基于MEMS的外层型人工视网膜刺激信号发生器的初步研究

视网膜是中枢神经系统的重要部分，在早期视觉信息处理中发挥重要作用。视网膜变性疾病的致盲率很高。人工视网膜植人物是值得关注的新研究方向。在基于MEMS的外层型人工视网膜的设计中，刺激信号发生器，即微光电二极管阵列（MPDA），是植人芯片的核心。首先，介绍一种双反相池张振荡器的设计原理。其次，对国外设计方案进行改进，设计出一款基于双反相弛张振荡器的新电路模型。然后对模型进行了仿真。结果表明该电路设计基本能实现预期的功能，对外层型人工视网膜植人物的后续研究奠定基础。     

Photocurrent and excess microwave reflectivity at semiconductor|electrolyte junctions: I. Minority carrier profiles

The photocurrent density j_Ph and the excess microwave reflectivity, ΔR_M, at the semiconductor|electrolyte boundary are calculated using analytical expressions for the excess minority carrier profiles. The model incorporates diffusive terms in the space charge region, depending on the surface parameter values, K_r (charge transfer velocity), and S_r (surface recombination velocity), the influence of small electrical fields in the so-called neutral region and the back contact recombination velocity, S_rb. Photocurrent–voltage and excess microwave reflectivity–voltage curves are calculated depending on K_r and S_r showing that it is possible to determine these surface kinetic parameters from simultaneous measurements of j_Ph and ΔR_M. It is shown that for cases in which the quantum yield is high, microwave measurements exhibit a higher-sensitivity to changes in experimental parameters.     

Spectral sensitivity of single cones in rainbow trout (Oncorhynchus mykiss): A whole-cell voltage clamp study

The UVS cone mechanism is known to light adapt at low intensities in comparison to other cones. We were interested in whether this property was related to higher sensitivity in UVS cones or to network adjustments in sensitivity. We investigated spectral sensitivity of 107 individual cone photoreceptors in rainbow trout (Oncorhynchus mykiss) using a whole-cell voltage clamp technique. Mean time-to-peak response was 339 ± 90 ms and flash sensitivity for a 100 ms flash was 4.37 × 10⁻³ ± 2.50 × 10⁻³ pA photons⁻¹ μm², with no significant differences between the UVS, SWS, MWS and LWS cone classes. The spectral sensitivity of the UVS, SWS and LWS cones conformed to the expression of SWS1, SWS2 and LWS opsin genes. The spectral sensitivity of MWS cones, however, showed clear evidence of co-expression of RH2a and RH2b opsin pigments. The fish used in this study bridged the ontogenetic stage where the MWS cones shift their expression from RH2b to RH2a.     

Study of passive films formed on Sn in the 7–14 pH range

The electronic, structural and morphological characteristics of passive layers formed anodically on tin in the 7–14 pH range were studied. Photocurrent spectroscopy measurements showed that Sn passive films behave as highly disordered n-type semiconductors. The measured band gap values, E_g, diminish as the formation pH is increased. Fourier transform infrared reflection–absorption spectroscopy results indicate that the ratio between SnOH and SnOSn bonds in the passive films increases with increasing formation pH. At pH >12.5 the films contained appreciable amounts of lattice water. A correlation between E_g and the hydroxylation degree of the films was proposed.     

Mode of origin of large extracellular photovoltages from the retinal rods of the frog

Arrays of extracellular microelectrodes were advanced into the receptor layer of isolated frog retinae to measure interstitial voltage and resistance. In addition, current was locally injected through one of the micro-electrodes and spatial profiles of local resistance were recorded. A large increase of local resistance in the proximal half of the receptor layer was frequently associated with the occurrence of large extracellular voltages arising from rods. The large responses presumably occur at sites where the micropipette tip makes tight contact with membrane processes of the rod inner segment and a quasi-intracellular mode of recording results.     

