Journ al o f Me d ical Engine erin g & Tech no logy, Vo lum e 23, Nu m b er 6, ( Novem be r / Dece m ber 1999 ) , p ages 222- 224

Fo r c e tr a n s d u c e r f o r m e a s u r e m e n t o f m u s c le c o n tr a c tio n M. B u n c * ² , D . SÏu p u t ²

an d J. R o zm an ³

² In stitute of Pathophysiology, School of Medicine, ZalosÏka 4, 1000 Lju bljana, Slovenia ³ ITIS d.o.o. Ljubljan a, Cen tre for Implan tabile Techn ology an d Sen sors, Lepi pot 11, 1000 Lju bljana, Slovenia

Sin gle chan n el force tran sdu cers, in ten ded for measu remen t an d evalu ation cu rves of precon diti on ed ® bres in mu scles con tractin g syn ergi stically du rin g direct or in di rect electric stimu lati on of isolated mu scle, were design ed, dev eloped an d experimen tally tested. The force tran sdu cers were made u p of a fu ll Wheatston e bridge composed of fou r semicon du ctor strain gau ges bon ded on a specially design ed can tilever. The tran sdu cers with a n atu ral frequ en cy 350 Hz an d compli an ce ± 1 of 0.25 l m g represen ts a v ery lin ear depen den ce of the ou tpu t v oltage upon the load givi ng a sen siti vi ty for the tran sdu cers of ± 1 0.5 mV mN with a bridge excitation v oltage of 5 V. The n omi n al ran ge of each tran sdu cer i s 0± 70 mN. The system is able to record even a con tracti on of on ly a few mu scle ® bres, both sin gle twi tches as well as su stain ed tonic con traction s.

In tro d u ctio n For basic re se arch in ph ysiology m e asure m e n ts of m uscle con tractions, force s are ve ry im portan t for studie s of contraction m e ch an ism s. Force tran sduce rs m ade of strain gau ge s give th e opportun ity to de ve lop force se nsitive an d re liable tran sduce rs. Th e buildin g of tran sduce rs to m e asu re m uscle con tractions h as a lon g h istory ove r at le ast 30 ye ars [ 1± 6] an d m an y de vice s are n ow com m e rcially availab le ( Ugo Basile , Italy, Harvard Instrum e nts, UK) . Th e force tran sduce r sh ould ful® ll th e followin g re quire m e n t: ( a ) it should be able to e valu ate a force produce d by the contraction of m uscle s, ( b ) th e e le ctrical re spon se should be as lin e ar as possible ove r th e wh ole ran ge of force s an d ( c ) the tran sduce r sh ould re act fast e n ough to be able to re pre se n t con traction re liably [ 7, 8] . The goal of our work was to de ve lop a force tran sduce r th at would ® t all of th e above re quire m e n ts an d be e asy to m an ipulate , as part of a de vic e , wh e n te stin g m uscle s th at are dif® cult to acce ss.

Mate rials an d m e th o d s The desi gn of the can ti lever Th e tran sduce r sh ould be able to re ach the de sire d con tractin g tissue s, the re fore we de signe d th e can tile-

* Au thor for corresponden ce.

