6.8 Wednesday, Jan. 4 Optimizing work and power production of a Manduca sextalarval locomotory muscle WOODS, W. A.*; SCHULER, F. R. ; YEE, A. L. ; TRIMMER, B. A. ; Tufts University; Tufts University; Tufts University; Tufts University email@example.com
Because caterpillar locomotory muscle develops relatively high stress over a large strain range, and because larval muscle cultured from primary myocytes shows better longevity and environmental tolerance than vertebrate muscle, it is a promising candidate for cultured bioactuators. Since such constructs lack motor neurons, we optimized muscle work and power production of native larval ventral interior longitudinal muscle under direct stimulation. In anticipation of powering devices producing rotary motion, we used sinusoidal strain cycling, which is unlike strain cycling during in vivo crawling. We varied stimulus train timing, duration and frequency, as well as pulse duration and voltage. In physiological saline mimicking hemolymph, no combination of parameters yielded positive work during any portion of the strain cycle; the very slow relaxation of the muscle at termination of stimulus caused prohibitively high stress during relengthening. Because of the potential channel blocking properties of the physiological saline Mg2+, we carried out subsequent experiments using saline optimized for desheathed nerves; this reduced the muscle relaxation time constant sixfold. During cycling at 0.25 Hz (approximating in vivo crawl cycle frequency), muscles produced up to 1.7 x 10-5 J per cycle, yielding 2.59 W kg-1 power output; peak stress during shortening was 90 kPa. Higher cycling frequencies reduced work without increasing power, while lower frequencies reduced power. Optimal performance was achieved when 40 Hz trains of 0.1 ms 40 v pulses were applied for 0.6 s beginning 0.1 s after shortening from 1.14 to 0.86 lengths commenced. Both work and power production values were over an order of magnitude higher that those for simulated in vivo crawling conditions.