Transduction steps which characterize retinal cone current responses to flash stimuli

There are three prominent characteristics in current responses of retinal cones to flash stimuli when compared with those of rods. First, the time-to-peak of the photocurrent is two to four times faster than for the rod. Second, the photosensitivity of the cone is more than one order of magnitude lower than that of the rod. Third, current responses of cones resemble characteristic undershoot in the recovery phase. At present, however, it is not known what kinds of mechanisms underlie these characteristics. In the present study, critical steps which characterize cone responses have been investigated by changing rate constants in the model for the signal transduction scheme of cone outer segments under the assumption that the scheme is basically the same as for rods. The computer simulations have shown that rate constants were divided into three categories: those which do not appreciably change photocurrent, those which change only photosensitivity, and those which change both sensitivity and speed. Furthermore, rate constants which characterize undershoot were identified. From these simulations it has been revealed that characteristics of the cone response could be reproduced by changing the rate constants. The critical ones are rate of rhodopsin deactivation, rate of GTP hydrolysis, and deactivation rate of activated phosphodiesterase. These results suggest possibilities that the signal transduction mechanisms in cones are basically the same as those for rods, while rate constants are different.     

Abnormal photocurrent of the anodic film on lead in sulfuric acid solution

The abnormal photocurrent of the anodic film formed on lead in 4.5 mol dm ^?3 H₂SO₄ at 1.4 V (vs. Hg|Hg₂SO₄) for 1 h was investigated by several electrochemical methods. The experimental results show the abnormal photocurrent is due to a two-phase system in the film, i.e. a p-type semiconductor t-PbO and an n-type semiconductor PbO_1,4]. The composition of the two-phase system is about PbO_1,19 when the photocurrent is zero.     

Effects of Ag-photodeposition on photocurrent of an ITO electrode modified by a hybrid film of TiO2 nanosheets

We performed the photodeposition of Ag clusters onto a TiO₂ nanosheet hybridized with an amphiphlic Ru(II) complex (denoted by TiO₂–Ru/Ag). Firstly a floating monolayer of [Ru(phen)₂(dC18bpy)](ClO₄)₂ (phen = 1,10-phenanthroline, dC18bpy = 4,4^′-dioctadecyl-2,2^′-bipyridyl) was formed on an aqueous dispersion of exfoliated layered titanate, (TBAOH)_xTi_2?x/4□_x/4O₄ · H₂O (x～0.7;□, vacancy; TBAOH = tetrabutylammonium hydroxide) (denoted as TiO₂ nanosheet). The monolayer acted as the organic part for the hybridization of TiO₂ particles in an aqueous phase. A hybrid film of TiO₂ nanosheet and an amphiphilic metal complex (denoted by TiO₂–Ru) was transferred onto an indium tin oxide substrate by the vertical dipping method. Thereafter the electrode was immersed in 1.0 M AgNO₃ for 10 min and rinsed with water. The electrode was irradiated for 30 min by a Xe-lamp (150 W). Atomic force microscope, X-ray photoelectron spectra and cyclic voltammometry showed the formation of silver nanoparticles with a radius of 20–150 nm. The photocurrent response was measured in the potential range of 0.2–1.2 V (vs. Ag|AgCl|KCl(sat)) under the illumination of light (>350 nm). The current increased 1.7 times when the layer number of hybrid films increased from one to two. It should be emphasized that such an increase of photocurrent with a layer number was not observed in the absence of Ag particles.     

Chemical and electrochemical interaction of acidic H2O2 solutions with (100) InP

The chemical and electrochemical interactions between the (100) InP surface and acidic aqueous H₂O₂ solutions were studied by etch rate, voltammetric, IMPS and electroluminescence measurements. Etching of InP by H₂O₂ appears to occur at a very low rate through a chemical mechanism. Photocurrent enhancement caused by H₂O₂ is observed both at the p-InP cathode and at the n-InP anode. Whereas the former effect is ascribed to reduction of H₂O₂ by the well-known current-doubling mechanism, for the latter effect a reaction mechanism is proposed in which intermediates of the photoanodic dissolution reaction of InP are modified by H₂O₂ so that they can more easily inject electrons into the conduction band of the semiconductor.     

Electrochemical and photoelectrochemical behaviour of poly-3-methylthiophene and polybithiophene in non-aqueous solutions: 1. Dark and light processes in systems with and without cathodic doping

The electrochemical as well as the photoelectrochemical behaviour of poly-3-methylthiophene and polybithiophene in systems with and without the possibility of cathodic doping was studied. Using electrochemical impedance measurements it was demonstrated that in both types of systems the photoelectrochemical processes on such polymers involved photoelectrochemical undoping of the polymer itself as well as the photoelectrochemical reduction of a component of an external redox couple in the solution. In systems with cathodic doping, in addition to these two processes, photoassisted cathodic doping of the polymer is observed. The model of photoelectrochemical processes on polymer electrodes is discussed.