Weird Science

Pneumatic Artificial Muscles in Our Lab

Polish ver­sion is here

The fol­lo­wing article was ori­gi­nally publi­shed in the jour­nal for edu­ca­tors Bio­lo­gia w Szkole (Bio­logy in School) (3/2021):

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Ples M., Sztuczne mięśnie w naszej pra­cowni (eng. Pneu­ma­tic Arti­fi­cial Musc­les in Our Lab), Bio­lo­gia w Szkole (Bio­logy in School), 3 (2021), Forum Media Pol­ska Sp. z o.o., pp. 52-56

In our explo­ra­tions, we have fre­qu­en­tly dealt with the topic of loco­mo­tion, exa­mi­ning the mecha­ni­sms of move­ment and rela­ted phe­no­mena among repre­sen­ta­ti­ves of the plant king­dom (Planta) and the ani­mal king­dom (Ani­ma­lia) [1] [2] [3] [4]. So far, we have focu­sed pri­ma­rily on plant move­ment because those mecha­ni­sms often feel more unfa­mi­liar. This makes sense, given that we our­se­lves belong to the ani­mal king­dom. Today I invite you, Dear Rea­der, to join me in a few sim­ple expe­ri­ments invo­lving musc­les. These tis­sues pro­vide the essen­tial force that dri­ves move­ment in ani­mal bodies, whe­ther it invo­lves loco­mo­tion or other types of motion.

The muscle muscu­lus is an organ endo­wed with the abi­lity to con­tract acti­vely. It is one of the struc­tu­ral and func­tio­nal com­po­nents of the loco­mo­tor sys­tem as a whole. Nota­bly, musc­les form the active, or drive, ele­ments of this sys­tem. They are found in higher inver­te­bra­tes Inver­te­brata and in all ver­te­bra­tes Ver­te­brata.

The tis­sue that builds these organs con­si­sts of highly spe­cia­li­zed cells. Musc­les are con­nec­ted in a spe­ci­fic way to the ske­le­ton, which acts as a sup­por­ting struc­ture: inter­nal in ver­te­bra­tes and exter­nal in arth­ro­pods Arth­ro­poda, for instance. By chan­ging their dimen­sions, they cause indi­vi­dual ske­le­tal ele­ments to move rela­tive to one ano­ther. The source of energy for musc­les is che­mi­cal sub­stan­ces, pri­ma­rily the gly­co­gen sto­red in them as well as glu­cose sup­plied by the cir­cu­la­tory sys­tem. The shape and struc­ture of a muscle stric­tly depend on the func­tion it per­forms in the orga­nism [5].

Musc­les can be clas­si­fied in many ways depen­ding on their struc­ture, func­tion, or shape. Howe­ver, the most essen­tial divi­sion reco­gni­zes three main types:

In terms of struc­ture, the sim­plest musc­les in the human body are the smo­oth musc­les respon­si­ble for move­ments inde­pen­dent of our will, such as pupil dila­tion or the peri­stal­tic move­ments of the inte­sti­nes. In con­trast, stria­ted musc­les are much more com­plex, so it is no sur­prise that they repre­sent a signi­fi­can­tly later evo­lu­tio­nary inno­va­tion and ena­ble our loco­mo­tion. A sepa­rate type display­ing uni­que cha­rac­te­ri­stics is the car­diac muscle that pumps blood.

Ske­le­tal (stria­ted) muscle is a type of muscle tis­sue com­po­sed of stron­gly elon­ga­ted cylin­dri­cal cells. The cell nuc­lei are loca­ted peri­phe­rally, while in the cen­ter of the cell there are nume­rous myo­fi­brils that extend along its entire length. These myo­fi­brils are com­po­sed of alter­na­ting thin actin and thick myo­sin fila­ments. This par­ti­cu­lar fea­ture of their struc­ture appe­ars under the micro­scope as the cha­rac­te­ri­stic stria­tion. Both actin and myo­sin are motor pro­te­in­s—they have the abi­lity to move in rela­tion to each other, essen­tially “sli­ding” past one ano­ther. This results in a reduc­tion in the length of the muscle fibers, whe­reas the dimen­sions of the fila­ments them­se­lves remain unchan­ged. Stria­ted musc­les work in a man­ner depen­dent on the will, but they become fati­gued rela­ti­vely quic­kly [6].

Musc­les as dri­ving ele­ments cau­ght the atten­tion of scien­ti­sts invo­lved in bio­mi­me­tics during the second half of the 20th cen­tury.