ve r an d its h olde r in such a way th at it could be m ove d lin e arly an d rotated in a lim ite d space . Th e afore m e ntion e d ch aracte ristics of the tran sduce r e n able the m e asu re m e n ts of the con tractin g tissue s always to be pe rpe n dicular to th e axis of th e m e asu re d m uscle con traction . Th e can tileve r ( ® gure 1 ) was m ad e of h igh ly te m pe re d stain le ss ste e l ribbon solde re d to a stainle ss ste e l bar using silve r solde r. The dim e nsions of th e can tile ve r section , wh ich could be be nt upon ap plyin g force , we re de ® n e d accordin g to the spe ci® cation of th e gau ge ’ s m an ufacture r ( Ce le sco, USA ) an d our n e e d to de ve lop a tran sduce r that would be sen sitive e n ough to m e asu re th e force s produce d. The sensors Th e se nsor was m ad e up of a full Wh e atstone bridge com pose d of four sem icon ductor strain gau ge s ( re sistan ce in ohm s: 500.0 pm 6 0.3% ) , at a bridge e xcitation voltage of 5 V, bonde d on th e can tile ve r. Strain gauge s we re bon de d accordin g to th e proce dure spe ci® e d by th e m an ufacture rs of the strain gau ge s an d the ad he sive , re spe c tive ly. Th e m e ch an ical ten sion produce d by th e force produce s an e lastic de form ation of th e can tile ve r, thus re sultin g in a chan ge of th e output voltage , wh ich could be am pli® e d an d con ne cte d to an A/ D con ve rte r an d an IBM com patible pe rson al com pute r. Testin g of the sensor stati c an d dyn ami c characteri stics Th e static ch aracte ristic of th e tran sduce r was obtain e d sim ply by h an gin g we ights of 0.1, 1, 2, 3, 4, 5, 6 an d 7 g on th e tran sduce r at a le ve l assum e d to be th e acting poin t of the force s. The com plian ce was m e asu re d by a h an ging we ight of 1 g an d m e asu ring th e be nd ing of the can tile ve r with a m icrom e te r. Dyn am ic be h aviour of th e se nsor was de ® n e d by producing m e ch an ical vibration in the can tile ve r by ¯ ickin g it with on e ’ s ® n ge r. Th e sign al from the tran sduce r was am pli® e d at a gain of 100 an d re corde d as de scribe d above . The re corde d data was an alyse d usin g a software package ( Matlab, USA) e nablin g Fourie r an alysis. Mu scle preparation s Diaphragm. Male Wistar rats we igh in g 280± 31 0 g we re use d in all e xpe rim e n ts. Th e an im als we re an ae sthetize d with a m ixture of Rom pun ( Baye r, Le ve rkuse n ) ± 1 ( Parke-Davis, 15 m g kg an d Ke tan e st Be rlin ) ± 1 100 m g kg body we igh t. The substance s we re ap plie d i.p. toge th e r with He parin ( Krka, Novo m e sto ) 10 IU / 100 g body we igh t as de scribe d be fore [ 9, 10] . The

22 2 Journ al of Medical En gin eerin g & Techn ology ISSN 0309-1902 prin t/ ISSN 1464-522X on lin e h ttp:/ / www.tand f.co.uk / JNLS / m e t.h tm http:/ / www.tayloran dfrancis.com / JNLS / m e t.h tm

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1999 Taylor & Fran cis Ltd

M. Bu n c et al.

Force tran sduce r for m easure m en t of m uscle con trac tion

diap hragm s for in vitro e xpe rim e n ts we re isolate d from rats th at we re an ae sth e tize d an d killed by e xan gu in ation through th e com m on carotid arte ry ( n 5 4 ) . Afte r th at th e che st was ope n e d an d the le ft diap hrag m with n . ph re n icus was isolate d an d tran sferre d to a bath ® lle d with Rin ge r solution at 37 8 an d oxyge n ate d with carboge n. N. ph re n icus was stim ulate d with a suction e le ctrode by 2± 3 V in divid ual im pulse s th at laste d 0.1 s at 0.1 Hz. Afte r an e quilibration pe riod, diffe re nt con ce n tration s of e quato xin II we re ad de d to th e bath [ 11 ] . Th e ten sion of th e isolate d diap h ragm was m e asu re d by th e m e ch an o-e le ctric tran sduce r.

