Time-dependent theoretical model for terahertz wave detector using a parametric process.

We have presented a time-dependent theoretical model to describe the time behavior of a quasi-monochromatic nanosecond terahertz detector reported by Guo et al. [Appl. Phys. Lett. 93, 021106 (2008)]. The temporal input-output characteristic of the detector is investigated numerically by taking the system parameters close to the experimental ones, and the calculated pulse width for the incident terahertz wave agrees well with the experimental one. Our results demonstrate that the energy and width of an output idler wave pulse are proportional to those of the incident terahertz wave pulse. This study provides a strict theoretical basis and could be used to guide the design and optimization for the highly sensitive coherent terahertz detector.

High-efficiency direct-pumped Nd:YVO4 laser operating at 1.34 microm.

We report a high-efficiency Nd:YVO(4) laser operating at 1342 nm pumped by an all-solid-state Q-switched Ti:Sapphire laser at 879 nm. A plano-concave cavity was optimized to obtain high efficiency and good beam quality. Output power for two Nd:YVO(4) crystals with 1.0- and 3.0- at.% Nd(3+) doping under 879-nm pumping was measured respectively. Comparative results obtained by traditional pumping at 808 nm into the highly absorbing (4)F(5/2) level were presented, showing that the slope efficiency of the 1.0-at.% Nd:YVO(4) laser under 879-nm pumping was 10.5% higher than that of 808-nm pumping. In a 4-mm-thick, 1.0-at.% Nd:YVO(4)4 crystal, a high slope efficiency of 64% was achieved under 879-nm pumping, with an optical-to-optical conversion efficiency of 41.3%.

Generation of 3.5W high efficiency blue-violet laser by intracavity frequency-doubling of an all-solid-state tunable Ti:sapphire laser.

In this paper, we report a high power, high efficiency blue-violet laser obtained by intracavity frequency-doubling of an all-solid-state Q-switched tunable Ti:sapphire laser, which was pumped by a 532 nm intracavity frequency-doubled Nd:YAG laser. A beta-BaB2O4 (BBO) crystal was used for frequency-doubling of the Ti:sapphire laser and a V-shape folded three-mirror cavity was optimized to obtain high power high efficiency second harmonic generation (SHG). At an incident pump power of 22 W, the tunable output from 355 nm to 475 nm was achieved, involving the maximum average output of 3.5 W at 400 nm with an optical conversion efficiency of 16% from the 532 nm pump laser to the blue-violet output. The beam quality factor M(2) was measured to be Mx(2)=2.15, My(2)=2.38 for characterizing the tunable blue laser.

Stem cells for tissue engineering of articular cartilage.

Articular cartilage injuries are one of the most common disorders in the musculo-skeletal system. Injured cartilage tissue cannot spontaneously heal and, if not treated, can lead to osteoarthritis of the affected joints. Although a variety of procedures are being employed to repair cartilage damage, methods that result in consistent durable repair tissue are not yet available. Tissue engineering is a recently developed science that merges the fields of cell biology, engineering, material science, and surgery to regenerate new functional tissue. Three critical components in tissue engineering of cartilage are as follows: first, sufficient cell numbers within the defect, such as chondrocytes or multipotent stem cells capable of differentiating into chondrocytes; second, access to growth and differentiation factors that modulate these cells to differentiate through the chondrogenic lineage; third, a cell carrier or matrix that fills the defect, delivers the appropriate cells, and supports cell proliferation and differentiation. Stem cells that exist in the embyro or in adult somatic tissues are able to renew themselves through cell division without changing their phenotype and are able to differentiate into multiple lineages including the chondrogenic lineage under certain physiological or experimental conditions. Here the application of stem cells as a cell source for cartilage tissue engineering is reviewed.

Cross-linking to improve THR wear performance.

Polyethylene wear and associated osteolysis can limit the longevity of total hip replacement. In recent years, many improvements have been made in the consolidation, manufacture, and sterilization of polyethylene acetabular components. These improvements provided reduced polyethylene wear and prolonged usefulness of total hip replacement. Recent advances in extensively cross-linking polyethylene offer the possibility to substantially further reduce wear in total hip replacement. Hip simulator wear testing demonstrates an order of magnitude reduction in wear resulting from cross-linking GUR 1050 polyethylene by exposure to 100 kGy of electron beam radiation followed by annealing to encourage cross-linking and to reduce residual free radicals. Clinical investigation will be required to validate the wear advantage of these materials in vivo. (Hip International 2002; 2: 103-7).

Ultrasonic measurement of the thickness of human articular cartilage in situ.

