micromachines Review Vorticella: A Protozoan for Bio-Inspired Engineering Sangjin Ryu 1,*, Rachel E. Pepper 2, Moeto Nagai 3 and Danielle C. France 4 1 Department of Mechanical and Materials Engineering, University of Nebraska-Lincoln, Lincoln, NE 68588, USA 2 Department of Physics, University of Puget Sound, Tacoma, WA 98416, USA; [email protected] 3 Department of Mechanical Engineering, Toyohashi University of Technology, Toyohashi 441-8580, Japan; [email protected] 4 National Institute of Standards and Technology, Boulder, CO 80305, USA; [email protected] * Correspondence: [email protected]; Tel.: +1-402-472-4313 Academic Editors: Hyoung Jin Cho and Sung Kwon Cho Received: 28 October 2016; Accepted: 20 December 2016; Published: 26 December 2016 Abstract: In this review, we introduce Vorticella as a model biological micromachine for microscale engineering systems. Vorticella has two motile organelles: the oral cilia of the zooid and the contractile spasmoneme in the stalk. The oral cilia beat periodically, generating a water flow that translates food particles toward the animal at speeds in the order of 0.1–1 mm/s. The ciliary flow of Vorticella has been characterized by experimental measurement and theoretical modeling, and tested for flow control and mixing in microfluidic systems. The spasmoneme contracts in a few milliseconds, coiling the stalk and moving the zooid at 15–90 mm/s. Because the spasmoneme generates tension in the order of 10–100 nN, powered by calcium ion binding, it serves as a model system for biomimetic actuators in microscale engineering systems. The spasmonemal contraction of Vorticella has been characterized by experimental measurement of its dynamics and energetics, and both live and extracted Vorticellae have been tested for moving microscale objects. We describe past work to elucidate the contraction mechanism of the spasmoneme, recognizing that past and continuing efforts will increase the possibilities of using the spasmoneme as a microscale actuator as well as leading towards bioinspired actuators mimicking the spasmoneme. Keywords: Vorticella; oral cilia; feeding current; contractile stalk; spasmoneme; Ca2+-powered contraction; bioinspired engineering 1. Introduction 1.1. Overview of Vorticella Among various microorganisms, protozoa have been widely studied as model systems for biology, physics, engineering and biomimetics. They form a diverse group of unicellular eukaryotic organisms including ciliates, flagellates and amoeboids. Among such protists, Vorticella has drawn the attention of scientists since Anthony van Leeuwenhoek first described its unique motility in his letter dated 9 October 1676 [1,2]. In a recent study, Vorticella was discovered in a fossil that is more than 200 million year old, bolstering the fossil record of soft-bodied organisms [3]. While we primarily describe biomechanical and biomimetic aspects of Vorticella, uncovered since the 1950s, it is worth noting that Vorticella has played a long and ongoing role in many fields of biological, physical and engineering study. Vorticella is a suspension-feeding ciliate that lives in two forms: free swimming telotroch and sessile stalked trophont [4]. A sessile Vorticella consists of the zooid (inverted-bell-shaped cell body; usually about 30–40 µm in diameter when contracted) and the stalk (3–4 µm in diameter and about 100 µm in length) (Figure1A). The zooid has two bands of cilia around the peristome, the mouth-like Micromachines 2017, 8, 4; doi:10.3390/mi8010004 www.mdpi.com/journal/micromachines MicromachinesMicromachines 20172016,,8 8,, 44 22 ofof 2525 mouth-like part of the zooid, which are used for suspension feeding. The cilia of the inner band partgenerate of the water zooid, flow which to aredraw used food for particles suspension towa feeding.rd the zooid The cilia (Figure of the 1B), inner and band these generate particles water are flowfiltered to drawby the food cilia particles of the towardouter band. the zooid An (Figureexample1B), of and such these a feeding particles current are filtered observed by the in cilia the oflaboratory the outer band.shows Ana examplevortex (Figure of such a1B), feeding and currentit was observed observed in thethat laboratory this flow shows could a vortexmove (Figuremicro-diameter1B), and itparticles was observed at least that450 thisμm flowaway could from movethe peristome micro-diameter with the particles maximum at leastflow 450velocityµm awayof 360 from μm/s the [5,6]. peristome with the maximum flow velocity of 360 µm/s [5,6]. Figure 1. Sessile ciliate protozoan Vorticella. (A) Structure of Vorticella convallaria (reproduced from [7]); Figure 1. Sessile ciliate protozoan Vorticella.