Re su lts

Pig coron ary artery. Pigs ( 80± 100 kg ) we re kille d an d e xsan gu in ate d in a local slau gh terh ouse . He arts ( 260 ± 39 0 g, n 5 10 ) we re rap idly e xcise d an d place d in ic e cold Kre bs± He n se le it ( K± H ) solution an d transporte d to th e laboratory. Th e le ft an terior de sce n din g ( LAD ) coron ary arte ry was isolate d an d thre e ad jace n t rings ( 5 m m wid e ) , 5 m m at the be gin n in g of th e LAD coron ary arte ry, we re cut an d transfe rre d to a K± H solution with th e followin g com position ( m M ) : NaCl 11 8, KCl 4.7, KH 2 PO 4 1,2, MgSO 4 1,2, CaCl 2 2.5, NaHCO 3 25 an d glu cose 11. The K± H solution ( pH =7.4 5 ) was m ain tain e d at 37 8 an d gasse d with 95% O 2 an d 5% CO 2 . Th e rings we re stre tch e d gradu ally ove r 1 h to obtain a stable 50 m N initial te nsion . Rin gs we re contracte d with 60 m M KCl [ 12] . Th e con tractions we re m e asu re d isom e trically with th e m e ch an o- e le ctric tran sduce r.

Measu remen ts of con traction s on isolated rat diaphragm

Static an d dyn ami c characteristi cs of the sen sor It is obvious from the static ch aracte ristic obtain e d th at th e com ple te d transduce r gave a ve ry lin e ar de pe nde n ce of th e output voltage with the load actin g square ly on th e can tile ve r. O ve r the n om in al ran ge of 0± 70 m N ± 1 th e sen sitivity of the tran sduce rs was 0.5 m V m N . The ( n atu ral fre que n cy of th e tran sduce r is 350 Hz ® gure 2 ) ± 1 an d its com plian ce is about 0.25 l m g .

With the tran sduce r it is possible to m e asu re not only sin gle twitch e s produce d by e le ctrostim u lation but also a sum m ation of twitch e s such as subtetan ic contraction . Fast ch an ge s of m uscle te n sion up to 10 Hz stim ulation fre que nc ie s are de tec ted with out artefacts, as se e n in ® gu re 3. Measu remen ts of su stain ed ten sion on isolated coron ary artery Isolate d coron arie s we re con tracte d afte r addition of KCl, wh ich cau se d de polarization of m uscle m e m bran e s an d in¯ ow of calcium in to m uscle ® bre s. Sustaine d con traction was suitably re corde d, with out an y drift, by th e tran sduce r ( se e ® gu re 4 ) .

Discu ssio n According to th e re quire m e n ts de te rm in e d in the m e th ods se ction we can con clude th at all of th e se we re m e t. Be cau se of its high n atu ral fre que n cy of 350 Hz an d h igh se nsitivity, th e system is able to re cord fre que nt ch an ge s of m uscle ten sion , e ve n for on ly a fe w m uscle ® bre s, with out artefacts. It is also suitable for sustain e d tonic con traction s of a m uscle . Th e lim itation of th e tran sd uce r is that it is de signe d for th e m e asure m e n t of contraction s up to 100 m N.

Figu re 1. A pi cture of the mechan o-electric tran sdu cer.

Figu re 2. Dyn ami c characteristic of the tran sdu cer. The frequ en ci es of the can tilever v ersu s the in ten sity of the sign al are presen ted. It cou ld be seen that the n atu ral frequ en cy of the tran sdu cer is equ al to 350 Hz. Below the n atu ral frequ en cy of the tran sdu cer the cu rve i s relativ ely lin ear, en abli n g accu rate measu remen ts of forces within a ban d from 0 to 350 Hz. 223

M. Bu n c et al.

Force tran sduce r for m easure m en t of m uscle con trac tion

pe rpe n dicular to th e axis of th e m e asu re d m uscle con traction . O ur tran sduce r is re lative ly sim ple , re liable an d low cost. Ackno wle d ge m e n ts Th is work was ® n an ce d by Re se arch Gran ts L4-7400 , RU-230 4 an d J3-79 18 from the Min istry of Scie n ce an d Te ch nology.

Re fe re n ce s

Figu re 3. Measu remen ts of ten sion an d frequ en cies of con traction on isolated rat diaphragm stimu lated throu gh N. ph re nicus. Frequ en cies of stimu lati on are marked abov e. Certain time peri ods are also presen ted as A, B an d C.

