David Cheneler

Mechanical properties of alginate hydrogels manufactured using external gelation

Alginate hydrogels are commonly used in biomedical applications such as scaffolds for tissue engineering, drug delivery, and as a medium for cell immobilisation. Multivalent cations are often employed to create physical crosslinks between carboxyl and hydroxyl moieties on neighbouring polysaccharide chains, creating hydrogels with a range of mechanical properties. This work describes the manufacture and characterisation of sodium alginate hydrogels using the divalent cations Mg(2+), Ca(2+) and Sr(2+) to promote gelation via non-covalent crosslinks. Gelation time and Young׳s modulus are characterised as a function of cation and alginate concentrations. The implications of this work towards the use of environmental elasticity to control stem cell differentiation are discussed.

On the calibration of rectangular atomic force microscope cantilevers modified by particle attachment and lamination

A simple but effective method for estimating the spring constant of commercially available atomic force microscope (AFM) cantilevers is presented, based on estimating the cantilever thickness from knowledge of its length, width, resonant frequency and the presence or absence of an added mass, such as a colloid probe at the cantilever apex, or a thin film of deposited material. The spring constant of the cantilever can then be estimated using standard equations for cantilever beams. The results are compared to spring constant calibration measurements performed using reference cantilevers. Additionally, the effect of the deposition of Cr and Ti thin films onto rectangular Si cantilevers is investigated.

Degradation of polymer films

In this review paper the current state of research into the physical degradation of polymer films is elucidated. Modern applications of polymer films and the implication of their degradation are discussed. Recent investigations into solid interactions such as abrasion, adhesion, fatigue and other failure modes as well as plasma and photonic interactions are examined. This investigation highlights key degradation mechanisms as well as areas where controversy over these mechanisms lies, and suggests directions for future research.

Smart Pipes—Instrumented Water Pipes, Can This Be Made a Reality?

Several millions of kilometres of pipes and cables are buried beneath our streets in the UK. As they are not visible and easily accessible, the monitoring of their integrity as well as the quality of their contents is a challenge. Any information of these properties aids the utility owners in their planning and management of their maintenance regime. Traditionally, expensive and very localised sensors are used to provide irregular measurements of these properties. In order to have a complete picture of the utility network, cheaper sensors need to be investigated which would allow large numbers of small sensors to be incorporated into (or near to) the pipe leading to so-called smart pipes. This paper focuses on a novel trial where a short section of a prototype smart pipe was buried using mainly off-the-shelf sensors and communication elements. The challenges of such a burial are presented together with the limitations of the sensor system. Results from the sensors were obtained during and after burial indicating that off-the-shelf sensors can be used in a smart pipes system although further refinements are necessary in order to miniaturise these sensors. The key challenges identified were the powering of these sensors and the communication of the data to the operator using a range of different methods.

Spherical indentation analysis of stress relaxation for thin film viscoelastic materials

The mechanical testing of thin layers of soft materials is an important but difficult task. Spherical indentation provides a convenient method to ascertain material properties whilst minimising damage to the material by allowing testing to take place in situ. However, measurement of the viscoelastic properties of these soft materials is hindered by the absence of a convenient yet accurate model which takes into account the thickness of the material and the effects of the underlying substrate. To this end, the spherical indentation of a thin layer of viscoelastic solid material is analysed. It is assumed that the transient mechanical properties of the material can be described by the generalised standard linear solid model. This model is incorporated into the theory and then solved for the special case of a stress relaxation experiment taking into account the finite ramp time experienced in real experiments. An expression for the force as a function of the viscoelastic properties, layer thickness and indentation depth is given. The theory is then fitted to experimental data for the spherical indentation of poly(dimethyl)siloxane mixed with its curing agent to the ratios of 5:1, 10:1 and 20:1 in order to ascertain its transient shear moduli and relaxation time constants. It is shown that the theory correctly accounts for the effect of the underlying substrate and allows for the accurate measurement of the viscoelastic properties of thin layers of soft materials.

