Micromechanics of Engineered and Biological Systems

Micromechanics of engineered and biological systems G K ANANTHASURESH1 and SUMAN CHAKRABORTY2 1Mechanical Engineering, Indian Institute of Science, Bangalore, India. e-mail: [email protected] 2Mechanical Engineering, Indian Institute of Technology, Kharagpur, India. e-mail: [email protected] This review paper is concerned with the mechanics issues at the micron scale. The topics consi- dered here are microsystems (microelectromechanical systems – MEMS), microfluidic devices, and biological cells. Mechanics is important for all the three fields. Micromechanics of solids is important for microsystems while microfluid mechanics is essential for microfluidics. Understanding the mechanics of cells requires both and much more. We highlight here some of the major advances and important challenges in all the three fields. 1. Introduction phenomena and systems – engineered or biological. Another critical aspect that ties the three topics of This review paper focuses on three aspects of this paper together is microfabrication and micromechanics of systems of small size: microsystems machinery. Without the ability to make intricate (microelectromechanical systems – MEMS – as structures of small size, to actuate and to control they are popularly known), microfluidics, and bio- them, these devices would not have come into exis- logical cells. The three topics are substantially tence. Without the micromachinery, which includes different from each other but there is significant sensors and actuators, quantitative investigation of interplay among them. The critical dimension of the behaviour of cells is unthinkable. To make it micron size is of course common to all three. How- all work, we need an understanding of mechanics ever, when we consider devices and systems, the issues and that is the focus of this paper. size sometimes tends to be much bigger than a micron; and sometimes it can be much smaller than a micron going down to a nanometer when 1.1 Microsystems we try to understand a phenomenon or to make an idea work in a device. The bigger than micron Here, we use the word ‘microsystem’ rather than size mainly comes because of microsystems. While ‘MEMS’ because the latter is restrictive as it the critical dimensions of micromachined sensors, only conveys mechanical and electrical aspects. actuators, and devices are indeed of micron size, Microsystems are good examples of integrated their packaged size tends to be much bigger. Cells engineered systems of small size. Although this are of micron size but their mechano-transduction field started in early 1970s, its formal identity came takes place because of entities much smaller than only in late 1980s [1]. In the two recent decades a micron – down to proteins of nanometer size. of its intense activity and growth, the emphasis Tissues are necessarily bigger than cells and they has been on sensors and actuators and some inte- exhibit behaviour that can be understood from grated systems. Undoubtedly, rapid advances in the investigations on the single cells but not all microfrabrication techniques for silicon and other of it. So, it suffices to say that they are all small materials have played a major role in the develop- and hence different from the traditional ‘macro’ ment of the microsystems field. It is also pertinent Keywords. Microsystems; MEMS; microfluidics; cell mechanics. 83 84 G K ANANTHASURESH AND SUMAN CHAKRABORTY to note that mechanics too has played an equally deformable structure is released by dissolving a important role. The origin of much of the diffi- sacrificial (fugitive) layer that holds the releasable culty in simulating and designing a microsystems structure until the end of the process. The surface component or device lies in two things: coupling tension forces arising from the liquid in the narrow with other energy domains and scaling effects. gaps is large enough to pull the deformable struc- There is not a useful microsystems device that ture down and make it stick to the substrate. Mois- operates without involving more than one energy ture and electrical charge accumulation also lead domain. A simple beam-based switch needs actu- to stiction. Narrow gaps also lead to another com- ation to work. If this actuation is electrostatic plication because the fluid (mostly air surrounding force, coupling between elastic structures and elec- the device) causes squeezed film effects [4]. In gen- trostatic field ensues. A diaphragm-based micro- eral, flow behaviour is intrinsically coupled with pump involves a strong coupling between solid the microsystems area. More details on microflu- and fluid mechanics fields and also couples with idics follow next. a field associated with what actuates it. Often the coupling between the fields is strong and, in 1.2 Microfluidics some cases, unprecedented at the macroscale. It is important to note that mechanics couples with The ever-increasing possibilities of creating struc- other energy domains in all these problems. Con- tures and patterns on small length scales with sequently, new approaches – both analytical and ultra-tunable precision have triggered a wide numerical – are warranted to efficiently simulate or range of scientific investigations involving the design microsystems devices. A key component of transportation and manipulation of fluids in this effort is the need for model reduction. As mul- miniaturized devices and systems. These types tiple energy domains are solved simultaneously to of investigations, broadly classified under the simulate the behaviour of an integrated microsys- theme of microfluidics (concerning flow length tems device, usually requiring the solution of two scales over micrometer ranges) or nanofluidics or more coupled partial differential equations in (concerning flow length scales over nanometer spatial and temporal domains, the computational ranges), have rekindled interests in a classical complexity tends to be very high. The degrees of area of fluid dynamics: low-Reynolds-number flows, freedom involved will be large too. The results with a fusion and agglomeration of concepts of such simulations are useful to see if a device from molecular physics, surface chemistry, and works or not but they are not necessarily amenable life sciences. As a commonly acceptable metric, for gaining insight and for designing efficiently. microfluidics refers to the control of small volumes −15 −6 Hence, microsystems researchers often resort to of liquids (fL–μL, 10 –10 L). Traditionally, model reduction. It can be done analytically in silicon micromachining methods have been used some cases [2] but not always. Therefore, tech- to fabricate micron-scale channels from silicon niques for model reduction are developed. Some of and glass for transporting these ultra-small fluidic these are available today in microsystems design volumes. Other types of materials (such as poly- software but there is a lot more to be desired in this dimethylsiloxane (PDMS), polymethyl mythacry- direction. late (PMMA)) are also being extensively used. Scaling effects are many as we move down to The latter systems offer potential advantages of the micro size. Although continuum mechanics faster design times, low cost, the ability to fabricate assumptions do not break down at this scale for nanoscale features, and the possibilities of obtain- deformable solids, a number of problems come to ing deformable shapes. the fore because of the small size. Residual stresses The principal components of the present day and the stress gradients are common in micro- microfluidics are lab-on-a-chip devices, which are fabrication processes. These tend to be very high nothing but miniaturized analytical laboratories, as compared with the stiffness of the microme- typically consisting of a network of microchannels chanical structures. Hence, unintended deforma- and reservoirs on small glass or plastic chips. tions occur if the effects of residual stresses are Additionally, these chips may contain electrodes, not properly accounted for. The inhomogeneity of sensors and electrical circuits, for performing the residual stresses is hard to characterize and control desired protocols. Such devices are capable of in a microfabrication process. The same is true of duplicating some specialized functionalities as their anisotropic properties of materials. There is often larger-sized counterparts in the macroscopic world, disagreement on basic properties such as Young’s such as clinical diagnostics, DNA scanning, cel- modulus when it is determined with different lular analysis, and separation processes. How- methods [3]. Stiction is another major problem ever, with a dramatically reduced sample volume in deformable structures with features separated requirement, the lab-on-a-chip concept leads to by narrow gaps. This is especially so when a much shorter reaction and analysis times, higher MICROMECHANICS OF ENGINEERED AND BIOLOGICAL SYSTEMS 85 throughput, automation, design flexibility, and first time and coined the term ‘cell’. Since then portability. Modern developments in the design mechanics and biology have moved on more or less and utilization of microfluidic devices have found separately but are now coming together to enrich many applications, ranging from the life sciences each other. It is also pertinent to note that Francis industries for pharmaceuticals and biomedicine Crick,theco-discovererofthedoublehelixstruc- (drug design, delivery and detection, diagnostic ture of DNA also has the credit of performing devices) to industrial applications of combinatorial one of the earliest mechanics

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