Figu re 4. Measu remen ts of ten sion on isolated coron ary arteries after addi tion of 60 mM KC1 . Stars mark the time when KC1 was washed ou t by the K-H solu tion . ± 1

Th e tran sd uce r, with com plian ce 0.2 5 ¹ m g , e n able s re cording of force s in a m ixe d isom e tric an d isoton ic m ode . Th e high e r th e force the m ore isoton ic con traction ap pe ars. The ad van tage of th e tran sd uce r is that it could be orie n tate d in space to re ach an y poin t of a h e m isph e re with a diam e ter of 15 cm from the ve rtical h olde r. Be cau se of th is the m e asu re m e n ts of con tracting tissue s we re always

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1. H UXLEY, A. F., and SIMMO NS, R. M., 1968, A capacitan cegauge te nsion tran sduce r. Jou rn al of Physiology, 197 ( 1 ) , 12P. 2. MEISS , R. A., 1974, A ve rsatile tran sduce r syste m for m e chanical studie s of m uscle . Jou rn al of Appli ed Physiology, 37 ( 3 ) , 459± 463. 3. MEISS , R. A., and SO NNENBLICK, E. H., 1972, Con trolle d sh orte n ing in h e art m uscle : ve locity-force an d active-state prope rtie s. American Jou rn al of Physiology, 222 ( 3 ) , 630± 639. 4. MEISS , R. A., 1971, An isom e tric m uscle force tran sduce r. Jou rn al of Appli ed Physiology, 30 ( 1 ) , 158± 160. 5. SO DEN , P. D., and KERSHAW, I., 1974, Te n sile te sting of con n e ctive tissu e s. Medi cal & Biological En gin eerin g, 12 ( 4 ) , 510± 518. 6. RO ZMAN, J., ZO RKO , B., and NGHIEM, T., 1994, Isom e tric twitch con trac tion s of se le ctive ly stim ulate d m uscle s in dog’ s le g. Basic an d Applied Myology, 4 ( 2 ) , 155± 163. 7. RO ZMAN, J., BRATANICÏ , J., SO VINEC , B., LENART , B., JEGLICÏ , A., an d FEFER, D., 1993, Four chan ne l tran sduce r for e valu ation of m uscle contraction s, e dite d by R. Jae ge r and T. Bajd Proceedin gs of the FES Con ference, Lju bljan a ( Ljubljan a: Slove n ian Socie ty of Me dical and Biological Engin e e ring ) , pp. 126± 129. 8. B UNC , M., RO ZMAN, J., an d SÏUPUT , D., 1998, Me asu re m e n ts of gill m ove m e nt in ® sh . Proceedin gs of the 6th Vien n a Workshop on FES: basics, technology, appli cation (Vien n a: Univ ersity of Vienn a ), pp. 279± 282. 9. B UNC , M., DREVENSÏ EK, G., B UDIHNA, M. V., an d SÏUPUT , D., 1999, Effe cts of bolus in je ction of e quin atoxin II on isolate d rat h e arts: th e im portan ce of dire ct card iotoxic e ffe cts in e quin atoxin II le th ality. Toxicon , 37, 109± 123. 10. BUNC, M., FRANGEZÏ, R., T URK, T., an d SÏUPUT , D., 1994, Effe cts of e qinatoxin in vivo. An n als of the New York Academy of Science, 710, 162± 167. 11. BUNC, M., FRANGEZÏ, R., RO ZMAN, J., an d SÏUPUT , D., 1998, Th e e ffe cts of e quin atoxin II on ne urom uscular jun ctionÐ possible m e chanism of th e toxin le th ality. Eu ropean Jou rn al of Physiology ( Fe bruary 2000 ) . 12. JAPELJ, I., BUNC, M., BUDIHNA, M. V., an d SÏUPUT , D., 1998, Equinatoxin II con trac ts th e large e picardial porcin e coron ary arte ry on ly in the pre se nce of th e in tact e ndothe lium . Pharmacological Toxicology, 83 ( 1 ) , 97.

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