Mechanical characterisation of hydrogel materials

Interest in hydrogel materials is growing rapidly, due to the potential for hydrogel use in tissue engineering and drug delivery applications, and as coatings on medical devices. However, a key limitation with the use of hydrogel materials in many applications is their relatively poor mechanical properties compared with those of (less biocompatible) solid polymers. In this review, basic chemistry, microstructure and processing routes for common natural and synthetic hydrogel materials are explored first. Underlying structure-properties relationships for hydrogels are considered. A series of mechanical testing modalities suitable for hydrogel characterisation are next considered, including emerging test modalities, such as nanoindentation and atomic force microscopy (AFM) indentation. As the data analysis depends in part on the material's constitutive behaviour, a series of increasingly complex constitutive models will be examined, including elastic, viscoelastic and theories that explicitly treat the multiphasic poroelastic nature of hydrogel materials. Results from the existing literature on agar and polyacrylamide mechanical properties are compiled and compared, highlighting the challenges and uncertainties inherent in the process of gel mechanical characterisation. © 2014 Institute of Materials, Minerals and Mining and ASM International.

Properties of a-C:H films deposited from a methane electron cyclotron wave resonant plasma

An electron cyclotron wave resonant methane plasma discharge was used for the high rate deposition of hydrogenated amorphous carbon (a-C:H). Deposition rates of up to ∼400 Å/min were obtained over substrates up to 2.5 in. in diameter with a film thickness uniformity of ∼±10%. The deposited films were characterised in terms of their mass density, sp3 and hydrogen contents, C-H bonding, intrinsic stress, scratch resistance and friction properties. The deposited films possessed an average sp3 content, mass density and refractive index of ∼58%, 1.76 g/cm3 and 2.035 respectively.Mechanical characterisation indicated that the films possessed very low steady-state coefficients of friction (ca. 0.06) and a moderate shear strength of ∼141 MPa. Nano-indentation measurements also indicated a hardness and elastic modulus of ∼16.1 and 160 GPa respectively. The critical loads required to induce coating failure were also observed to increase with ion energy as a consequence of the increase in degree of ion mixing at the interface. Furthermore, coating failure under scratch test conditions was observed to take place via fracture within the silicon substrate itself, rather than either in the coating or at the film/substrate interface. © 2003 Elsevier B.V. All rights reserved.

Characterisation of cell adhesion to substrate materials and the resistance to enzymatic and mechanical cell-removal

Cell-implant adhesive strength is important for prostheses. In this paper, an investigation is described into the adhesion of bovine chondrocytes to Ti6Al4V-based substrates with different surface roughnesses and compositions. Cells were cultured for 2 or 5 days, to promote adhesion. The ease of cell removal was characterised, using both biochemical (trypsin) and mechanical (accelerated buoyancy and liquid flow) methods. Computational fluid dynamics (CFD) modelling has been used to estimate the shear forces applied to the cells by the liquid flow. A comparison is presented between the ease of cell detachment indicated using these methods, for the three surfaces investigated. © 2008 Materials Research Society.

Resonant Raman characterisation of ultra-thin nano-protective carbon layers for magnetic storage devices

Carbon thin films are very important as protective coatings for a wide range of applications such as magnetic storage devices. The key parameter of interest is the sp3 fraction, since it controls the mechanical properties of the film. Visible Raman spectroscopy is a very popular technique to determine the carbon bonding. However, the visible Raman spectra mainly depend on the configuration and clustering of the sp2 sites. This can result in the Raman spectra of different samples looking similar albeit having a different structure. Thus, visible Raman alone cannot be used to derive the sp3 content. Here we monitor the carbon bonding by using a combined study of Raman spectra taken at two wavelengths (514 and 244 nm). We show how the G peak dispersion is a very useful parameter to investigate the carbon samples and we endorse it as a production-line characterisation tool. The dispersion is proportional to the degree of disorder, thus making it possible to distinguish between graphitic and diamond-like carbon. © 2003 Elsevier B.V. All rights reserved.

Characterisation and fluidisation of synthetic pit latrine sludge

Half of the world’s urban population will live in informal settlements or “slums” by 2030. Affordable urban sanitation presents a unique set of challenges as the lack of space and resources to construct new latrines makes the de-sludging of existing pits necessary and is something that is currently done manually with significant associated health risks. Therefore various mechanised technologies have been developed to facilitate pit emptying, with the majority using a vacuum system to remove material from the top of the pit. However, this results in the gradual accumulation of unpumpable sludge in the pit, which eventually fills the latrine and forces it to be abandoned. This study has developed a method for fluidising unpumpable pit latrine sludge, based on laboratory experiments using a harmless synthetic sludge. Such a sludge consisting of clay and compost was developed to replicate the physical characteristics of pit latrine sludges characterised in Botswana during the 1980s. Undrained shear strength and density are identified as the critical parameters in controlling pumpability and a method of sludge characterisation based on these parameters is reported. In a series of fluidisation tests using a one fifth scale pit emptying device the reduction in sludge shear strength was found to be caused by i) dilution, which increases water content, and ii) remoulding, which involves mechanical agitation to break down the structure of the material. The tests demonstrated that even the strongest of sludges could be rendered “pumpable” by sufficient dilution. Additionally, air injection alone produced a three-fold decrease in strength of consolidated samples as a result of remoulding at constant water content. The implications for sludge treatment and disposal are discussed, and the classification of sludges according to the equipment required to remove them from the latrine is proposed. Possible field tests to estimate sludge density and shear strength are suggested. The feasibility of using low cost vacuum cleaners to replace expensive vane pumps is demonstrated. This offers great potential for the development of affordable pit emptying technologies that can remove significantly stronger sludges than current devices through fluidising the wastes at the bottom of the pit before emptying