Scaffold Gradients: Finding the Right Environment for Developing Cells

People often have
strong opinions on the “right” firmness of mattresses for themselves,
and, as it turns out, some cell types have similar preferences for their
support structures. Now a research team from the National Institute of
Standards and Technology (NIST) and the National Institutes of Health
(NIH) has developed a way to offer cells a three-dimensional scaffold
that varies over a broad range of degrees of stiffness to determine
where they develop best. Their recently published technique* is a way to
rapidly optimize 3D cell growth media to meet the developmental needs
of specific cell types for a wide variety of potential
tissue-replacement therapies.

Tissue engineering is a relatively new field
that is developing methods to grow or regenerate bodily tissues—skin,
bone, cartilage, blood vessels, perhaps one day even whole organs—to
replace those damaged by injury or disease. One of the key challenges in
the field is developing appropriate three-dimensional “scaffolds,”
artificial materials that can hold tissue progenitor cells and allow
them to be nurtured and supported while they multiply and develop into
desired tissues. Research has shown that cells often need to develop in a
3D environment if they are to mature and differentiate properly.

Hydrogels—most familiar for their use in
soft contact lenses—are a promising material for tissue scaffolds. They
consist of a loose network of polymer chains that is swollen with water;
in fact, like the majority of the body’s tissues, they are mostly
water.

Experimental hydrogel scaffolds prepared at NIST for culturing bone cells clearly show the dependence of bone mineralization on the stiffness of the gel. Deposited minerals are denser at the bottoms of the gradient gels, which are progressively stiffer from top to bottom. Strips are approx. 1 X 6 cm. (Color added for emphasis.) Credit: Chatterjee, NIST View hi-resolution image

But, says NIST materials scientist Kaushik
Chatterjee, deciding on a hydrogel is just the beginning. “Now you’ve
got these gels, what sort of properties do you want? What gets you the
best kind of whatever tissue you’re after—in our case, bone? We focused
on stiffness because cells are known to sense and respond to changes in
the stiffness of their environment.”

To test this, the research team developed a
method to create samples of a typical hydrogel used in biomedical
research, PEGDM**, where the stiffness of the gel increases smoothly
from one end of the sample to the other. This approach, using smoothly
varying gradients of compounds to test many possible combinations
simultaneously, is called combinatorial screening. NIST has pioneered
such techniques for a variety of materials problems***, but this
research is one of the first applications of combinatorial screening to
3D scaffolds for tissue engineering. The team tested the technique on
mouse osteoblasts—cells responsible for building bone—mixed in with the
PEGDM gel. Interestingly, although cell survival rates were higher
at the softer end of the test strips and got progressively worse
towards the stiffer ends, cell differentiation and mineralization, which
are measures of how well the cells actually develop into bone tissue,
did the reverse. Fewer cells survive in a stiff gel, but those that do
are much more active in building bone. That result, of course, is
specific to osteoblasts, says Chatterjee, “These are bone cells and they
seem to like the stiffer environment more than softer ones, but you
could apply something similar to, say, nerve cells, and they might like
the softer ones more.”

In addition, the researchers note, the gel
stiffness gradient induced a matching gradient in the tissue
mineralization. This is potentially important, they say, because tissue
gradients often occur naturally at the interfaces of, for example, teeth
or ligaments, so 3D scaffold gradients could be a valuable tool for
engineering graded tissues for regenerative medicine.

The research was supported by NIST and NIH.

* K. Chatterjee, S. Lin-Gibson, W.E.
Wallace, S.H. Parekh, Y.J. Lee, M.T. Cicerone, M.F. Young and C.G.
Simon Jr. The effect of 3D hydrogel scaffold modulus on osteoblast
differentiation and mineralization revealed by combinatorial screening. Biomaterials
31 (2010), doi:10.1016/j.biomaterials.2010.03.024.

** poly(ethylene glycol) dimethacrylate

*** See, for example, “NIST
Study Finds a Decade of High-Payoff, High-Throughput Research,” NIST
Tech Beat, May 20, 2009 at http://www.nist.gov/msel/polymers/throughput_052009.cfm.

Media Contact: Michael Baum, michael.baum@nist.gov, (301)
975-2763