The biomechanics toolbox: experimental approaches for living cells and biomoleculesIntroductionCell population techniquesSubstrate deformationSubstrate compositionSingle cell and single molecule techniquesEmbedded particle trackingMagnetic twisting cytometryMicropatterned substratesMicropipette aspirationOptical trapsOptical stretcherMagnetic trapsMicroneedleAtomic force microscopy and force spectroscopyBiomembrane force probeConcluding remarksAcknowledgementsReferencesActa Materialia 51 (2003) 5881–5905www.actamat-journals.comThe biomechanics toolbox: experimental approaches forliving cells and biomolecules夽K.J. Van Vlieta,b,∗, G. Baoc, S. SureshaaDepartment of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, USAbDepartment of Surgical Research, Children’s Hospital and Harvard Medical School, Boston, MA 02115, USAcDepartment of Biomedical Engineering, Georgia Institute of Technology and Emory University, Atlanta, GA 30332, USAAccepted 31 August 2003AbstractThe mechanical behavior of biological materials has been studied extensively at the tissue, organ and systems levels.Emerging experimental tools, however, enable quantitative studies of deformation of individual cells and biomolecules.These approaches also facilitate the exploration of biological processes mediated by mechanical signals, with force anddisplacement resolutions of 0.1 pN and 0.1 nm, respectively. As a result of these capabilities, it is now possible toestablish the structure-function relationships among the various components of a living cell. In order to fully realizethis potential, it is necessary to critically assess the capabilities of current experimental methods in elucidating whetherand how the mechanics of living cells and biomolecules, under physiological and pathological conditions, plays a majorrole in health and disease. Here, we review the operating principles, advantages and limitations, and illustrative examplesof micro- and nano-scale mechanical testing techniques developed across many research communities to manipulatecell populations, single cells, and single biomolecules. Further, we discuss key opportunities for improved analysis ofsuch experiments, as well as future directions and applications. 2003 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.Keywords: Biomechanics; Macromolecular materials; Elastic behavior; Stress-rupture1. IntroductionThe structure and function of many living cellsdepend directly on their global and local mechan-ical environment. The importance of this mechan-ical stimulus can be appreciated at the tissue level∗Corresponding author.E-mail address: [email protected] (K.J. Van Vliet).夽The Golden Jubilee Issue—Selected topics in MaterialsScience and Engineering: Past, Present and Future, edited byS. Suresh.1359-6454/$30.00  2003 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.doi:10.1016/j.actamat.2003.09.001through well-known examples such as muscle atro-phy and bone resorption in the absence of skeletalloading, and has been implicated at the cellularlevel in terms of processes including adhesion,motility and differentiation. Fundamental under-standing of these basic cellular processes, and ofthe pathological responses of the cell, will befacilitated greatly by developments in the fields ofcell and molecular biomechanics. Despite thesophistication of experimental and computationalapproaches in cell and molecular biology, themechanisms by which cells sense and respond to5882 K.J. Van Vliet et al. / Acta Materialia 51 (2003) 5881–5905mechanical stimuli are poorly understood. To alarge extent, the intricate coupling between the bio-chemical and mechanical processes of the cellimpedes research efforts. In particular, applicationof external mechanical stimuli can induce bio-chemical reactions, including the synthesis of newbiomolecules and the enhanced interaction amongbiomolecules that can generate mechanical forces.Likewise, changes in chemical stimuli, includingpH, temperature, and biomolecular activity, canalter the structure and mechanical integrity of thecell, even in the absence of mechanical stimuli.In contrast with most material systems, themechanical behavior of a living cell cannot becharacterized simply in terms of fixed “properties”,as the cell structure is a dynamic system that adaptsto its local mechanochemical environment. Mech-anistic understanding of the relationships amongextracellular environment and intracellular struc-ture and function, however, requires meaningfulquantification of these closely coupled fields. Tothat end, researchers from such diverse disciplinesas molecular biology, biophysics, materialsscience, chemical, mechanical and biomedicalengineering have developed an impressive array ofexperimental tools that can measure and imposeforces as small as a few fN (10⫺15N) and displace-ments as small as a few Angstroms (10⫺10m).Thus, it is now possible to probe the interactionforces between individual molecules that comprisethe cell and its local environment, as well as themechanical response of the entire cell (See Fig. 1for schematics of prokaryotic and eukaryotic cells,as well as key molecules, that have been analyzedexperimentally with these tools).Due to the rich history and unique perspectivesof the fields that contribute to advances in theexperimental mechanics of living cells and biomo-lecules, the questions of interest, approaches, toolsand even vocabulary particular to these communi-ties can impede communication and progress inthis inherently interdisciplinary venture. To thisend, we review in detail experimental tools andassociated analytical/computational models withthe aim to identify opportunities and challenges forinterdisciplinary collaboration and achievements inthis important field of experimental micro- andnanomechanics of biological materials.There exist a variety of techniques to manipulatethe mechanical environment of cell populations,individual living cells, and individual biomolec-ules. These approaches differ in three importantrespects: operating principles, force and displace-ment maxima and resolutions, and extent of defor-mation (i.e., global vs. local). Table 1 summarizesthe abbreviations, relevance to biological struc-tures, and applications for each of the experimentaltools discussed below. Fig. 2 indicates the rangeof force and displacement covered by theseapproaches, as compared to the range relevant