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Biomedical subjects

J T Mortimer

Publications and source records attributed to J T Mortimer.

66 records · Page 4Linked to original sources
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Electrical properties of implant encapsulation tissue.

The purpose of this study was to determine the electrical properties of the encapsulation tissue that surrounds electrodes chronically implanted in the body. Two four-electrode arrays, fabricated from either epoxy or silicone rubber, were implanted in each of six adult cats for 82 to 156 days. In vivo measurements of tissue resistivity using the four-electrode technique indicated that formation of the encapsulation tissue resulted in a significant increase in the resistivity of the tissue around the arrays. In vitro measurements of tissue impedance using a four-electrode cell indicated that the resistivity of the encapsulation tissue was a function of the tissue morphology. The tight layers of fibroblasts and collagen that formed around the silicone rubber arrays had a resistivity of 627 +/- 108 omega-cm (mean +/- SD; n = 6), which was independent of frequency from 10 Hz to 100 kHz, and was significantly larger than the resistivity of the epoxy encapsulation tissue at all frequencies between 20 Hz and 100 kHz. The combination of macrophages, foreign body giant cells, loose collagen, and fibroblasts that formed around the epoxy arrays had a frequency-dependent resistivity that decreased from 454 +/- 123 omega-cm (n = 5) to 193 +/- 98 omega-cm between 10 Hz and 1 kHz, and was independent of frequency between 1 kHz and 100 kHz, with a mean value of 195 +/- 88 omega-cm. The results indicate that the resistivity of the encapsulation tissue is sufficient to alter the shape and magnitude of the electric field generated by chronically implanted electrodes.

Animals↗
PubMedDate not supplied

Extraction forces and tissue changes during explant of CWRU-type intramuscular electrodes from rat gastrocnemius.

Intramuscular electrodes are currently in use for clinically implementing several electrotherapeutic and neuroprosthetic protocols. A decrease in motor recruitment is often reported in these systems due to movement of the electrode tip from the initial implant site. In the study reported here, multistrand intramuscular electrodes of the CWRU design were implanted aseptically in the gastrocnemii of adult rats under anesthesia. These electrodes were explanted immediately after implant in one group and after periods of 1 and 4 hr; 1, 3, and 5 days; 1 week; 10 days; and 2 and 4 weeks in others. Force as a function of displacement was recorded during explantation. Analysis of the results showed that there was a significant increase in the force required to dislodge the electrode tip between 5 and 7 days of implant. Electrodes seemed to be vulnerable to movement in the first 5 days when the barb provided the only fixation. Histology of muscles from which electrodes had been explanted did not show any increase in the area of tissue changes, compared with control muscles in which the electrode remained in situ. These results indicated that electrode removal occurred within the encapsulation tissues, and the surrounding muscle was mainly unaffected by the explant process.

Analysis of Variance↗
PubMedDate not supplied

Alteration in the force and fatigability of skeletal muscle in quadriplegic humans following exercise induced by chronic electrical stimulation.

The paralyzed forearm finger flexor muscles of 10 quadriplegic subjects were exercised via electrical stimulation to determine changes in the contractile force and fatigability of the muscle that could be externally induced. The force of contraction initially produced by most muscles was less than that required for functional activity. Following exercise, the strength of all muscles was increased above the level for functional use. In general, no correlation was noted between the amount of exercise and the force change. Two subjects had high contraction forces after prolonged periods before stimulation began, and 4 subjects who were followed after the stimulation program maintained near the contractile force achieved at the end of the program in the absence of stimulation. Fatigue of the muscle during the course of the exercise program was increased to above that recorded initially, although this was more variable and sometimes dropped to less than that recorded initially. Generally, no correlation was demonstrated between usage and fatigue. It is concluded that exercise induced by electrical stimulation alters the cantractile properties of the muscle toward a state usable for functional, tonic activation.

Animals↗