Flexural Response of Degraded Polyurethane Foam Core Sandwich Beam with Initial Crack between Facesheet and Core.

Structural systems developed from novel materials that are more durable and less prone to maintenance during the service lifetime are in great demand. Due to many advantages such as being lightweight as well as having high strength, corrosion resistance, and durability, the sandwich composites structures, in particular, have attracted attention as favorable materials for speedy and durable structural constructions. In the present research, an experimental investigation is carried out to investigate the flexural response of sandwich beams with a pre-cracked core-upper facesheet interface located at one end of the beam. During the development of the sandwich beams, an initial pre-cracked debond was created between the core and facesheet by placing a Teflon sheet at the interface. Both three-point and four-point flexural tests were conducted to characterize the flexural behavior of the sandwich beams. The effects of the loading rate, core thickness, and placement of the initial interfacial crack under a compressive or tensile stress state on the response and failure mechanism of Carbon Fiber-Reinforced Polymer (CFRP)/Polyurethane (PU) foam sandwich beams were investigated. It was found that the crack tip of the initial debonding between the upper facesheet and the core served as a damage initiation trigger followed by the fracture failure of the core due to the growth of the initial crack into the core in an out-of-plane mode. Finally, this leads to facesheet damage and rupture under flexural loadings. An increase in the core thickness resulted in a higher peak load, but the failure of the sandwich beam was observed to occur at significantly lower displacement values. It was found that the behavior of sandwich beams with higher core thickness was loading rate-sensitive, resulting in stiffer response as the loading rate was increased from 0.05 to 1.5 mm/s. This change in stiffness (10-15%) could be related to the squeezing of all pore space, resulting in the collapse of cell walls and thereby making the cell behave as a solid material. As a result, the occurrence of the densification phase in thick core beams occurs at a faster rate, which in turn makes the thick cored sandwich beams exhibit loading rate-sensitive behavior.

Comparative assessment of iridium oxide and platinum alloy wires using an in vitro glial scar assay.

The long-term effect of chronically implanted electrodes is the formation of a glial scar. Therefore, it is imperative to assess the biocompatibility of materials before employing them in neural electrode fabrication. Platinum alloy and iridium oxide have been identified as good candidates as neural electrode biomaterials due to their mechanical and electrical properties, however, effect of glial scar formation for these two materials is lacking. In this study, we applied a glial scarring assay to observe the cellular reactivity to platinum alloy and iridium oxide wires in order to assess the biocompatibility based on previously defined characteristics. Through real-time PCR, immunostaining and imaging techniques, we will advance the understanding of the biocompatibility of these materials. Results of this study demonstrate iridium oxide wires exhibited a more significant reactive response as compared to platinum alloy wires. Cells cultured with platinum alloy wires had less GFAP gene expression, lower average GFAP intensity, and smaller glial scar thickness. Collectively, these results indicated that platinum alloy wires were more biocompatible than the iridium oxide wires.

Nanopatterning effects on astrocyte reactivity.

An array of design strategies have been targeted toward minimizing failure of implanted microelectrodes by minimizing the chronic glial scar around the microelectrode under chronic conditions. Current approaches toward inhibiting the initiation of glial scarring range from altering the geometry, roughness, size, shape, and materials of the device. Studies have shown materials which mimic the nanotopography of the natural environment in vivo will consequently result in an improved biocompatible response. Nanofabrication of electrode arrays is being pursued in the field of neuronal electrophysiology to increase sampling capabilities. Literature shows a gap in research of nanotopography influence in the reduction of astrogliosis. The aim of this study was to determine optimal feature sizes for neural electrode fabrication, which was defined as eliciting a nonreactive astrocytic response. Nanopatterned surfaces were fabricated with nanoimprint lithography on poly(methyl methacrylate) surfaces. The rate of protein adsorption, quantity of protein adsorption, cell alignment, morphology, adhesion, proliferation, viability, and gene expression was compared between nanopatterned surfaces of different dimensions and non-nanopatterned control surfaces. Results of this study revealed that 3600 nanopatterned surfaces elicited less of a response when compared with the other patterned and non-nanopatterned surfaces. The surface instigated cell alignment along the nanopattern, less protein adsorption, less cell adhesion, proliferation and viability, inhibition of glial fibrillary acidic protein, and mitogen-activated protein kinase kinase 1 compared with all other substrates tested.

Characterization of astrocyte reactivity and gene expression on biomaterials for neural electrodes.

