The Scanning Kelvin Probe is a flat-bed scanner that has traditionally been utilised for imaging the electric fields, subsurface currents, surface charge and voltages on conducting specimens (metals and semi-conductors). However a Surface Charge Imaging mode, developed by the author for imaging charge distribution on silicon wafers, works extremely well on both isolating samples, for example, 'artificial skin' (ptfe membranes) and living tissue. The technical developments of this system including design philosophy, digital oscillator, electronics schematics and example voltage and sample height 3D topographies were recently submitted to Review of Scientific Instruments. A further development of the traditional technique, using the metallic tip to probe the dielectric constant of the bulk underlying medium, e.g. human tissue, is scheduled for experimental testing in the forthcoming period. Lastly, before the end of the current grant cycle, we anticipate producing images of cell topograp hy and cell voltage/charge distribution using a Atomic Force Microscope (AFM), at local, i.e., 20-50 nm, resolution. In order to examine the feasibility of utilising surface voltage/charge methods as a tool for human skin diagnostics an initial study was conducted on artificial skin membranes (microporous ptfe), as a precursor to imaging electric current fields on living tissue. This investigation was also designed to determine and assess the instrumentation requirements, experimental procedures and control parameters required in order to non-destructively examine living tissue. The sample membranes were supplied by a Scottish company, Mupor Limited, with whom I have collaborated for several years. I received sets of 6 samples consisting of 2 untreated membranes, having bulk termination (type A) having pore sizes of 1 and 3 (m respectively. Further, 2 sets of 2 samples, (type B, C) having contrasting surface functionality, i.e., chemically modified, were investigated. Details of the functional coating and its specific mode of deposition were considered sensitive information by the supplier and are not reported here. The Scanning Kelvin Probe (SKP) was used to provide voltage/charge images of 12.5 x 12.5 mm2 sections of the membranes at 0.3 - 0.5 mm step sizes. All these membranes display significant surface charges, particularly the untreated surfaces which produced extremely high surface electric fields having regions of both positive and negative charge. This is thought to be induced by their handling history. As the bulk material is an excellent insulator and is best classified as a dielectric having a complex dielectric constant, (, it is classically improper to ascribe the term 'work function' to its surface: rather we probe the variations of surface potential due to patches of (static charge) on its surface. There are few, if any, mobile charge carriers, the SKP is effectively measuring the electric field gradient (-((z/(z) normal to the surface, that the skin cells (fibroblasts) will be exposed to during deposition. In contrast, voltage/charge images of the type A functionalised membranes, displayed relatively low magnitude, negative, flat, profiles. The implication here is that some form of limited 2D charge transfer is possible which produces a stable charge and voltage profile. Separate studies have shown that deliberate charging of the type A membrane by the SKP tip produce a charge pulse that decayed away over several hours, revealing the original charge pattern. Thus, in terms of surface charge, these membranes can be considered stable-they will eventually expel external charge of either sign. The other membrane functionality, type B, showed areas of both positive and negative surface charge, although considerably smaller in magnitude than the untreated material. At this time it is not known whether these regions are due to incomplete coverage of the functionality or variable functionality thickness. Video-microscope images of stained HSF cultured on these 6 membranes showed: (i) extremely poor growth (only isolated cells) on the untreated membranes, (ii)some growth on type B membranes, with the smaller pore size having best coverage, although in both cases cells were randomly orientated, (iii) excellent growth, indeed confluence, in type A membranes, which also exhibited a preferential growth direction, i.e., the fibroblasts were aligned in a specific direction. We may tentatively conclude that this encouraging study demonstrates that the surface charge, along with surface topography and chemistry, plays a major role in HSF adhesion and growth. The reasons for a specific growth direction in the case of the specimens attaining confluence is unknown. Although orientational effects have been observed due to atomic steps, terrace arrangement, etc., these membranes are both amorphous and display much greater surface roughness ( 300 nm. We cannot, at this stage, rule out a surface voltage induce effect due to relative polarisation of the sample surface in the culture media. Thisstudy will now continue via to SKP imaging of living tissue. The main emphasis here is to distinguish between living and dead tissue and characterise the voltage patterns (if any) present. It may well prove fruitful to utilise the scanning tip (in the constant separation mode) as an alternating current (AC), variable frequency, signal injector, so as to prove the local ( vector, utilising both surface charge and material (bulk) dielectric constant.
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