Self-assembly of collagen bundles and enhanced piezoelectricity induced by chemical crosslinking.

The piezoelectricity of collagen is purported to be linked to many biological processes including bone formation and wound healing. Although the piezoelectricity of tissue-derived collagen has been documented across the length scales, little work has been undertaken to characterise the local electromechanical properties of processed collagen, which is used as a base for tissue-engineering implants. In this work, three chemically distinct treatments used to form structurally and mechanically stable scaffolds-EDC-NHS, genipin and tissue transglutaminase-are investigated for their effect on collagen piezolectricity. Crosslinking with EDC-NHS is noted to produce a distinct self-assembly of the fibres into bundles roughly 300 nm in width regardless of the collagen origin. These fibre bundles also show a localised piezoelectric response, with enhanced vertical piezoelectricity of collagen. Such topographical features are not observed with the other two chemical treatments, although the shear piezoelectric response is significantly enhanced upon crosslinking. These observations are reconciled by a proposed effect of the crosslinking mechanisms on the molecular and nanostructure of collagen. These results highlight the ability to modify the electromechanical properties of collagen using chemical crosslinking methods.ERC, Bill and Melinda Gates Foundation, Geistlich Pharma A

Time-resolved open-circuit conductive atomic force microscopy for direct electromechanical characterisation.

Studying nanomaterial piezoelectricity and triboelectricity is attractive for energy and sensing applications. However, quantitative characterisation of electromechanical effects in nanomaterials is challenging due to practical limitations and possible combination of effects, resulting in contradicting reports at times. When it comes to piezoelectricity at the nanoscale, piezoresponse force microscopy (PFM) is the default characterisation tool. In PFM the converse piezoelectric effect is measured - the conversion from electrical signal to mechanical response. However, there is an underlying desire to measure the direct piezoelectric effect - conversion of mechanical deformation to an electrical signal. This corresponds to energy harvesting and sensing. Here we present time-resolved open-circuit conductive atomic force microscopy (cAFM) as a new methodology to carry out direct electromechanical characterisation. We show, both theoretically and experimentally, that the standard short-circuit cAFM mode is inadequate for piezoelectric characterisation, and that resulting measurements are governed by competing mechanisms. We apply the new methodology to nanowires of GaAs, an important semiconductor, with relatively low piezoelectric coefficients. The results suggest that time-resolved operation distinguishes between triboelectric and piezoelectric signals, and that by measuring the open-circuit voltage rather than short-circuit current, the new methodology allows quantitative characterisation of the vertical piezoelectric coefficient. The result for GaAs nanowires, ∼ 1-3 pm V-1, is in good agreement with existing knowledge and theory. This method represents a significant advance in understanding the coexistence of different electromechanical effects, and in quantitative piezoelectric nanoscale characterisation. The easy implementation will enable better understanding of electromechanics at the nanoscale

The effect of crystal structure on the electromechanical properties of piezoelectric Nylon-11 nanowires.

Crystal structure is crucial in determining the properties of piezoelectric polymers, particularly at the nanoscale where precise control of the crystalline phase is possible. Here, we investigate the electromechanical properties of three distinct crystalline phases of Nylon-11 nanowires using advanced scanning probe microscopy techniques. Stiff α-phase nanowires exhibited a low piezoelectric response, while relatively soft δ'-phase nanowires displayed an enhanced piezoelectric response

Au - Ge Alloys for Wide-Range Low-Temperature On-Chip Thermometry

We present results for a Au - Ge alloy that is useful as a resistance-based thermometer from room temperature down to at least 0.2 K. Over a wide range, the electrical resistivity of the alloy shows a logarithmic temperature dependence, which simultaneously retains the sensitivity required for practical thermometry while also maintaining a relatively modest and easily measurable value of resistivity. We characterize the sensitivity of the alloy as a possible thermometer and show that it compares favorably with commercially available temperature sensors. We experimentally identify that the characteristic logarithmic temperature dependence of the alloy stems from Kondo-like behavior induced by the specific heat treatment it undergoes.J.R.A.D., P.C.V., G.J.C., and V.N. acknowledge funding from the Engineering and Physical Sciences Research Council, United Kingdom. G.J.C. and S.E.R. acknowledge funding from the Royal Society, United Kingdom. J.F.O. thanks the Brazilian Agency CNPq. A.D. and S.K-N. acknowledge financial support through a European Research Council Starting Grant (Grant No. ERC-2014- STG-639526, NANOGEN)

Aerosol-jet-printed, conformable microfluidic force sensors.

