This post presents an overview of my work for the last 3 years. Essentially I’ve been developing new techniques for structuring materials on the sub milimeter scale by a combination of laser ablation, plasma chemistry and standard wet chemistry.
Silicon has been the wonder material of the late 20th century and looks set to continue well into the 21th, mono-crystalline silicon underpins much of the electronics now taken for granted and is also used in the developing Micro-Electro-Mechanical Systems (MEMS) technology. The structure and properties of bulk mono-crystalline silicon have been intensively studied since it was first produced.
The properties and applications of silicon at the meso- and nano-scales, however, have been less intensively exploited. Intensive research has demonstrated that the electronic and optical properties of nanoscale silicon structures change dramatically with respect to the bulk and these changes may lead to significant advances in devices and sensor. Furthermore, silicon mesostructures, which are already being used in the accelerometers that trigger automotive airbags, exhibit great promise for applications in, eg, antireflection coatings, microfluids and biomedical applications.
Chemically enhanced laser ablation of silicon provides a mechanism to quickly produce large numbers of near-identical meso-scale silicon structures which can be easily converted to nano-scale structures via standard wet chemistry procedures. Starting from a flat or patterned silicon substrate, pillars and cones will spontaneously form when the substrate is irradiated by a pulsed laser beam.
Applications of the structured surfaces.
The main body of my work is the production and characterisation of the structures I produce. I have provided samples to various groups interested in using them for various applications ranging from biological applications to electrode arrays, photonic structures and basic materials research. In this post I’ll concentrate on the biological applications.
The potential for localised delivery and subsequent activity of drugs [McAllister2000] and gene [Coulman2005] therapeutics within the skin and for fluid extraction for diagnostics [Torabi-pour2004] has stimulated research into cheap, painless ways to deliver the macro-molecules through the tough outer, stratum corneum (SC), layer of the skin as shown in fig. 1.
Figure 1: Schematic diagram showing how the microneedles should penetrate the Stratum Corneum allowing drug delivery to the Viable Epidermis, avoiding contacting the blood vessels and nerves of the Dermis layer. (Image courtesy of C. Allender, personal communication)
There is considerable interest in using silicon needles to puncture the SC producing microconduits though to the viable epidermis layer of the skin, through which macro-molecule treatments can be introduced.
The lithographic fabrication of microstructures on a silicon surface is a well established process, but requires a clean room, toxic chemicals or access to reactive ion etching equipment and producing large areas of needles is a time consuming process. This is in contrast to the relatively quick and low technology method we employ to produce large arrays of silicon pillars. As was seen in the previous chapter, wet etching of silicon pillars produced in SF6 leads to well defined, short, but sharp needles.
The structures formed by the chemically enhanced laser ablation of silicon may provide a cheap, easy way to produce large numbers of needles for experimentation. Silicon pillars covered with a layer of porous silicon may be ideal for the transfer of material into cells, the pillars acting as needles and the porous layer acting as a reservoir of the material to be injected.
There is still much work do be done in this area, there isn’t a whole lot more than can be done to alter the pillar shapes, so muuch of the future work will be lab trials investigating the usage of the pillars as needles.