Range Of Physiological Stresses Research Articles

467 A PHYSIOLOGIC ROBOT REACTOR SYSTEM TO SIMULATE IN VIVO CONDITIONS

Purpose: Success in cartilage tissue engineering and cartilage repair strategies depends on the formation of a hyaline cartilage tissue. In any circumstances, the interaction of at least the four components cells, scaffold/ matrix, biochemical, and biomechanical factors, is of utmost importance to induce or maintain the cells in a chondrocytic phenotype, which is a prerequisite to form hyaline cartilage tissue. Any significant in vitro evaluation, such as testing different cell-scaffold constructs, has to be performed under the harsh conditions encountered in vivo within synovial joints. Therefore, many different bioreactor systems have been developed with the aim to simulate these conditions. However, two main shortcomings have been identified in these systems: (i) the mechanical stimulation units do not operate within a physiological stress range and are limited in the applicable motion pattern, and (ii) most systems lack an ambient control and therefore no hypoxic environment is generated as encountered in synovial joints. We have addressed these shortcomings by designing a fully autonomic modular Physiologic Robot Reactor System (PRRS). Methods: We have engineered a reactor system that comprises a mechanical stimulation unit (MSU), an automatic sample changer (ASC), and an environmental control box (ECB) (Figure 1). The MSU is designed with three linear (orthogonal axes) and one rotational degree of freedom (around z-axis; a rotational component around y-axis is pending). The load generated by the MSU is transferred via an exchangeable plunger on a sample tissue placed in a sample holder (Figure 2). Highly accurate force-feedback and motion systems are controlled by ultrafast Field Programmable Gate Array (FPGA) and real-time components which continuously monitor all system parameters. The ASC is designed as a carrousel providing space for 24 sample holders, which allows for individual piloting of the samples with their own stimulation pattern. The ASC and the MSU are integrated in the ECB in which humidity, temperature, gas composition (O2, CO2), and pressure are actively controlled. In addition, an automated media exchange is also implemented in the system, which enables a prolonged uninterrupted cultivation of sample tissues. Results: The complex physiological motion and load pattern of a knee joint were closely simulated by combining sinusoidal and linear motions. Loading forces of up to 500 N in z-axis were achieved, closely matching the physiological forces encountered in the knee. Within the ECB, the climate is accurately controlled and maintained (deviations of less than 0.1% and 0.1°C from given gas concentrations and temperature, respectively) within the range of the detectors.

Conduction block during and after ischaemia in chronic inflammatory demyelinating polyneuropathy.

A previous study suggested that axonal hyperpolariza tion produced by maximal voluntary contraction could accentuate conduction block in symptomatic patients with chronic inflammatory demyelinating polyneuropathy (CIDP). If this is so, conduction block should occur with hyperpolarization due to other causes such as the release of ischaemia. The effects of ischaemia on axonal excitability and on impulse conduction were therefore studied in 12 healthy control subjects and seven patients with symptomatic CIDP. The compound muscle action potential (CMAP) of abductor pollicis brevis was recorded in response to supramaximal stimuli to the median nerve at the wrist alternating with measurements of axonal excitability before, during and after ischaemia for 10 min produced by inflation of a sphygmomanometer cuff around the arm. During ischaemia, the amplitude/area of the maximal CMAP was reduced in the patients by 10% and, after release of ischaemia, it was attenuated by 19%. There were only slight changes in the CMAPs in the healthy controls. The attenuation of the CMAP during ischaemia presumably results from depolarization-induced inactivation of Na(+) channels in axons critically dependent on the number of functioning Na(+) channels for action potential generation. The attenuation of the CMAP after release of ischaemia paralleled the post-ischaemic hyperpolarization and was probably precipitated by it. This study provides suggestive evidence that axonal depolarization can produce conduction block in CIDP, in addition to providing confirmation that axonal hyperpolarization can also do so. In patients with chronic demyelinating disorders, conduction block can probably result from a wider range of physiological stresses than previously appreciated, such as natural activity, ischaemia or recovery from transient ischaemia--all of which could produce fluctuations in symptoms.

Thermo-mechanical behavior the the cornea

Shrinkage of corneal collagen is used during thermokeratoplasty, a method to remodel the corneal curvature. The goals of our investigations were to determine the optimal temperature range for maximal shrinkage of the collagen fibers with minimal damage. By means of a commercially available stress-strain-measuring device with a paraffin oil bath of temperatures varying from 35 degrees C to 120 degrees C strips of pig cornea 5 mm in width and 9 mm in length were investigated in the physiological stress range from sigma = (0,5-12,5) . 10(4) N/m2 by stress-strain, stress relaxation and creep measurements. Biomechanical properties of the cornea remain unchanged in the temperature range from 30 to 50 degrees C. Starting at 60 degrees C shrinkage occurs that increases up to 90 degrees C. The maximal rate of shrinkage of (57 +/- 12)% was measured at temperatures of 75 to 80 degrees C. Above 100 degrees C this effect is reduced by the destruction of intermolecular bonds between the collagen fibers. The stress-strain curves of the shrunk corneas are flatter than that of native corneas, which means, the Young's modulus is significantly reduced. In order to realize optimal shrinkage during thermokeratoplasty temperatures of 65-85 degrees C should be achieved in the coagulated tissue. Higher temperatures cause also a shrinkage effect but also a destruction of tissue.

Mechanical Effects of a Cushioned Plate on Bone Fixation

In vitro and in vivo experiments were carried out to study mechanical effects of the cushioned plate fixation in which a cushion was placed between a metal plate and bone. The cushioned plate fixation was found to be flexible in the physiological stress range and to reduce the stick slip between bone and the plate. A blood-soaked compression test indicated that the cushion moderates the stress shielding effect which occurs in the vicinity of the rigid plate, this being consistent with the result of in vivo experiments.

Human HSP70 promoter contains at least two distinct regulatory domains.

The expression of the human HSP70 gene is induced by a wide range of physiological stresses, including exposure to heat shock and heavy metals, or under nonstress conditions, such as in response to serum stimulation. We have previously demonstrated that in either case the regulated expression is at the primary level of transcription. To determine whether transcription is mediated through a single or multiple genetic elements, we have dissected the sequences upstream of the transcription start site of the human HSP70 gene by constructing chimeric genes retaining variable amounts of 5' flanking regions fused to the bacterial gene encoding chloramphenicol acetyltransferase. Transcription from the chimeric genes was determined by S1 nuclease analysis of separate stable transfectants. The sequences required for heat shock and cadmium induction lie between -107 and -68. Within this region is the sequence CTGGAATATTCCCG, which is identical in 12/14 positions with the heat shock element of Drosophila heat shock genes, and a separate sequence, CGNCCCGG, which is homologous to the core of the human metallothionein II metal-responsive element. The sequences required for serum-stimulated transcription are distinct from the heat shock element. The sequence CCAAT at -68 is required for high levels of correctly initiated transcripts, and a purine-rich sequence, GAAGGGAAAAG, at -58 is required for serum stimulation. The human HSP70 promoter contains at least two regulatory domains--a distal domain responsive to heat shock or cadmium and a proximal domain responsive to stimulation by serum.

A hemodynamic model for relating phasic pressure and flow in the large arteries.

A model of arterial hemodynamics has been developed which encompasses both windkessel and pulse propagation effects and is intended to be valid over a wide range of physiological stresses including hemorrhage. Initial exploratory results suggest that the model has considerable potential for application to the hemodynamic manipulation of research animals and to clinical monitoring.