Electron transfer reactions at gold nanoparticles.

It is demonstrated that scanning electrochemical microscopy can be used to investigate the kinetics of electron transfer reactions catalysed by metal nanoparticles supported on an insulating substrate.

Electrochemical imaging of diffusion through single nanoscale pores.

A combined scanning electrochemical-atomic force microscope (SECM-AFM) has been used to probe the diffusional transport of target electroactive solutes in isolated nanopores of a track-etched membrane. A polycarbonate membrane (100-nm-diam pore size) hydrated with an electrolyte solution, containing a redox-active probe molecule, such as IrCl6(3-) or Fe(phen)3(2+), functions as the model membrane system. The use of a mobile Pt-coated AFM probe enables individual solution-filled pores to be topographically identified.

Proton diffusion at phospholipid assemblies.

A new scanning electrochemical microscopy proton feedback method has been developed for investigating lateral proton diffusion at phospholipid assemblies: specifically Langmuir monolayers at the water/air interface. In this approach, a base is electrogenerated by the reduction of a weak acid (producing hydrogen) at a "submarine" ultramicroelectrode (UME) placed in the aqueous subphase of a Langmuir trough close to a monolayer. The electrogenerated base diffuses to and titrates monolayer-bound protons and is converted back to its initial form, so enhancing the current response at the UME.

Noncontact electrochemical imaging with combined scanning electrochemical atomic force microscopy.

Combined scanning electrochemical atomic force microscopy (SECM-AFM) is a recently introduced scanned probe microscopy technique where the probe, which consists of a tip electrode and integrated cantilever, is capable of functioning as both a force sensor, for topographical imaging, and an ultramicroelectrode for electrochemical imaging. To extend the capabilities of the technique, two strategies for noncontact amperometric imaging-in conjunction with contact mode topographical imaging-have been developed for the investigation of solid-liquid interfaces. First, SECM-AFM can be used to image an area of the surface of interest, in contact mode, to deduce the topography.

High resolution imaging of the distribution and permeability of methyl viologen dication in bovine articular cartilage using scanning electrochemical microscopy.

Scanning electrochemical microscopy (SECM) has been used in the induced transfer (SECMIT) mode to image the permeability of a probe cation, methyl viologen (MV(2+)), in samples of articular cartilage. An ultramicroelectrode (UME), scanned just above the surface of a sample, is used to amperometrically detect the probe solute. The resulting depletion of MV(2+) in solution induces the transfer of this cation from the sample into the solution for detection at the UME.

Endoprosthetic replacement of the proximal tibia.

We have performed endoprosthetic replacement after resection of tumours of the proximal tibia on 151 patients over a period of 20 years. During this period limb-salvage surgery was achieved in 88% of patients with tumours of the proximal tibia. Both the implant and the operative technique have been gradually modified in order to reduce complications.

Cementless fixation for primary segmental bone tumor endoprostheses.

To combat the high incidence of aseptic loosening for young patients and for patients with failed implants after resection for bone tumors, intramedullary cementless fixation of massive tumor implants was investigated. These implants consist of a hydroxyapatite coated titanium stem. To date, 47 of these prostheses have been inserted for the treatment of primary bone tumors.

Scanning electrochemical microscopy as a local probe of oxygen permeability in cartilage.

The use of scanning electrochemical microscopy, a high-resolution chemical imaging technique, to probe the distribution and mobility of solutes in articular cartilage is described. In this application, a mobile ultramicroelectrode is positioned close ( approximately 1 microm) to the cartilage sample surface, which has been equilibrated in a bathing solution containing the solute of interest. The solute is electrolyzed at a diffusion-limited rate, and the current response measured as the ultramicroelectrode is scanned across the sample surface.

Combined scanning electrochemical-atomic force microscopy.

A combined scanning electrochemical microscope (SECM)-atomic force microscope (AFM) is described. The instrument permits the first simultaneous topographical and electrochemical measurements at surfaces, under fluid, with high spatial resolution. Simple probe tips suitable for SECM-AFM, have been fabricated by coating flattened and etched Pt microwires with insulating, electrophoretically deposited paint.

New electrochemical techniques for probing phase transfer dynamics at dental interfaces in vitro.

Phase transfer reactions such as dissolution, precipitation, sorption, and desorption are important in a wide range of processes on dental hard tissue surfaces. An overview is provided of several new complementary electrochemical techniques which are capable of probing the dynamics of such processes at solid/liquid interfaces from millimeter- to nanometer-length scales, with a variable time resolution down to the sub-millisecond level. Techniques considered include channel flow methods with electrochemical detection, which allow reactions at solid/liquid interfaces to be studied under well-defined and calculable mass transport regimes.

Quantitative spatially resolved measurements of mass transfer through laryngeal cartilage.

The scanning electrochemical microscope (SECM) is a scanned probe microscope that uses the response of a mobile ultramicroelectrode (UME) tip to determine the reactivity, topography, and mass transport characteristics of interfaces with high spatial resolution. SECM strategies for measuring the rates of solute diffusion and convection through samples of cartilage, using amperometric UMEs, are outlined. The methods are used to determine the diffusion coefficients of oxygen and ruthenium(III) hexamine [Ru(NH3)6(3+)] in laryngeal cartilage.

Determination of the diffusion coefficient of hydrogen in aqueous solution using single and double potential step chronoamperometry at a disk ultramicroelectrode.

