Science of Water Leaks: Validated Theory for Moisture Flow in Microchannels and Nanochannels.

Water is ubiquitous; the science of its transport in micro- and nanochannels has applications in electronics, medicine, filtration, packaging, and earth and planetary science. Validated theory for water vapor and two-phase water flows is a "missing link"; completing it enables us to define and quantify flow in a set of four standard leak configurations with dimensions from the nanoscale to the microscale. Here we report the first measurements of water vapor flow rates through four silica microchannels as a function of humidity, including under conditions when air is present as a background gas. An important finding is that the tangential momentum accommodation coefficient (TMAC) is strongly modified by surface layers of adsorbed water molecules, in agreement with previous work on the TMAC for nitrogen molecules impacting a silica surface in the presence of moisture. We measure enhanced flow rates for two-phase flows in silica microchannels driven by capillary filling. For the measurement of flows in nanochannels we use heavy water mass spectrometry. We construct the theory for the flow rates of the dominant modes of water transport through each of the four standard configurations and benchmark it against our new measurements in silica and against previously reported measurements for nanochannels in carbon nanotubes, carbon nanopipes, and porous alumina. The findings show that all behavior can be described by the four standard leak configurations and that measurements of leak behavior made using other molecules, such as helium, are not reliable. Single-phase water vapor flow is overestimated by a helium measurement, while two-phase flows are greatly underestimated for channels larger than 100 nm or for all channels when boundary slip applies, to an extent that depends on the slip length for the liquid-phase flows.

Pt-Al2O3 interfaces in cofired ceramics for use in miniaturized neuroprosthetic implants.

A core element to miniaturized, hermetic encapsulations for neuroprosthetic implants with high numbers of stimulation channels is the creation of electrical feedthroughs. Platinum (Pt) and alumina (Al2 O3 ) are necessary to connect the sealed electronics to external components including electrode arrays that provide a neural interface function, as well as to sources of power or data. Combined with laser micro-processing, high-density feedthrough arrays were created with up to 2500 channels per cm(2) . The chemistry, micro structure, and crystallography of the Pt-Al2 O3 interface created by the cofiring of Pt particles and Al2 O3 particulate in binder were studied by transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS) and selective area electron diffraction (SAED) to determine the nature of the Pt-Al2 O3 bond. While Pt-Al2 O3 interfaces only occurred in cases where the different grains were in distinct orientations where the crystal lattices matched, the addition of glass additives allowed for bonding nonmatching orientations by devitrification to form Pt-glass-Al2 O3 interfaces. The conditions for the formation of both mechanisms were determined, and it was shown that higher order crystal planes than previously described can be matched. Analyzing the lattice matches in detail showed the ability of the material compound to compensate for mismatches by the formation of dislocations, out-of-angle matching, lattice distortion, and the existence of semi-coherent interfaces in case of integer misfit ratios to create domain matching.

Pt-Al2O3 interfacial bonding in implantable hermetic feedthroughs: morphology and orientation.

The present work examines the chemistry, microstructure, and crystallography of a Pt-Al(2)O(3) joint used in implantable hermetic feedthrough designs in neural prostheses. Pt was joined to Al(2)O(3) by passing Pt pins through green Al(2)O(3) disks and then sintering in air. This created a Pt-Al(2)O(3) joint that was prepared for the current investigation by gross sectioning and then polishing and sectioning into slices using focused ion beam milling. The slices were examined by scanning electron microscopy, transmission electron microscopy, and energy dispersive spectroscopy. Two types of interfaces in the sintered material were identified: Vitreous-bonded Pt-glass-Al(2)O(3) and direct-bonded Pt-Al(2)O(3). In the case of the former, glass formation owing to the presence of glass-forming additives (to enhance densification and suppress grain growth) and consequent wetting of both the Pt and Al(2)O(3) facilitated interfacial bonding. In the case of the latter, the interfacial planes were (002)(Pt) // (022)Al(2)O(3) [rhombohedral] or (002)(Pt) // (022 2)Al(2)O(3) [hexagonal]. The lattice mismatch was calculated to be 11% (based on the calculated d spacings) or 15% (based on the literature d spacings). Both of these suggest the establishment of semicoherent interfaces.

Process development for dry etching polydimethylsiloxane for neural electrodes.

In order to create high density electrode arrays, a reactive ion (dry) etching process was developed using sulphur hexafluoride (SF(6)) and oxygen (O(2)) plasma to pattern micro-structures in medical grade polydimethylsiloxane (PDMS). The surface topography and etch performance were analyzed by employing surface profilometry, scanning electron micrographs (SEM) and atomic force miscroscopy (AFM). The maximum etch rate was approximately 0.22 µm/min. The chemical modification of the PDMS structure in SF(6) and O(2) plasma was investigated through x-ray photoelectron spectroscopy (XPS). Micro-scale openings in PDMS were achieved using a dry etching method to allow charge injection at the electrode-tissue interface.

Force application during cochlear implant insertion: an analysis for improvement of surgeon technique.

Highly invasive surgical procedures, such as the implantation of a prosthetic device, require correct force delivery to achieve desirable outcomes and minimize trauma induced during the operation. Improvement in surgeon technique can reduce the chances of excessive force application and lead to optimal placement of the electrode array. The fundamental factors that affect the degree of success for cochlear implant recipients are identified through empirical methods. Insertion studies are performed to assess force administration and electrode trajectories during implantations of the Nucleus 24 Contour and Nucleus 24 Contour Advance electrodes into a synthetic model of the human Scala Tympani, using associated methods. Results confirm that the Advance Off- Stylet insertion of the soft-tipped Contour Advance electrode gives an overall reduction in insertion force. Analysis of force delivery and electrode positioning during cochlear implantation can help identify and control key factors for improvement of insertion method. Based on the findings, suggestions are made to enhance surgeon technique.

