Piezoelectric polymers are capable of interconverting mechanical and electrical energy, and are therefore candidate materials for biomedical applications such as sensors, actuators, and energy harvesters. In particular, nanowires of these materials are attractive as they can be unclamped, flexible and sensitive to small vibrations. Poly-l-lactic acid (PLLA) nanowires have been investigated for their use in biological applications, but their piezoelectric properties have never been fully characterised, even though macroscopic films and fibres have been shown to exhibit shear piezoelectricity. This piezoelectric mode is particularly interesting for in vivo applications where shear forces are especially relevant, and is similar to what has been observed in natural materials such as bone and DNA. Here, using piezo-response force microscopy (PFM), we report the first direct observation of shear piezoelectricity in highly crystalline and oriented PLLA nanowires grown by a novel template-wetting method. Our results are validated using finite-element simulations and numerical analysis, which importantly and more generally allow for accurate interpretation of PFM signals in soft nanostructured materials. Our work opens up the possibility for the development of biocompatible and sustainable piezoelectric nanogenerators and sensors based on polymer nanowires.

The choice of crosslinking is shown to enhance the piezoelectric response of a collagen construct. In particular, EDC-NHS crosslinking induces the self-assembly of collagen bundles which present a localised piezoelectric response.

It has become increasingly evident that the mechanical and electrical environment of a cell is crucial in determining its function and the subsequent behaviour of multicellular systems. Platforms through which cells can directly interface with mechanical and electrical stimuli are therefore of great interest. Piezoelectric materials are attractive in this context due to their ability to inter-convert mechanical and electrical energy, and piezoelectric nanomaterials in particular are ideal candidates for tools within mechanobiology, given their ability to both detect and apply small forces on a length scale that is compatible with cellular dimensions. The choice of piezoelectric material is crucial to ensure compatibility with cells under investigation, both in terms of stiffness and biocompatibility. Here, we show that poly-L-lactic acid nanotubes, grown by a melt-press template wetting technique, can provide a “soft” piezoelectric interface onto which human dermal fibroblasts readily attach. Interestingly, by controlling the crystallinity of the nanotubes, the level of attachment can be regulated. In this work, we provide detailed nanoscale characterization of these nanotubes to show how differences in stiffness, surface potential and piezoelectric activity of these nanotubes result in differences in cellular behaviour.

To better understand the formation mechanism of ohmic contacts to GaN-based heterostructure field effect transistors, we have compared in detail Ti/Al/Ti/Au and Ti/Al/Ti/TiN contacts. Transmission electron microscopy and electron dispersive X-ray spectroscopy revealed that following anneal, TiN islands penetrated through the AlGaN barrier, as already well known, in the gold-based ohmic contacts but not in the gold-free contacts. We hence conclude that gold facilitates the formation of the TiN islands and propose that the role of gold is extraction of gallium from the semiconductor, providing a gallium depleted region for TiN island formation. For the case of the gold-free contacts, a 8 nm thick semi continuous TiN layer was formed following 900 °C anneal. A 2 nm thick TiN layer was observed in the as deposited samples and remained intact after anneal up to 825 °C. The different ohmic contact formation mechanism of gold-based and gold-free contacts is also manifested by our finding that a discontinuous AlN nitride spacer layer between the barrier and the bulk may lead to non-uniformity in contact behavior across the wafer in the case of gold-free contacts. For gold-based contacts, ohmic contact behavior was uniform across the wafer.

Mechanical and piezoelectric properties of Nylon-11 nanowires strongly depend on their crystal structure, which can be precisely controlled by template-assisted growth methods.