Biomedical Engineering Reference
In-Depth Information
16.4.3 Cell Mechanics and rheometry
AFM. has. been. extensively. used. as. a. nanotool. to. study. static. mechanical. properties. of.
living.cells.(Costa.2003),.membranes.(Müller.2008),.supported.lipid.bilayers.(Picas.et.al..
2012),. and. a. variety. of. soft. substrates.. In. basic. stiffness. measurements,. the. substrate.
or. cellular. stiffness. is. determined. from. the. slope. of. a. force-distance. curve.. A. steeper.
slope. corresponds. to. a. stiffer. cell,. such. as. that. shown. for. vascular. endothelial. cells. in.
Figure.16.3B.(Callies.et.al..2011)..In.another.popular.approach,.force.indentation.curves.
are.recorded.and.indentation-depth.dependent.moduli.for.cell.stiffness.are.determined.
(Fuhrmann.et.al..2011).
Dynamic.mechanical.measurements.using.AFM.can.also.yield.viscoelastic.behavior.of.
cells..The.main.component.of.AFM-based.viscoelastic.measurements.is.a.small.amplitude.
high-frequency.oscillatory.drive.signal.superposed.with.the.slowly.changing.force.curve.
signal.(Mahaffy.et.al..2004)..AFM.microrheology.has.been.used.to.map.the.viscoeleastic.
behavior. of. ibroblasts. (Mahaffy. et. al.. 2004;. Hale. et. al.. 2009),. neutrophils. (Lee. et. al..
2011),.and.cartilage.tissue.(Stolz.et.al..2004)..More.recently,.Raman.et.al..have.proposed.a.
dynamic.AFM.method.to.quantitatively.map.the.nanomechanical.properties.of.live.cells.
with. a. throughput. that. is.∼10. to. 1000. times. higher. than. that. achieved. with. quasi-static.
AFM.techniques.(Raman.et.al..2011)..Here,.the.local.properties.of.a.cell.are.derived.from.
the.zeroth,. irst,.and.second.harmonic. components. of.the.Fourier. spectrum. of.the.AFM.
cantilevers. interacting. with. the. cell. surface.. Local. stiffness,. stiffness. gradient,. and. the.
viscoelastic.dissipation.of.live.
Escherichia coli
.bacteria,.rat.ibroblasts,.and.human.red.blood.
cells.were.all.mapped.by.Raman.et.al..in.buffer.solutions.using.this.method..In.another.
approach,.FM-AFM.has.been.used.to.image.as.well.as.map.the.local.mechanical.properties.
of.virus.particles.with.a.force.sensitivity.of.20.pN.(Martinez-Martin.et.al..2012).
16.4.4 Micromechanics of Single Fibers
AFM. serves. as. a. useful. tool. to. characterize. the. mechanical. properties. of. single. ibers.
composed.of.biological.materials..The.modulus.of.the.iber's.material.can.be.obtained.by.
nanoindentation.protocols.as.outlined.in.Sections.16.4.2.and.16.4.3..Newer.models.of.com-
mercial.AFMs.can.now.enable.simultaneous.imaging.and.measurement.of.nanomechani-
cal.properties.of.ibers.and.surfaces.using.modes.such.as.the.harmonic.force.microscopy.
(HarmoniX).and.PeakForce.QNM.(Table.16.2)..These.modalities.allow.extraction.of.mechan-
ical.parameters.of.the.surface.with.a.lateral.resolution.and.speed.comparable.to.dynamic.
AFM.imaging..Figure.16.3C.shows.how.mechanical.properties,.as.well.as.morphology.of.
amyloid.ibrils,.have.been.studied.using.these.dual.capabilities.of.AFM.(Sweers.et.al..2011).
To.obtain.a.more.complete.mechanical.characterization,.the.AFM.probe.can.also.be.used.
to. pull. or. push. a. single. iber. and. determine. the. related. mechanical. responses.. A. single.
iber,. for. instance,. can. be. attached. between. the. AFM. probe. and. the. substrate;. both. the.
static.and.the.dynamic.mechanical.responses.of.the.iber.can.be.studied.as.the.AFM.can-
tilever. is. ramped. vertically. up. and. down. to. extend. and. relax. the. iber. (Svensson. et. al..
2010)..AFM.can.also.be.used.as.a.stylus.to.extend.or.push.a.suspended.iber.vertically.or.
laterally,.analogous.to.a.nanoscale.three-point.bending.test..By.combining.the.AFM.stylus.
with. an. LM,. the. change. in. iber. length. can. be. measured. and. iber. extensibility. can. be.
ascertained..The.AFM.has.been.used.to.vertically.bend.and.measure.the.elastic.proper-
ties.of.suspended.bacterial.cellulose.ibers.(Guhados.et.al..2005).and.collagen.ibrils.(Yang.
et.al..2007)..Elastic.moduli.and.extensibility.of.single.ibers.of.ibrin.(Liu.et.al..2010).have.
been.measured.by.laterally.stretching.these.ibers.with.an.AFM.cantilever.and.observing.