(A) Structure of Vorticella convallaria (reproduced (B) pathlines of water flow generated by cilia beating of V. convallaria (reproduced from [8]); from [7]); (B) pathlines of water flow generated by cilia beating of V. convallaria (reproduced from [8]); (C) sequential images of stalk contraction of V. convallaria (reproduced from [9]). The stalk begins to (C) sequential images of stalk contraction of V. convallaria (reproduced from [9]). The stalk begins to coilcoil atat 00 ms.ms. In the sessile form, the vertex or scopula of the zooid is connected to the proximal end of the stalkIn while the sessilethe distal form, end the is vertexrooted oron scopulaa habitat of su therface zooid (Figure is connected 1A). Therefore, to the proximalthe stalk endtethers of thethe stalkzooid while to the the habitat. distal endThe is stalk rooted also on coils a habitat very surfacequickly (Figure and moves1A). Therefore, the zooid the toward stalk tethersthe surface the zooidthrough to thethehabitat. action of The its stalk contractile also coils organelle, very quickly the spasmoneme and moves the (Figure zooid toward1C). Afterwards, the surface the through stalk theslowly action relaxes of its and contractile moves the organelle, zooid away the from spasmoneme the surface (Figure over a1 fewC). Afterwards,seconds [10–12]. the stalkAlthough slowly the relaxesstalk contraction-relaxation and moves the zooid cycle away fromof Vorticella the surface has overbeena hypothesized few seconds [10as– 12a ].way Although either theto avoid stalk contraction-relaxationdangers, or to mix the cyclesurrounding of Vorticella fluid,has no been explanation hypothesized is completely as a way satisfactory. either to avoid dangers, or to mix theDuring surrounding stalk contraction, fluid, no explanation Vorticella stops is completely ciliary beating satisfactory. and contracts the zooid to a nearly sphericalDuring shape, stalk and contraction, the stalkVorticella shortensstops to 20%–40% ciliary beating of its andextended contracts length the zooidin less to than a nearly 10 ms spherical (Figure shape,1C). Therefore, and the stalk Vorticella shortens contracts to 20%–40% at the of its average extended rate length of in10–20 less thanmm/s, 10 msand (Figure its zooid1C). Therefore,reaches a Vorticellamaximumcontracts speed atof the60–90 average mm/s rate [9,12–14]. of 10–20 In mm/s, terms and of itsspecific zooid velocity reaches a(body maximum length/s), speed this of 60–90maximum mm/s speed [9,12– corresponds14]. In terms ofto specific~1200 body velocity length/s, (body length/s),making Vorticella this maximum among speed the fastest corresponds living tocreatures ~1200 body using length/s, this metric making [12].Vorticella It has alsoamong been the estimated fastest living that creatures in terms using of specific this metric power [12 ].(power It has alsodissipation been estimated per mass) that the in termsstalk ofof specific Vorticella power performs (power better dissipation than both per mass) automotive the stalk engines of Vorticella and performsstriated muscle better than[15]. both Recent automotive measurements engines showed and striated that musclelive spasmonemes [15]. Recent measurementscould generate showed a peak thatcontractile live spasmonemes force of ~30 could nN with generate maximum a peak contractilepower of ~1.6 force nW of ~30during nN withnormal maximum contraction, power and of ~1.6isometric nW during tension normal of 150–350 contraction, nN [9,16]. and isometric tension of 150–350 nN [9,16]. InIn additionaddition to to its its remarkable remarkable motility, motility, the energythe energy source source for the for stalk the contraction stalk contraction makes Vorticella makes unique.Vorticella While unique. most biologicalWhile most contractile biological systems contract dependile onsystems ATP (adenosine depend triphosphate)on ATP (adenosine for their biochemicaltriphosphate) fuel, for thetheirVorticella biochemicalspasmoneme fuel, the Vorticella is powered spasmoneme by calcium is ions powered [17]. In by the calcium absence ions of ATP,[17]. extractedIn the absenceVorticella of ATP,stalks extracted can repeat Vorticella the contraction–relaxation stalks can repeat the cycle cont andraction–relaxation generate contractile cycle force, and generate contractile force, driven only by external calcium ions diffusing into the spasmoneme [17– Micromachines 2017, 8, 4 3 of 25 Micromachines 2016, 8, 4 3 of 25 23].driven Because only byof externalits surpassing calcium contractilit ions diffusingy and into unique the spasmonemeenergy source, [17 therefore,–23]. Because the Vorticella of its surpassing stalk is regardedcontractility as a and candidate unique for energy Ca2+-powered source, therefore, cell motility the andVorticella biomimeticstalk is actuating regarded materials as a candidate [24–29].