Project Title:
Mechanical Behavior of Biological Materials
Project Objective: To
develop, validate and disseminate methods to quantify the mechanical properties
and response of biological materials at the salient length scales: cell, cell + matrix, and tissue.
Project Description: Mechanical
measurements of tissues, cells+matrix (as with cells cultured on scaffolds),
and cells are needed to identify the mechanisms of disease, for diagnoses and
treatment, and for establishing baselines and targets for tissue-engineered
constructs. The pathology of diseases
can be manifested from the subcellular to the tissue levels. Therefore, depending on the particular
disease and condition, the tools to assess properties or behaviors may be needed
by researchers at any of these length scales.
Quantifying the behaviors at the cellular and the cell+matrix levels
provide the building blocks for the structure-property relations of tissue and
organs, natural and engineered. In the
context of functional tissue engineering, it is necessary to control all
important variables and nondestructively track mechanical properties during the
entire growth cycle and while it is developing functionality.
Cellular Level: A challenge is to develop mechanical
tools that can be integrated with currently used biological techniques for the
evaluation and measurement of cellular response (e.g., gene expression, cell
morphology, area of adhesion). The
importance of the environment becomes apparent when one considers that
engineered tissues have mechanical properties inferior to those of naturally
grown tissues. This is possibly a bulk effect, but is clearly related to
processes at the cellular level. Without a quantitative understanding of the
mechanics and functionality of the building blocks (cells), the bulk properties
of the tissues cannot be fully understood and modeled.
Cell+Matrix Level: Functional tissue engineering aims to develop tissues with
sufficient structure and function for transplantation into a living body.
Quality assessment is generally performed by measurement of the mechanical
properties of the tissue-engineered construct. Current efforts by the tissue
engineering (TE) community aim to develop monitoring techniques that permit
quality assessment during incubation of the TE construct in a bioreactor. The
availability of online monitoring techniques enables feedback control
capabilities and avoids the issue of contamination of the construct when
removed from the bioreactor for monitoring.
Tissue Level: At the
tissue level, we use
the complementary techniques of mechanical testing, quantitative ultrasonic
characterization, and histology to measure the response of tissue to the onset
of disease. Mechanical tests are
conducted in a biaxial bubble inflation device. The tissue is pressurized and digital
images of the inflated tissue membrane are captured as the pressure is
increased; stress-strain behavior of the tissue membrane can be determined from
the pressure and deformation. Ultrasound
has the potential to interrogate biological materials at all length scales of
interest. Quantitative ultrasonic
characterization of biological materials aims to reduce health care costs
through the development of improved techniques for diagnosis of pathologies and
monitoring of therapies. Computational
tools and standard reference materials facilitate design and interpretation of
laboratory and clinical measurements, and validation of proposed diagnostic and
monitoring techniques, respectively. Optical
histology of stained tissue specimens enables investigation of the structural
properties of biological materials. We
quantify the areal fraction of each of the major constituents of the tissue,
including extracellular matrices and cells.
Relative thicknesses, morphology, layup, and absolute counts of each
cell type per unit area are measured.
Area of Application: Health and Medical Products and Services. The medical research community has found that
mechanical factors affect processes in healthy and diseased tissues and cells. Therefore, they seek measurement solutions
that are customarily in the realm of engineering. We offered capabilities in mechanical test
and stimulation development that complement the biological and medical
expertise of the traditional health care industry.
Project Accomplishments: We built and
calibrated an optical tweezer; built bio-MEMS devices for adhesion
measurement, and for mechanical stimulation of single cells on-site; seeded
cells onto bioMEMS devices; designed, built, and
tested a novel bioreactor; performed mechanical tests on biodegradation
of PCL scaffolds using lipase enzymes; developed mechanical tests and empirical
models for a random pore PCL scaffold using small samples; predicted initial
elastic properties from first principles using microstructure; completed and analyzed >150 mechanical tests on rat
pulmonary arteries from 4 populations for pulmonary hypertension; 3
manuscripts are in prep; developed fundamental relation between attenuation
coefficient and phase velocity via the Kramers-Kronig relations; completed >80
measurements on pulmonary arteries; published 2 papers on the ultrasonics work,
1 is in press, and 2 more submitted; presented results at 5 conferences and 2
invited talks; are quantifying the histology of the rat pulmonary arteries for
populations under investigation.
Future Plans: (1)
perform uniaxial and biaxial mechanical measurements of single cells; (2) optimize
the mechanical stimulation of vascular smooth muscle cells seeded on a PGA/PLLA
copolymer scaffold to enhance the mechanical properties using novel bioreactor
and in situ monitoring techniques;
(3) develop ultrasound measurement systems that integrate with mechanical
measurement systems (elasticity microscope) and tissue engineering bioreactors
(online monitoring); (4) measure the mechanical properties and histology of the
major constituents of soft tissue (including, functionally graded tissue) to
build structure-property relations.
Relevant Links: http://www.boulder.nist.gov/cmbbm/
Recent publications: http://www.boulder.nist.gov/cmbbm/pubs.htm
External
collaborators: http://www.boulder.nist.gov/cmbbm/collabs.htm
Coordinator: Elizabeth Drexler
NIST,
325 Broadway, MS 853,
303-497-5350,
drexler@boulder.nist.gov