Hydrogel polymers comprise a novel category of synthetic materials being investigated for use in cartilage replacement. One candidate compound, a poly(ethylene glycol)/poly(acrylic acid) (PEG/PAA) interpenetrating polymer network (IPN), was developed for use in corneal prostheses and was recently engineered for potential orthopedic use. The current study examined the effects of particles of this compound on two cell lines (MG-63 osteoblast-like cells and RAW 264.7 macrophages) over a 48-h time course. To mimic the effects of wear debris, particles of the compound were generated and introduced to the cells. In the MG-63 cell line, the particles had no significant effect on cell viability measured by PicoGreen assay and trypan blue exclusion. In contrast, a significant decrease in cell viability was detected in the Raw 264.7 macrophage cells at the final timepoint with the highest concentration of hydrogel (3.0% v:v). A concentration- and time-dependent increase in TNF-alpha release characteristic of other known biocompatible materials was also detected in RAW 264.7 cells, but nitric oxide and interleukin (IL)-1beta showed no response. In addition, the MG-63 cell line demonstrated no IL-6 response. Particles of the PEG/PAA IPN thus seem to stimulate biological responses similar to those in other biocompatible materials.

The objectives of this study were to determine whether residual tensile strains exist in the dura mater of mammals in vivo, and whether the strains are age-dependent. We made incisions in the parietal dura mater of immature and mature rats, and measured the retraction of the dura mater from each incision. We then used a finite-element model to calculate the strain present in the parietal dura mater of each rat. We found that age-dependent residual tensile strains are present in the dura mater of rats. The mean average residual strain of the immature rats was significantly larger than that of the mature rats (4.96+/-1.54% (s.d.) versus 0.39+/-0.13%, p<0.0001), with the mean strain calculated in the mature rats of the order of the minimum measurement that could be made using our experimental approach. In addition, in the immature rats mean residual strain in the longitudinal direction was significantly larger than mean residual strain in the transverse direction (6.11+/-3.62% versus 3.82+/-2.64%, p=0.0218). Our findings show that age-dependent residual tensile strains exist in the dura mater of rats. We speculate that these strains may reflect the rate and direction of cranial growth and may also influence cranial healing.

Changes in regional blood flow and surface temperatures closely paralleled onsets of focus, rest, and withdraw periods and appear to be volitional activations of known vasomotor mechanisms underlying non-hairy skin regions such as the hands and face. Changes in blood flow through these vascular structures are generally autonomic thermoregulatory responses, not normally under voluntary control, but may also represent a relaxation response.

It has been proposed that periosteal residual tensile strains influence periosteal bone apposition and endochondral ossification. The role of bone growth rates on the development of residual strains is not well known. This study examined the relationships between specific growth rate and residual strains in chick tibiotarsi. We measured length and circumference during embryonic days 11-20 using microCT. Bones grew faster in length, with longitudinal and circumferential specific growth rates decreasing from 17 to 9% and 14 to 8% per day, respectively. To calculate residual strains, opening dimensions of incisions through the periosteum were analysed using finite element techniques. Results indicate that Poisson's ratio for an isotropic material model is between 0 and 0.04. For the model with Poisson's ratio 0.03, longitudinal and circumferential residual strains decreased from 46.2 to 29.3% and 10.6 to 3.9%, respectively, during embryonic days 14-20. Specific growth rates and residual strains were positively correlated (p<0.05).