This field, also known as bio­nics, is a mul­ti­di­sci­pli­nary science that stu­dies the struc­ture and prin­ci­ples of how orga­ni­sms func­tion. The goal is to adapt these prin­ci­ples for use in engi­ne­e­ring and the design of tech­ni­cal devi­ces that repli­cate orga­ni­sms or, in prac­tice, merely cer­tain ele­ments the­reof. Bio­mi­me­tics aims to inve­sti­gate, thro­ugh scien­ti­fic expe­ri­men­ta­tion, the pro­ces­ses gover­ning the func­tio­ning of orga­ni­sms and to use them in various areas of human acti­vity, such as auto­ma­tion, elec­tro­nics, mecha­nics, and con­struc­tion. An intri­gu­ing exam­ple is the inven­tion of Vel­cro in 1941 by George de Mestral, com­monly used today for so-cal­led hook-and-loop faste­ners. This solu­tion mimics the method by which seeds of the gre­a­ter bur­dock Arc­tium lappa disperse in nature: they attach them­se­lves to ani­mal fur and tra­vel along with it, a note­wor­thy exam­ple of zoo­chory [7].

Today, we com­monly use various types of drive mecha­ni­sms, such as inter­nal com­bu­stion engi­nes in vehic­les or elec­tric motors in small devi­ces. These sys­tems typi­cally pro­duce use­ful work by rota­ting a motor shaft. This method is quite dif­fe­rent from the way living orga­ni­sms gene­rate force and motion, which relies on the action of musc­les. Despite this dif­fe­rence, natu­ral musc­les are remar­ka­bly effi­cient, offe­ring high force out­put rela­tive to their small mass and volume. These cha­rac­te­ri­stics make them highly attrac­tive for engi­ne­e­ring and indu­strial appli­ca­tions. As a result, scien­ti­sts in the field of bio­mi­me­tics are wor­king inten­si­vely on the deve­lop­ment of arti­fi­cial musc­les. These are desi­gned to repli­cate at least some of the pro­per­ties of bio­lo­gi­cal musc­les and to serve as func­tio­nal drive sys­tems. One nota­ble exam­ple is an arti­fi­cial muscle made from nylon, which was descri­bed in the jour­nal Science [8]. Photo 1 shows a sam­ple arti­fi­cial muscle fiber (AMF) cre­a­ted in my lab.

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Phot.1 – Arti­fi­cial Muscle Made From Nylon Fiber (AMF)

The arti­fi­cial nylon muscle is ela­stic and stret­ches under a load, as shown in Phot.2A.

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Phot.2 – AMF; A – rela­xa­tion phase under the influ­ence of a 60 g, approx. 2.12 oz, wei­ght; B – con­trac­tion phase

Aro­und the muscle, an addi­tio­nal pla­stic thread coa­ted with metal­lic silver has been wound. When an elec­tri­cal cur­rent is pas­sed thro­ugh it, the entire struc­ture heats up sli­gh­tly, cau­sing the muscle to con­tract and lift the wei­ght, thus gene­ra­ting usa­ble work (Phot.2B). The muscle’s action is rever­si­ble: once the cur­rent flow is swit­ched off, the muscle cools quic­kly and rela­xes.

Ilustracja
Anta­go­ni­stic action of two arti­fi­cial nylon musc­les
Ani­ma­tion: sup­ple­men­tary mate­rial

Aside from the expe­ri­men­tal arti­fi­cial nylon muscle descri­bed above, there are also other tech­no­lo­gies alre­ady in use on an indu­strial scale today. One of these is pneu­ma­tic arti­fi­cial musc­les (some­ti­mes cal­led “mu­sc­les”). Their prin­ci­ple of ope­ra­tion and gene­ral con­struc­tion are not overly com­pli­ca­ted, so we can even attempt to build a sim­ple model of such a drive device.

Con­struc­tion and Obse­rva­tions

To build a wor­king model of a pneu­ma­tic arti­fi­cial muscle, we need the fol­lo­wing mate­rials:

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Phot.3 – Mate­rials nee­ded to build the device; see descrip­tion in the text

The dimen­sions of all com­po­nents sho­uld be selec­ted accor­ding to indi­vi­dual needs.

It is worth discus­sing the bra­i­ded sle­eve (Phot.3C), which is used for pro­tec­ting and orga­ni­zing various types of cables (for exam­ple, in com­pu­ter assem­blies). It is com­po­sed of inter­sec­ting pla­stic fibers (Phot.4).

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Phot.4 – Close-up of the bra­i­ded sle­eve struc­ture; A – elon­ga­tion, B – com­pres­sion

An impor­tant fea­ture of this bra­i­ded sle­eve, which plays a key role in con­struc­ting an arti­fi­cial muscle, is that it beha­ves as a dyna­mic struc­ture. The fibers inter­sect at dif­fe­rent angles depen­ding on exter­nal con­di­tions. When the bra­i­ded sle­eve is com­pres­sed, the fibers form angles clo­ser to 90 degrees (Photo 4B). After being stret­ched, the angles change signi­fi­can­tly (Photo 4A). This results in a change in dimen­sions: the length of the bra­i­ded sle­eve beco­mes shor­ter while its dia­me­ter incre­a­ses, and vice versa (Photo 5).

