations. Kelvin Probe Force Microscopy (KPFM) highlighted the difference in surface potential between amorphous and crystalline nanotubes. PFM data also demonstrated the piezoelectric activity of both crystalline and amorphous nanotubes. The interaction between PLLA nanotubes and Human Dermal Fibroblast (HDF) cells was also investigated. Cell attachment was found to be significantly higher for nanotubes in comparison to bulk films, and a further increase in attachment was observed between amorphous and crystalline nanotubes. Electrodes embedded into the nanotube devices allowed for electrical stimulation to be applied during cell growth. Preliminary observations suggest that this stimulation improves cell attachment and/or proliferation, and the use of Aerosol Jet Printing (AJP) to pattern the electrodes can lead to directed cell growth.
Supervisor: Kar-Narayan, Sohini Sponsor: Cambridge Commonwealth ; European and International Trust
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
Keywords: piezoelectric polymers ; polymer nanostructures ; bio-piezoelectric interface