Penetrating Microindentation of Hyper-soft, Conductive Silicone Neural Interfaces in Vivo Reveals Significantly Lower Mechanical Stresses

ABSTRACTThere is growing evidence that minimizing the mechanical mismatch between neural implants and brain tissue mitigates inflammatory, biological responses at the interface under long-term implant conditions. The goal of this study is to develop a brain-like soft, conductive neural interface and use an improvised, penetrating microindentation technique reported by us earlier to quantitatively assess the (a) elastic modulus of the neural interface after implantation, (b) mechanical stresses during penetration of the probe, and (c) periodic stresses at steady-state due to tissue micromotion around the probe. We fabricated poly- dimethylsiloxane (PDMS) matrices with multi-walled carbon nanotubes (MWCNTs) using two distinct but carefully calibrated cross-linking ratios, resulting in hard (elastic modulus∼484 kPa) or soft, brain-like (elastic modulus∼5.7 kPa) matrices, the latter being at least 2 orders of magnitude softer than soft neural interfaces reported so far. Subsequent loading of the hard and soft silicone based matrices with (100% w/w) low-molecular weight PDMS siloxanes resulted in further decrease in the elastic modulus of both matrices. Carbon probes with soft PDMS coating show significantly less maximum axial forces (-587 ± 51.5 µN) imposed on the brain than hard PDMS coated probes (-1,253 ± 252 µN) during and after insertion. Steady-state, physiological micromotion related stresses were also significantly less for soft- PDMS coated probes (55.5 ± 17.4 Pa) compared to hard-PDMS coated probes (141.0 ± 21.7 Pa). The penetrating microindentation technique is valuable to quantitatively assess the mechanical properties of neural interfaces in both acute and chronic conditions.

Impact of Climatic Ageing on the Basic Mechanical Properties of Viscoelastic Sealants

The aim of the research presented here was to assess the influence of climatic loading on the basic mechanical properties of polymer sealants, from the viewpoint of their application in creating and repairing outdoor mosaics. Six promising sealants were selected for the measurements. They were tested in their native state and also after four different model types of physical-chemical ageing were applied. We used a combination of ultra-violet radiation with the influence of enhanced temperature, moisture and also soaking in water and freezing. The Hysitron TI 750D TriboIndeterTM system, equipped with a Berkovich indenter, was used for tests. The basic mechanical characteristics were measured using the Basic QS Trapezoid quasi-static mode. Time-dependent properties of the sealants were assessed by the indentation creep method. The results have confirmed that microindentation technique offers a suitable methodology for assessing changes in the basic mechanical time-dependent properties of the tested sealants due to photo-degradation processes and climatic effects.

MICROHARDNESS EVALUATION OF NON ENZYMATICALLY GLYCATED BOVINE FEMUR CORTICAL BONE

Purpose: The aim of this study is to investigate deterioration in mechanical integrity of the collagen network of bovine bone formed by non-enzymatic glycation (NEG), a process that mimics aging by microindentation technique. Methods: Young and old bovine cortical bone specimens were rested in solutions for four weeks for the process of NEG and were grouped as ribosylated and non-ribosylated. A series of indentations were made on bone specimens weekly for each of 3-masses of 50 g, 100 g and 200 g for 10 s to detect the effect of indentation load and for each of five durations of 5 s, 10 s, 20 s, 30 s for 100 g to study the effect of indentation duration. The applied load was increased to 300 g, 500 g, 1000 g and 2000 g for 10 s to be able to make microcracks. Specimens were tested in the wet and dry state to study the effects of hydration on microhardness measurement. Results: Loads of 50 g, 100 g and 200 g for 10 s were able to differ ribosylated bone from non-ribosylated bone for the young and old bovine bones. Microhardness values increased with increasing incubation period. Microhardness of dry specimens were found to be statistically higher than that of wet specimens. Presence of extrinsic toughening mechanisms including crack bridging due to uncracked ligaments and collagen fibers were directly observed by scanning electron microscope (SEM). Ribosylated bone was found to have lower number of collagen bridging compared to non-ribosylated bovine bone. Conclusion: Microhardness test by these are able to discriminate non-modified collagen structure from modified collagen. On the other hand, it is found that microindentation was not able to discriminate the degree of NEG.

Experimental Studies of Elastic Properties of Dental Enamel and Photopolymer Used for early Caries Treatment

The paper is concerned with the study of the elastic properties of dental enamel and its destruction by a microindentation technique. A series of experiments were performed to find differences in the mechanical properties of healthy and diseased dental enamel. It has been found that the times of dental enamel destruction in healthy and carious areas under a constant force are different. The effect of an infiltrated photopolymer on the hardness and elastic modulus of dental enamel was investigated by microindentation testing. A comparative analysis of the features of healthy, damaged by caries and treated by photopolymer enamel was carried out.

Investigation on Mechanical Properties of 6H-SiC Crystal

In this work, the mechanical properties, such as Vickers microhardnessHv, fracture toughnessKc, yield strengthσvand brittleness indexBi, of <0001> oriented 6H-SiC crystal are systematically evaluated using a microindentation technique under 0.1-2 kg applied load. It is found theHvis decreased as the applied load is increased which is mainly attributed to the effect of indenter penetration. TheHvvalue can be effectively presumed by Kicks law and the Meyers indexnis determined to be 1.73. However, theKcvalue is measured nearly a constant (~0.148 MPa.m1/2) which reveals the toughness of 6H-SiC(0001)crystal is much weaker than those of Si(100)and GaAs(100)crystals. The variation ofσvto the load is similar to that ofHv. The brittleness indexBialso exhibits deceasing tendency as the applied load is added.

Mechanical Properties of Enamel Nanocomposite

For adult Indian premolar teeth, we report for the first time ever the simultaneous evaluations of nanohardness, Young's modulus, and fracture toughness of the enamel nanocomposite. The nanohardness and Young's moduli were evaluated from near the beginning of the middle enamel region to within ~10 μm of the dentino-enamel junction (DEJ) and in the dentin region using the nanoindentation technique. The fracture toughness from near the middle of the enamel region to near the DEJ zone was measured using the microindentation technique. The deformation was studied using scanning electron microscopy (SEM) and field emission scanning electron microscopy (FESEM). The relative differences in the extents of biomineralization in the enamel and dentin regions were studied by the energy dispersive X-ray (EDS) technique. The variations of the toughness of the enamel as a function of the toughness of the protein matrix phase have been analyzed which showed that the predicted value of the toughness of the protein present in the nanocomposite was comparable to that of other bioproteins reported in the literature. Further, the work of fracture estimated from the measured value of toughness of the enamel nanocomposite agreed well with the experimental data reported in the literature.