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Phot.5 – Dimen­sio­nal chan­ges in the bra­i­ded sle­eve; top – elon­ga­tion, bot­tom – com­pres­sion

All the com­po­nents sho­uld be assem­bled accor­ding to the sche­ma­tic in Rys.1. The stret­cha­ble tube is pla­ced inside the bra­i­ded sle­eve, both of which are cut to the same length. We then insert one end of the fle­xi­ble tubing into the stret­cha­ble tube, and clamp eve­ry­thing toge­ther with zip ties (you can also use mul­ti­ple win­dings of strong thread). The con­nec­tion must be air­ti­ght to allow infla­ting the stret­cha­ble tube inside the bra­i­ded sle­eve. This com­ple­tes the con­struc­tion of the arti­fi­cial muscle model.

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Rys. 1 – Dia­gram of the arti­fi­cial muscle con­struc­tion; black – bra­i­ded sle­eve, yel­low – stret­cha­ble tube, blue – hose, gray – hose clamps

In this con­fi­gu­ra­tion, the arti­fi­cial muscle resem­bles ske­le­tal muscle: it has ele­ments ana­lo­gous to a belly and ten­dons (Phot.6).

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Phot.6 – Com­ple­ted arti­fi­cial muscle (a valve was added for mea­su­re­ment pur­po­ses); A – rela­xa­tion phase, B – con­trac­tion phase
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Ope­ra­tion of the pneu­ma­tic arti­fi­cial muscle
Ani­ma­tion: sup­ple­men­tary mate­rial

When the pres­sure inside the muscle equ­als atmo­sphe­ric value, the muscle is in the rela­xa­tion phase (Photo 6A). Incre­a­sing the pres­sure, for exam­ple by pum­ping air in with a syringe, cau­ses the muscle to con­tract; it expands radially and cle­arly shor­tens in length (Photo 6B).

To esti­mate the degree of con­trac­tion, we mea­sure the muscle in both sta­tes, as shown in Phot.7.

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Phot.7 – Mea­su­ring the length of the muscle in both pha­ses

As we can see, during rela­xa­tion, the muscle length is 80 mm, approx. 3.15 in, whe­reas during con­trac­tion, it is 61 mm, approx. 2.40 in. Its length the­re­fore decre­a­ses by about 25%, a result com­pa­ra­ble to that of cer­tain natu­ral musc­les.

As for the model’s capa­city for per­for­ming work, it is sur­pri­sin­gly high. The descri­bed model, just a few cen­ti­me­ters in length (about 1–2 in), easily lifts a 400 g, approx. 14.11 oz (0.88 lb), mass (Phot.8). In an extreme test, it mana­ged to lift a 2 kg, approx. 70.55 oz (4.41 lb), wei­ght, altho­ugh this requ­i­red higher pres­sure.

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Phot.8 – Arti­fi­cial muscle lifting a 0.4 kg (approx. 0.88 lb) load; A – rela­xa­tion, B – con­trac­tion

Such arti­fi­cial musc­les can be com­bi­ned with ske­le­tal models. It is also pos­si­ble to manu­fac­ture musc­les with more than one “belly.”

Expla­na­tion

As we see, buil­ding a func­tio­nal model of a pneu­ma­tic arti­fi­cial muscle is not dif­fi­cult. Impor­tan­tly, simi­lar actu­a­tors have been employed for seve­ral deca­des with incre­a­sing fre­qu­ency in indu­stry­—so this is not merely a curio­sity.

A pneu­ma­tic muscle has mini­mal mass mainly because its pri­mary com­po­nent is a stret­cha­ble con­duit. The loga­ri­th­mic rela­tion­ship between the inter­nal air level and the force gene­ra­ted by this com­po­nent is simi­lar to that obse­rved in real bio­lo­gi­cal sys­tems, which helps in repli­ca­ting muscle beha­vior and can be use­ful in pro­sthe­tics. More­o­ver, the ela­sti­city in both cases is com­pa­ra­ble, as shown in Photo 9.

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Phot.9 – Ela­sti­city of the arti­fi­cial muscle

Fur­ther­more, because gas is com­pres­si­ble, pneu­ma­tic musc­les allow for par­tial absorp­tion of exces­sive force, ena­bling more pre­cise ope­ra­tion. They would thus be sui­ta­ble for powe­ring active pro­sthe­tic limbs. Unfor­tu­na­tely, such devi­ces also have cer­tain draw­backs; the most signi­fi­cant is the chal­lenge of con­trol­ling them.

I hope that I have mana­ged to pique your inte­rest, Dear Rea­der, in this fasci­na­ting area of study known as bio­mi­me­tics.

Refe­ren­ces:

All pho­to­gra­phs and illu­stra­tions were cre­a­ted by the author.

This text has under­gone sli­ght edi­to­rial modi­fi­ca­tions com­pa­red to the ver­sion publi­shed in the jour­nal to bet­ter suit online pre­sen­ta­tion.

Marek Ples

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