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.

![](media/image1.png){width="2.0in"
height="1.4583333333333333in"}*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.

![](media/image2.png){width="2.0375in"
height="1.5083333333333333in"}*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, Boulder, CO 80305

303-497-5350, drexler@boulder.nist.gov