An assessment is made of single and double potential step chronoamperometry (SPSC and DPSC, respectively) at Pt disk ultramicroelectrodes (UMEs) as methods for determining the value of the diffusion coefficient of hydrogen in aqueous solutions. In SPSC, measured currents for the oxidation of dissolved hydrogen (at concentrations close to saturated solution values) comprise a significant contribution, at short to moderate times, from the oxidative desorption of adsorbed hydrogen as well as the diffusion-controlled oxidation of the solution species. Provided that the electrode is preconditioned using a well-defined potential cycling procedure, the behavior for the oxidative desorption step alone can be established in an Ar-saturated solution.

Aseptic loosening in cemented custom-made prosthetic replacements for bone tumours of the lower limb.

We have made a retrospective study of 1001 custom-made prostheses used as replacements after surgery for bone tumours. There were 493 distal femoral, 263 proximal femoral and 245 proximal tibial prostheses. Aseptic loosening was shown to be the principal mode of failure of the implants, and 71 patients had revision for aseptic loosening of a cemented intramedullary stem.

Extendible endoprostheses for the skeletally immature.

Aseptic loosening was identified as the predominant cause of implant related failure in a retrospective study of a consecutive series of 168 Stanmore custom made extendible endoprosthetic replacements used in skeletally immature patients. Most of the replacements were used in the treatment of bone tumor and the remainder for the revision of failed massive endoprosthetic replacements. Since the first Stanmore extendible endoprosthesis was inserted in 1976, 4 types of extension mechanisms have been used.

Distal femoral arthroplasty using custom-made prostheses. The first 218 cases.

The first 218 cases of limb salvage using cemented intramedullary Stanmore massive distal femoral arthroplasties with a single-axis hinged knee, followed for an average of 58 months with a maximum of 251 months, were reviewed. Loosening (5%), local recurrence (4%), intramedullary stem fracture (3%), and infection (2%) were the major complications requiring surgical intervention. Loosening and implant fracture were treated by the insertion of a second massive replacement in all but one case.

The CADCAM contribution to customized orthopaedic implants.

CADCAM (computer aided design/manufacture) production methods are often associated with mass production; working in the medical field at the Department of Biomedical Engineering, the requirement is for one-off, individualized implants. Using a knowledge-based system, implant designs are produced from X-ray data. Assembly from modular components has greatly reduced the production time of implants for bone tumour cases.

Chemical imaging of surfaces with the scanning electrochemical microscope.

Scanning electrochemical microscopy is a scanning probe technique that is based on faradaic current changes as a small electrode is moved across the surface of a sample. The images obtained depend on the sample topography and surface reactivity. The response of the scanning electrochemical microscope is sensitive to the presence of conducting and electroactive species, which makes it useful for imaging heterogeneous surfaces.

Sedation for endoscopy: midazolam or diazepam and pethidine?

One hundred patients received either diazepam given with pethidine, antagonized with naloxone, or midazolam alone in a double-blind randomized study of sedation for upper gastrointestinal endoscopy. Midazolam produced better amnesia for the procedure (P less than 0.0001) but diazepam and pethidine resulted in less retching during the procedure (P less than 0.

Two-dimensional structure of the light-harvesting chlorophyll a/b complex by cryoelectron microscopy.

The light-harvesting chlorophyll a/b complex (LHC-II) found in green plants has at least three functions: it absorbs light energy for transfer to the reaction centers, it is involved in keeping the photosynthetic membranes stacked, and it regulates energy distribution between the two photosystems. We have developed a procedure to produce large vesicles consisting almost exclusively of two-dimensional crystalline domains of LHC-II in which LHC-II is biochemically and structurally intact, as shown by SDS-PAGE, response to cations, and 77K fluorescence excitation spectra. The vesicles were examined by cryoelectron microscopy and analyzed, in projection, to a resolution of 17 A.

Gap junction structure and the control of cell-to-cell communication.

Gap junctions are collections of oligomeric membrane proteins (connexons), which interact across the space between neighbouring cells to form continuous cell-to-cell pathways for ions and small molecules. The connexon is constructed from six identical subunits, arranged symmetrically in the plane of the membrane and delineating the channel along their common sixfold axis. The subunits are rod-shaped and 7-8 nm long; they protrude about 1.

Location of exit channel for nascent protein in 80S ribosome.

Ribosomes crystallize on endoplasmic reticulum membranes in oocytes of the southern Italian lizard, Lacerta sicula, during winter. Electron crystallographic studies of the crystals have been made to elucidate the arrangement of the ribosomal subunits on the membrane surface. We have now obtained more extensive and better ordered crystals of the same habit, grown from chick embryo ribosomes, and report here on their native structure preserved by rapid freezing of the crystals in thin aqueous films.

Quaternary structure of the acetylcholine receptor.

The five membrane-spanning subunits of the acetylcholine receptor have been resolved in electron microscope images and are shown to lie at pentagonally symmetrical positions around the channel over a large fraction of their length. The channel consists of a wide synaptic portion and a narrow portion extending through the membrane into the interior of the cell.

Tubular crystals of acetylcholine receptor.

Well-ordered tubular crystals of acetylcholine receptor were obtained from suspensions of Torpedo marmorata receptor-rich vesicles. They are composed of pairs of oppositely oriented molecules arranged on the surface lattice with the symmetry of the plane group p2 (average unit cell dimensions: a = 90 A, b = 162 A, gamma = 117 degrees). The receptor in this lattice has an asymmetric distribution of mass around its perimeter, yet a regular pentagonal shape; thus its five transmembrane subunits appear to have different lengths, but approximately equal cross sections.

Two configurations of a channel-forming membrane protein.

The protein oligomer forming the gap junction channel has been analysed in two Ca2+-sensitive states by electron microscopy of membranes in frozen aqueous solutions. Switching between states occurs by a small cooperative rearrangement involving tilting of the subunits, which may be responsible for the effect of Ca2+ on channel permeability in vivo.