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Newsletter December 2025 | Menu of
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When measuring
qualities of interacting surfaces in relative motion, such as the
coefficient of friction, friction force, adhesion, wear volume, lubrication,
and deformation, the tribometer is the instrument of choice. Typically,
tribometers are used to characterize rigid substrates utilizing a rigid probe,
which measures friction forces through deflections of the cantilever
supporting the probe.
But characterizing soft surfaces such as cells or
other delicate tissues with tribometry presents a significant challenge. Hard
probes used to test such surfaces damage the cells and tissues. Contact
pressures and shear stresses from tribological testing may also induce
inflammatory cellular reactions.
"Soft implant materials increasingly
being used in biomedicine for contact lenses, clamps, catheters and soft
tissue prostheses present significant experimental challenges during
surface characterization with tribometry," says Prof. Angela Pitenis,
Interfacial Engineering Lab, University of California, Santa Barbara (UCSB).
"Over the last two years my lab has been investigating microscale contact areas
and forces between aqueous gels and cells. We want to understand how cells
respond to mechanical stimulation caused by soft implants."
"The need to
examine biocompatibility of these implant materials has introduced
opportunities to develop specialized instrumentation for tribological studies
of these low friction aqueous gels, cells and tissues," says Pitenis. "So we
developed our own custom instruments with the ability to observe and monitor
changes in the contact area within the deformations of these soft
materials."
"We design and fabricate precise in situ instrumentation
that interrogates microscopic contacts of cells under microscale forces (25 mN
to over 1,000 mN)," adds Pitenis. "A wide variety of cell and tissue types,
along with soft biocompatible materials are frequently used in combination to
create model interfaces from which to study fundamental mechanisms
associated with biological responses to contact and shear."
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Vibration
Isolation: The lab's biotribology research is conducted on top of an
air-cushioned optical table with 10 N and 25 N stages to provide passive
damping. Because the lab's research often measures very low friction
coefficients of approximately 0.01 and below, any vibrations from the
environment could interrupt measurements.asdfasdf"We measure forces on the
order of micronewtons," says Pitenis. "Vibration isolation is critical to our
research. Footfall from someone walking by, the closing of doors — even
with the optical table, these vibrations will still be
measured."
Consequently, the lab has added another layer of vibration
isolation designed to more thoroughly cancel out low-frequency
horizontal-direction vibrations coming up through the floor.
"We
selected a negative-stiffness vibration isolator from Minus K Technology," says
Pitenis. "It supports the biotribometer and the specimen container. The
isolator is positioned on the breadboard. The negative-stiffness isolator is
working fantastically," she says. "It is an excellent design choice for our
research."
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