Comparison of round window and cochleostomy approaches with a prototype hearing preservation electrode.

INTRODUCTION: Preservation of residual hearing in cochlear implant recipients has been demonstrated to be possible and provides the potential benefit of combined electric and acoustic auditory stimulation. A prototype 16-mm multichannel array has been designed to facilitate placement of 22 electrodes without damage to intracochlear structures. The electrode array is suitable for insertion via the round window membrane (RWM) or a small cochleostomy. AIM: To evaluate the insertion trajectory and the presence of trauma to intracochlear structures with the prototype electrode inserted by either the RWM or a scala tympani cochleostomy. MATERIALS AND METHODS: Eighteen fresh frozen human temporal bones were prepared for cochlear implantation using a standard transmastoid facial recess technique. Twelve electrodes were implanted at the University of Melbourne and 6 at the Medizinische Hochschule Hannover. In Melbourne fluoroscopy was used to monitor the insertions. Twelve prototype electrodes were inserted via the RWM. A further 6 electrodes were inserted via a small scala tympani cochleostomy. The cochleostomy was sited inferior to the RWM to avoid trauma to the basilar membrane and spiral ligament. Specimens were embedded and fixed with acrylic resin and the cochleae then examined histologically at 200-mum intervals using a grinding and polishing technique. RESULTS: Full insertion of the electrode was achieved without significant resistance in all RWM and cochleostomy specimens. In two RWM specimens fold-over of the electrode tip occurred, and in one specimen the electrode penetrated the spiral ligament to lie in an 'endosteal 'position. In one cochleostomy specimen the electrode was rotated within the cochlea to face laterally rather than towards the modiolus. The final electrode position differed for the two groups, with the electrodes inserted via the RWM lying in a more perimodiolar position along the first part of the basal turn. The average depth of insertion was 240 degrees for the RWM electrodes and 255 degrees for the cochleostomy electrodes. Histologic examination showed no damage in any specimen to the modiolus, osseous spiral lamina or basilar membrane. CONCLUSIONS: A prototype hearing preservation electrode array was inserted by either a RWM or a scala tympani cochleostomy without evidence of significant intracochlear trauma.

The effect of screw type on the biomechanical properties of SCARF and crescentic osteotomies of the first metatarsal.

The basilar crescentic osteotomy is a popular method for correcting moderate to severe hallux valgus. However, inadvertent dorsiflexion of the osteotomy can result from intraoperative malposition or from malunion after fixation failure. The mechanical properties of osteotomies are dependent on the nature of the osteotomy and the type of fixation. This study examines the mechanical properties of the SCARF and crescentic osteotomies of the first metatarsal by using a cannulated asymmetric pitched screw or AO cancellous screws. Sixteen human cadaveric first metatarsal specimens were tested in plantar to dorsiflexion cantilever bending by using a mechanical testing machine. The data was compared with our recent work on the mechanical properties of the SCARF and crescentic osteotomies. Ultimate load and stiffness of the SCARF osteotomy were superior to the crescentic osteotomy but were not dependent on screw type. Screw type was a prominent factor in the stiffness but not in the strength of the crescentic osteotomy. The ultimate load and the stiffness of SCARF osteotomy fixed with the cannulated asymmetric pitched screws were not significantly different compared with AO screws (ultimate load, 124.6 N [SD = 56.8] v 105.3 N [SD = 57.0]; stiffness, 52.0 N/mm [SD = 48.0] v 31.8 N/mm [SD = 19.0]). Modes of failure were fracture of the cortical bone bridge between the screw hole and the osteotomy in all crescentic osteotomies and fracture of the proximal dorsal bridge in all SCARF osteotomies. The superior mechanical properties of the SCARF osteotomy, fixed with cannulated asymmetric pitched screws, make this a more secure construct, with less risk of malunion than the crescentic osteotomy. Stiffness is an important mechanical factor that helps distinguish the mechanical performance of different osteotomy techniques.

Hydroxyapatite composite resin cement augmentation of pedicle screw fixation.

Pedicle screw stability is poor in osteopenic vertebrae attributable, in part, to low screw-bone interface strength. The current authors examined cement augmentation using a low curing temperature hydroxyapatite and bis-phenol-A glycidol methacrylate-based composite resin. This cement may stiffen the screw-bone interface and reduce the harmful effects associated with polymethylmethacrylate regarding temperature and toxic monomer. Thirty-five lumbar vertebrae from human cadavers were instrumented with pedicle screws, with one pedicle previously injected with cement and the other as the control. Caudocephalad toggling of +/- 1 mm for 1600 cycles was applied to the pedicle screws, and the resulting forces supported by the implant-bone interface were captured by a load cell. A curve was constructed from the peak caudal load for each cycle and three mechanical measures parameterized this curve: (1) initial load; (2) rate of load decay during the first 400 cycles; and (3) final load. The initial load increased by 16% as a result of cement augmentation, the final load increased by 65%, and the rate of load decay decreased by 59%. Cement augmentation of pedicle screws increased the stiffness and stability of the screw-bone interface.