Force sensors that are thin, low-cost, flexible, and compatible with commercial microelectronic chips are of great interest for use in biomedical sensing, precision surgery, and robotics. By leveraging a combination of microfluidics and capacitive sensing, we develop a thin, flexible force sensor that is conformable and robust. The sensor consists of a partially filled microfluidic channel made from a deformable material, with the channel overlaying a series of interdigitated electrodes coated with a thin, insulating polymer layer. When a force is applied to the microfluidic channel reservoir, the fluid is displaced along the channel over the electrodes, thus inducing a capacitance change proportional to the applied force. The microfluidic molds themselves are made of low-cost sacrificial materials deposited via aerosol-jet printing, which is also used to print the electrode layer. We envisage a large range of industrial and biomedical applications for this force sensor

Aerosol-jet printing facilitates the rapid prototyping of microfluidic devices with versatile geometries and precise channel functionalization.

Microfluidics has emerged as a powerful analytical tool for biology and biomedical research, with uses ranging from single-cell phenotyping to drug discovery and medical diagnostics, and only small sample volumes required for testing. The ability to rapidly prototype new designs is hugely beneficial in a research environment, but the high cost, slow turnaround, and wasteful nature of commonly used fabrication techniques, particularly for complex multi-layer geometries, severely impede the development process. In addition, microfluidic channels in most devices currently play a passive role and are typically used to direct flows. The ability to "functionalize" the channels with different materials in precise spatial locations would be a major advantage for a range of applications. This would involve incorporating functional materials directly within the channels that can partake in, guide or facilitate reactions in precisely controlled microenvironments. Here we demonstrate the use of Aerosol Jet Printing (AJP) to rapidly produce bespoke molds for microfluidic devices with a range of different geometries and precise "in-channel" functionalization. We show that such an advanced microscale additive manufacturing method can be used to rapidly design cost-efficient and customized microfluidic devices, with the ability to add functional coatings at specific locations within the microfluidic channels. We demonstrate the functionalization capabilities of our technique by specifically coating a section of a microfluidic channel with polyvinyl alcohol to render it hydrophilic. This versatile microfluidic device prototyping technique will be a powerful aid for biological and bio-medical research in both academic and industrial contexts

Poly-L-lactic acid nanotubes as soft piezoelectric interfaces for biology: controlling cell attachment via polymer crystallinity

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

Conformable and robust microfluidic force sensors to enable precision joint replacement surgery

Balancing forces within weight-bearing joints such as the hip during joint replacement is essential for implant longevity. Minimising implant failure and the corresponding need for expensive and difficult revision surgery is vital to both improve the quality of life of the patient and lighten the burden on overstretched healthcare systems. However, balancing forces during total hip replacements is currently subjective and entirely dependent on surgical skill, as there are no sensors currently on the market that are capable of providing quantitative force feedback within the small and complex geometry of the hip joint. Here, we solve this unmet clinical need by presenting a thin and conformable microfluidic force sensor, which is compatible with the standard surgical procedure. The sensors are fabricated via additive manufacturing, using a combination of 3D and aerosol-jet printing. We optimised the design using finite element modelling, then incorporated and calibrated our sensors in a 3D printed model hip implant. Using a bespoke testing rig, we demonstrated high sensitivity at typical forces experienced following implantation of hip replacements. We anticipate that these sensors will aid soft tissue balancing and implant positioning, thereby increasing the longevity of hip replacements. These sensors thus represent a powerful new surgical tool for a range of orthopaedic procedures where balancing forces is crucial

Surface Potential Driven Water Harvesting from Fog.

Access to clean water is a global challenge, and fog collectors are a promising solution. Polycarbonate (PC) fibers have been used in fog collectors but with limited efficiency. In this study, we show that controlling voltage polarity and humidity during the electrospinning of PC fibers improves their surface properties for water collection capability. We experimentally measured the effect of both the surface morphology and the chemistry of PC fiber on their surface potential and mechanical properties in relation to the water collection efficiency from fog. PC fibers produced at high humidity and with negative voltage polarity show a superior water collection rate combined with the highest tensile strength. We proved that electric potential on surface and morphology are crucial, as often designed by nature, for enhancing the water collection capabilities via the single-step production of fibers without any postprocessing needs

Electroactive polymers for sensing.

Electromechanical coupling in electroactive polymers (EAPs) has been widely applied for actuation and is also being increasingly investigated for sensing chemical and mechanical stimuli. EAPs are a unique class of materials, with low-moduli high-strain capabilities and the ability to conform to surfaces of different shapes. These features make them attractive for applications such as wearable sensors and interfacing with soft tissues. Here, we review the major types of EAPs and their sensing mechanisms. These are divided into two classes depending on the main type of charge carrier: ionic EAPs (such as conducting polymers and ionic polymer-metal composites) and electronic EAPs (such as dielectric elastomers, liquid-crystal polymers and piezoelectric polymers). This review is intended to serve as an introduction to the mechanisms of these materials and as a first step in material selection for both researchers and designers of flexible/bendable devices, biocompatible sensors or even robotic tactile sensing units.This is the final version of the article. It first appeared from The Royal Society Publishing via https://doi.org/10.1098/rsfs.2016.002