Application of Colloid Probe Atomic Force Microscopy to the Adhesion of Thin Films of Viscous and Viscoelastic Silicone Fluids

The adhesive characteristics of thin films (0.2-2 μm) of linear poly(dimethylsiloxane) (PDMS) liquids with a wide range of molecular weights have been measured using an atomic force microscope with a colloid probe (diameters 5 and 12 μm) for different separation velocities. The data were consistent with a residual film in the contact region having a thickness of ∼6 nm following an extended dwell time before separation of the probe. It was possible to estimate the maximum adhesive force as a function of the capillary number, Ca, by applying existing theoretical models based on capillary interactions and viscous flow except at large values of Ca in the case of viscoelastic fluids, for which it was necessary to develop a nonlinear viscoelastic model. The compliance of the atomic force microscope colloid beam was an important factor in governing the retraction velocity of the probe and therefore the value of the adhesive force, but the inertia of the beam and viscoelastic stress overshoot effects were not significant in the range of separation velocities investigated.

A Bio-Hybrid Tactile Sensor Incorporating Living Artificial Skin and an Impedance Sensing Array

The development of a bio-hybrid tactile sensor array that incorporates a skin analogue comprised of alginate encapsulated fibroblasts is described. The electrical properties are modulated by mechanical stress induced during contact, and changes are detected by a ten-channel dual-electrode impedance sensing array. By continuously monitoring the impedance of the sensor array at a fixed frequency, whilst normal and tangential loads are applied to the skin surface, transient mechanotransduction has been observed. The results demonstrate the effectiveness and feasibility of the preliminary prototype bio-hybrid tactile sensor.

Light-modulating pressure sensor with integrated flexible organic light-emitting diode

Organic light-emitting diodes (OLEDs) are used almost exclusively for display purposes. Even when implemented as a sensing component, it is rarely in a manner that exploits the possible compliance of the OLED. Here it is shown that OLEDs can be integrated into compliant mechanical micro-devices making a new range of applications possible. A light-modulating pressure sensor is considered, whereby the OLED is integrated with a silicon membrane. It is shown that such devices have potential and advantages over current measurement techniques. An analytical model has been developed that calculates the response of the device. Ray tracing numerical simulations verify the theory and show that the design can be optimized to maximize the resolution of the sensor.

Optimised determination of viscoelastic properties using compliant measurement systems

An analysis of a novel indentation model has been implemented to obtain master curves describing the optimal experimental parameters necessary to achieve the highest possible accuracy in the determination of viscoelastic properties of soft materials. The indentation model is a rigid indenter driven by a compliant measurement system, such as an atomic force microscope or optical tweezers, into a viscoelastic half space. The viscoelastic material is described as a multiple relaxation Prony series. The results have been extended via an application of a viscoelastic equivalence principle to other physical models such as poroelasticity. Optimisation of the indentation parameters has been conducted over many orders of magnitude of the velocity, viscoelastic moduli, spring stiffness, relaxation times and the duration of indentation resulting in a characteristic master curve. It is shown that using sub-optimal conditions gives the appearance of a more elastic material than is actually the case. For a two term Prony series the ideal ramp duration was found to be approximately one eighth of the relaxation. Also the ideal ramp duration for a three term Prony series was determined and shown to guarantee distinct relaxation times under specific conditions.

A Dynamic Model of the Jump-To Phenomenon During AFM Analysis

The measurement of the physical properties of surfaces on the nanoscale is a long-standing problem, and the atomic force microscope (AFM) has enabled the investigation of surface energies and mechanical properties over a range of length scales. The ability to measure these properties for softer materials presents a challenge when interpreting data obtained from such measurements, in particular because of the dynamics of the compliant AFM microcantilever. This work attempts to better understand the interaction between an AFM tip and samples of varying elastic modulus, in the presence of attractive van der Waals forces. A theoretical model is presented in which the dynamics of the approach of an atomic force microscope cantilever tip toward a surface, prior to and during the van der Waals-induced jump-to phenomenon, are included. The cantilever mechanics incorporates the motion of the air through which the cantilever moves, the acceleration, inertia, and torque of the cantilever, and the squeezing of the fluid between the cantilever tip and the surface, leading to elastohydrodynamic lubrication and deformation of the substrate. Simulations of the cantilever approach are compared to measurements performed using an atomic force microscope, and the effect of cantilever drive velocity is investigated. Cantilevers presenting (1) spherical colloid probe tips and (2) pyramidal tips are employed, and substrates exhibiting Youngs moduli of 3 MPa, 500 MPa, and 75 GPa are measured. The analysis presented could be extended to enhance understanding of dynamic phenomena in micro/nanoelectromechanical systems such as resonators and microrheometers, particularly those which contain soft materials and also where surface interactions are important.