OBJECTIVE: The objective of the present study was to explore the possibility of using the ultrasonic pulse-echo technique for the non-invasive measurement of cartilage thickness in situ during a joint arthroscopic examination. The accuracy of the ultrasonic measurement was assessed in vitro against that of an established needling technique which is destructive. METHODS: The velocity of sound in articular cartilage was measured in an in vitro study of one set of ipsilateral human ankle and hip joints at 69 test sites. Its variability was determined. RESULTS: The velocity of sound in human articular cartilage measured in situ varied widely (1419-2428 m/s; mean: 1892 m/s; S.D. 183 m/s) and therefore the error in the thickness of cartilage obtained from ultrasonic measurement based upon a constant velocity of sound could be as large as 33.6% (mean 7. 38%; S.D. 6.25%). CONCLUSIONS: The ultrasonic pulse-echo technique is not accurate for the measurement of the thickness of cartilage in situ. An alternative (albeit minimally invasive) would be the needling technique. This requires the development of a specialized probe.

Mechanical conditioning of articular cartilage to prevalent stresses.

The possible correlation between joint stresses and cartilage compressive modulus is examined. The stresses acting upon different areas of the joints and cartilage compressive modulus in these areas were obtained for 15 pairs of ipsilateral human ankle and knee autopsy joint specimens. It was found that the cartilage compressive modulus was significantly correlated with the mechanical stress (r = 0.889 at P < 0.02 level of significance) in such a manner that cartilage subjected to higher predominant stresses was significantly stiffer than that subjected to lower predominant stresses. This was true when comparing ankle with knee joints, and also when comparing different regions within one single joint. Such a correlation is significant in that it indicates that cartilage may well be conditioned mechanically by the prevalent stress it is subject to. However, this correlation of data obtained from autopsy specimens is a necessary, but not sufficient condition, for the above hypothesis to be true. It is, therefore, concluded that further work on animals is necessary.

Asperity lubrication in human joints.

The asperity lubrication in human joints is examined in the present paper, with particular reference to the tertiary undulation with wavelengths of around 20-45 microns. It was found that, under dynamic physiological loading conditions, the secondary waviness of the cartilaginous surface (typically 0.5 mm wavelength) could be effectively flattened to sustain a fluid film of 0.1-0.3 micron thick, while the tertiary waviness could be squashed to sustain a much thinner fluid film of 0.01 micron (10 nm) thick with normal synovial fluid as the lubricant. The calculated film thickness for the tertiary undulation was less than 5 nm when the ankle joint was lubricated by Ringer's solution or pathological synovial fluids, or when only quasi-static loading conditions were considered, while a sufficiently thick fluid film could still be formed when the secondary undulations were considered alone. It was thus suggested that the fluid film lubrication mechanism was operative for human joints with normal synovial fluid as the lubricant under physiological dynamic loading conditions and the mixed lubrication mechanism could take over when static loading conditions prevailed or when watery lubricants (eta approximately 0.001 Pa s) were used.

A new technique for measuring contact areas in human joints--the '3S technique'.

The need for estimating average stresses acting in human joints has prompted the measurement of contact areas. Various existing methods predict contact areas much larger than the actual ones occurring in vivo. This discrepancy is due to the inevitable flow of the interstitial water of articular cartilage in the form of creep deformation. Some of the tests are also too complicated and time consuming for them to be suitable for tests where a large body of data are to be collected. In this report, the authors present a new technique of indicating contact areas occurring in human synovial joints. The new technique employs silicone oil-carbon black powder suspension which is applied to one of the contact surfaces. The suspension is squeezed out of the contact area on applying the load leaving the contact area clearly indicated and possible to record photographically. This Silicone oil-carbon black powder Suspension Squeeze technique (the '3S technique' in short) has been validated and compared with another technique which is equally accurate but cannot be used in human joints. The technique the authors developed is simple, quick and allows accurate measurements of contact areas under physiological conditions with ease. Also, it leaves no permanent staining on cartilage surfaces and so allows one to repeat the test on the same joint under different loads and flexion angles of the joint. As an example, the change of contact areas in a tibio-talar joint during a normal level walking cycle is shown, which can be used to show the variation of the contact stress level at various instances within a walking cycle.

Room-temperature 1.06-0.53-microm second-harmonic generation with MgO:LiNbO(3).

Room-temperature 1.06-0.53-microm second-harmonic generation (SHG) achieved with LiNbO(3) doped with 7 mol. % MgO has been studied. Phase matching was achieved with angle tuning. SHG conversion efficiency of 45% was obtained with a 12-mm-long crystal and a fundamental peak-power density of 140 MW/cm (2). SHG performance of MgO:LiNbO(3) is compared with that of KTP and LBO crystals. Various phase-matching parameters of MgO:LiNbO(3) were calculated as functions of the fundamental wavelength, using the experimentally determined Sellmeier equations. It was found that room-temperature, noncritically-phase-matched Type I SHG can be achieved in this crystal at 1.053 microm, where Nd:YLF lasers operate.