Neural electrode devices hold great promise to help people with the restoration of lost functions. However, research is lacking in the biomaterial design of a stable, long-term device. Glial scarring is initiated when a device is inserted into brain tissue and an inflammatory response ensues. Astrocytes become hypertrophic, hyperplastic, and upregulate glial-fibrillary acidic protein. This study was designed to investigate the astrocyte proliferation, viability, morphology, and gene expression to assess the reactive state of the cells on different material surfaces. Although platinum and silicon have been extensively characterized both in vivo and in vitro for their biocompatibility with neuronal cells, this study used the novel usage of PMMA and SU-8 in neural electrodes by comparative analysis of materials' biocompatibility. This study has shown evidence of noncytotoxicity of SU-8. We have also confirmed the biocompatibility of PMMA with astrocytes. Moreover, we have established sound guidelines of which neural implant materials should meet to be depicted biocompatible.

Analysis of laser fabricated microjoint performance in cerebrospinal fluid using a computational approach.

Assessment of neural biocompatibility requires that materials be tested with exposure in neural fluids. We have studied the mechanical performance of laser bonded microjoints between titanium foil and polyimide film (TiPI) in artificial cerebrospinal fluid (CSF). The samples were exposed in CSF for two, four and twelve weeks at 37 °C. The laser microbonds showed initial degradation up to four weeks which then stabilized afterwards and retained similar strength until twelve weeks. To understand this bond degradation mechanism better, a finite element modeling approach was adopted. From the finite element results, it was revealed that bond degradation was not due to the hygroscopic expansion of polyimide. Rather, relaxation of the process induced residual stresses may have resulted in weakening of the bond strength as observed from experimental measurements.

A comprehensive review of surface modification for neural cell adhesion and patterning.

This comprehensive literature review covers recent studies on patterning neuronal cells by topographical modifications on material surfaces targeted for neural prostheses. We explore different materials that are used as the candidate surface for neuronal cell adhesion. Cell-material interactions are identified in both cases where the material surface was in direct contact with cells and where the materials were coated to facilitate cell adhesion. Commonly used coating materials and coating methods are discussed. The existing hypotheses behind mechanism of the response of neuronal cells to a specific topography are presented briefly. A few selected important studies have been presented to show the range of techniques employed and the extent of the research area.

Postimplantation pressure testing and characterization of laser bonded glass/polyimide microjoints.

The stability of the laser bonded titanium coated glass/polyimide microjoints were studied in vivo by implanting on a rat brain surface for 10 days. In the current state, the strength of the joints were measured by a specially designed instrument called "pressure test" equipment where the samples were subjected to a variable pressure load (using high pressure nitrogen) controlled by a pressure regulator. The strength of the joints seems to degrade by about 28% as a result of soaking in rat brain. The bond degradation in rat brain implants is similar compared with those soaked in artificial cerebrospinal fluid (CSF) solution. Polyimide uptakes water through existing pores in it and also water gets in the joint region through the edges of the samples. Water might have caused oxidation of the chemical bonds which are thought to have formed by the laser fabrication process. A separate set of samples were created using same parameters for testing the hermeticity of the laser bonds. The samples were also exposed to rat brain CSF and were tested for hermiticity at the end of 10 days exposure time. It was observed that the implanted samples retained their hermeticity although the bond strength degraded by about 28%.

A neural cell culture study on thin film electrode materials.

Functional neural stimulation requires good interface between the neural cells and the electrode surfaces. In order to study the effect of electrode materials and surface structure on cell adhesion and biocompatibility, we cultured cortical neurons on thin films of platinum and iridium oxide. We used both flat, as-deposited and laser micro-structured films. The laser micro-structuring consisted of creating regular arrays of micro-bumps or holes with diameters of 4-5 mum and height of about 1.5 mum. The micro-bumps were fabricated onto platinum and iridium film surfaces deposited on borosilicate glass substrates, using mask-projection irradiation with single nano-second pulses from a KrF excimer laser (lambda = 248 nm). Amorphous and crystalline (deposited at 250 degrees C) IrO(2) films were deposited onto the laser micro-structured iridium films by pulsed-DC reactive sputtering to obtain micro-structured IrO(2) films. Cortical neurons isolated from rat embryo brain were cultured onto these film surfaces. Our results indicate that flat and micro-structured film surfaces are biocompatible and non-toxic for neural cell growth. The use of poly-D: -lysine as a mediator for cell adhesion onto the thin film surfaces is also discussed.

Influence of nanoscale surface roughness on neural cell attachment on silicon.

The adherence and viability of neural cells (primary cortical cells) from rat embryo on silicon wafers with varying surface roughness (10 to 250 nm) at the nano scale were investigated. The roughnesses were achieved by using chemical etching. Atomic force microscopy was utilized to determine surface roughness. We examined the adherence and viability of neural cells by using scanning electron microscopy and fluorescence immunoassaying. Antineuron-specific enolase antibody was used for immunostaining. Results from this investigation show that for these specific neural cells, there is an optimum surface roughness range, R(a) = 20 to 100 nm, that promotes cell adhesion and longevity. For silicon-based devices, this optimum surface roughness will be desirable as a suitable material/neuron interface.