K. Seunarine

3D polymer scaffolds for tissue engineering

This review discusses some of the most common polymer scaffold fabrication techniques used for tissue engineering applications. Although the field of scaffold fabrication is now well established and advancing at a fast rate, more progress remains to be made, especially in engineering small diameter blood vessels and providing scaffolds that can support deep tissue structures. With this in mind, we introduce two new lithographic methods that we expect to go some way to addressing this problem.

A Biodegradable and Biocompatible Regular Nanopattern for Large‐Scale Selective Cell Growth

A biodegradable substrate with a regular array of nanopillars fabricated by electron-beam lithography and hot embossing is used to address the mechanisms of nanotopographical control of cell behavior. Two different cell lines cultured on the nanopillars show striking differences in cell coverage. These changes are topography- and cell-dependent, and are not mediated by air bubbles trapped on the nanopattern. For the first time, a strong cell-selective effect of the same nanotopography has been clearly demonstrated on a large area; while fibroblast proliferation is inhibited, endothelial cell spreading is visibly enhanced. The reduced fibroblast proliferation indicates that a reduction of available surface area induced by nanotopography might be the main factor affecting cell growth on nanopatterns. The results presented herein pave the way towards the development of permanent vascular replacements, where non-adhesive, inert, surfaces will induce rapid in situ endothelialization to reduce thrombosis and occlusion.

Investigation of the limits of nanoscale filopodial interactions

Mesenchymal stem cells are sensitive to changes in feature height, order and spacing. We had previously noted that there was an inverse relationship between osteoinductive potential and feature height on 15-, 55- and 90 nm-high titania nanopillars, with 15 nm-high pillars being the most effective substrate at inducing osteogenesis of human mesenchymal stem cells. The osteoinductive effect was somewhat diminished by decreasing the feature height to 8 nm, however, which suggested that there was a cut-off point, potentially associated with a change in cell–nanofeature interactions. To investigate this further, in this study, a scanning electron microscopy/three-dimensional scanning electron microscopy approach was used to examine the interactions between mesenchymal stem cells and the 8 and 15 nm nanopillared surfaces. As expected, the cells adopted a predominantly filopodial mode of interaction with the 15 nm-high pillars. Interestingly, fine nanoscale membrane projections, which we have termed ‘nanopodia,’ were also employed by the cells on the 8 nm pillars, and it seems that this is analogous to the cells ‘clinging on with their fingertips’ to this scale of features.

A Hierarchical Response of Cells to Perpendicular Micro- and Nanometric Textural Cues

In this paper, we report on the influence of shallow micro- and nanopatterned substrata on the attachment and behavior of a human fibroblast [human telomerase transfected immortalized (hTERT)] cells. We identify a hierarchy of textural guidance cues with respect to cell alignment on these substrates. Cells were seeded and cultured for 48 h on silicon substrates patterned with two linear textures overlaid at 90°, both with 24 ¿m pitch: a micrograting and a nanopattern of rows of 140- nm-diameter pits arranged in a rectangular array with 300 nm centre-to-centre spacing. The cell response to these textures was shown to be highly dependent on textural feature dimensions. We show that cells align to the stripes of nanopits. Stripes of 30-nm deep nanopits were also shown to elicit a stronger response from cells than 160-nm deep nanopits.

Progress towards tubes with regular nanopatterned inner surfaces

The repair of vascular tubing is an important task in tissue engineering. The behavior of cells is strongly influenced by the topology of the surfaces, on both a micrometric and a nanometric scale, in their vicinity. Thus the authors wish to make tubes that are patterned on the inner surface. One way to do this is to use the good depth of focus capabilities of x-ray exposure to print an array of dots, 200nm diameter and 400nm pitch, onto a curved surface coated in resist. A die made from this structure allows nanoembossing into a biodegradable polymer. A closed vessel can then be made by adding a lid, that also has a similar nanopatterned surface. Details of the accuracy of transfer are given. It is concluded that x-ray printing is a suitable approach for the formation of internally patterned tubing.

Review of optoelectronic oscillators based on modelocked lasers and resonant tunneling diode optoelectronics

Optoelectronic oscillators can provide low noise oscillators at radio frequencies in the 0.5-40 GHz range and in this paper we review two recently introduced approaches to optoelectronic oscillators. Both approaches use an optical fibre feedback loop. One approach is based on passively modelocked laser diodes and in a 40 GHz oscillator achieves up to 30 dB noise reduction. The other approach is based on resonant tunneling diode optoelectronic devices and in a 1.4 GHz oscillator can achieve up to 30 dB noise reduction.

3D fabrication methods for producing tissue engineering scaffolds

A new method for preparing 3D scaffolds for tissue engineering applications with highly controlled micro and nanotopography have been developed. A combination of UV and electron beam lithography was employed for master fabrication and nanoimprint lithography for the preparation of the 3D polymeric scaffolds.

Prospects for atomic magnetometers employing hollow core optical fibre

Presently, among the most demanding applications for highly sensitive magnetometers are Magnetocardiography (MCG) and Magnetoencephalography (MEG), where sensitivities of around 1pT.Hz-1/2 and 1fT.Hz-1/2 are required. Cryogenic Superconducting Quantum Interference Devices (SQUIDs) are currently used as the magnetometers. However, there has been some recent work on replacing these devices with magnetometers based on atomic spectroscopy and operating at room temperature. There are demonstrations of MCG and MEG signals measured using atomic spectroscopy These atomic magnetometers are based on chip-scale microfabricated components. In this paper we discuss the prospects of using photonic crystal optical fibres or hollow core fibres (HCFs) loaded with Rb vapour in atomic magnetometer systems. We also consider new components for magnetometers based on mode-locked semiconductor lasers for measuring magnetic field via coherent population trapping (CPT) in Rb loaded HCFs.

Sub-picosecond 9.8W peak power passively mode locked quantum well GaAs/AlGaAs laser

We present a monolithic passively mode-locked 795nm GaAs/AlGaAs quantum well laser with enlarged vertical mode profile and a low duty-cycle cavity design, emitting 710fs long pulses at a peak power of 9.8W per facet. Semiconductor laser, mode-locked laser, coherent population trapping.

Passively mode-locked semiconductor laser for coherent population trapping in 87 Rb

Recent growing interest in miniature atomic frequency references and precision magnetometers has motivated investigations of coherent population trapping (CPT) and its use in such applications [1]. System designs based on picosecond mode-locked lasers have been previously reported for generating the CPT effect, using Ti:Sapphire lasers passively mode-locked at a submultiple of the hyperfine splitting 87Rb levels [2], or laser diodes modulated electrically [3] or with an external electro-optic modulator [4] at the 87Rb splitting frequency. In this work we investigate a novel approach for achieving CPT in 87Rb vapour cells. We use a 795nm semiconductor laser passively mode-locked [5] at the 87Rb standard frequency (∼6.834GHz). This approach eliminates the need for any RF driving circuit, allowing a more compact, integrable and easy to drive implementation of CPT in 87Rb. For this purpose we have fabricated 5.678mm long lasers with 3µm wide and 1.2µm deep ridge waveguides, with a 150µm long saturable absorber (SA) at one facet. The laser material used is a 793nm GaAs/AlxGa1−xAs single quantum well (QW) graded-index separate confinement heterostructure (GRINSCH), with an epitaxial design similar to the one reported in [6].