The materials created with the help of viruses could eventually be used to create complex biological tissues, such as cornea, skin and bones. (Woo-Jae Chung photo) |
Researchers
at the University of California, Berkeley, have turned a benign virus
into an engineering tool for assembling structures that mimic collagen,
one of the most important structural proteins in nature. The process
they developed could eventually be used to manufacture materials with
tunable optical, biomedical and mechanical properties.
The
researchers, led by Seung-Wuk Lee, UC Berkeley associate professor of
bioengineering and faculty scientist at Lawrence Berkeley National
Laboratory (LBNL), describe their “self-templating material assembly”
process in the Oct. 20 issue of the journal Nature.
“We
took our inspiration from nature,” said Lee. “Nature has a unique
ability to create functional materials from very basic building blocks.
We found a way to mimic the formation of diverse, complex structures
from helical macromolecules, such as collagen, chitin and cellulose,
which are the primary building blocks for a wide array of functional
materials in animals and plants.”
Function follows form
For
instance, a number of blue-skinned animals, including the mandrill
monkey, derive their coloring not from pigment, but from the specific
scattering of light formed when thin fibers of collagen are bundled,
twisted and layered in its skin.
In
contrast, aligning collagen in a perpendicular, grid-like pattern
creates transparency, and is the basis of corneal tissue. And
corkscrew-shaped fibers, mineralized after interacting with calcium and
phosphate, can generate the hardest parts of our body: bones and teeth.
Woo-Jae Chung and Seung-Wuk Lee prepare solutions containing bacteria-eating M13 viruses. The researchers have developed a process that uses the viruses to assemble collagen-like materials. (Sarah Yang photo) |
“The
basic building block for all of these functional materials—corneas,
skin and teeth—is exactly the same. It’s collagen,” said Lee. “I was
mesmerized when I saw the brilliant skin color and sharp teeth of
blue-faced monkeys at the San Francisco Zoo. It is stunning that the way
the collagen fibers are aligned, twisted and shaped determine their
optical and mechanical functions. What had not been well understood,
however, is how such a simple building block can create such complicated
structures with diverse functions.”
The researchers began by studying the factors influencing the formation of hierarchical structures.
“We
noticed how collagen is secreted in confined spaces, and how its
assembly into tissues can be influenced by its environment,” said study
lead author Woo-Jae Chung, a post-doctoral researcher in Lee’s lab.
“Unfortunately, collagen is a difficult material to study because it is
hard to tune its physical and chemical structures. We needed a
convenient model system to solve this problem.”
Assembly with a twist
That
system was a soup of saline solution containing varying concentrations
of a common bacteria-attacking virus, the M13 bacteriophage. The
researchers chose the M13 virus—harmless to humans and a model organism
in research labs—because its long, “chopstick-like” shape with a helical
groove on its surface closely resembles collagen fibers.
The
technique the scientists developed entails dipping a flat sheet of
glass into the viral bath, then slowly pulling it out at precise speeds.
The sheet emerges with a fresh film of viruses attached to it. At a
pulling rate ranging from 10-100 ?m per minute, it could take 1-10 hours
for an entire sheet to be processed.
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By
adjusting the concentration of viruses in the solution and the speed
with which the glass is pulled, the researchers could control the
liquid’s viscosity, surface tension, and rate of evaporation during the
film growth process. Those factors determined the type of pattern formed
by the viruses. The researchers created three distinct film patterns
using this technique.
With
a relatively low viral concentration of up to 1.5 mg per milliliter,
regularly spaced bands containing filaments oriented at 90 degree angles
to each other were formed.
With
a slower pulling rate came increased physical constraints to the
movement and orientation of the viruses. The viruses spontaneously
bunched together, and as they stuck to the sheet, they started to twist
into helical ribbons, much like curled ribbon used for gift wrap.
The
most complex pattern—described as “ramen-noodle-like” by the
researchers—was formed using viral concentrations ranging from 4-6 mg
per milliliter. By using the Advanced Light Source at LBNL, the
researchers discovered that this highly ordered structure could bend
light like a prism in ways never before observed in nature or other
engineered materials.
Tune the system, then let the viruses do the work
“We
can determine the type of structure we get through this technique by
fine-tuning the factors that influence the kinetics and thermodynamics
of the assembly process,” said Chung. “We can control the levels of
order, direction of the twist, as well as the width, height and spacing
of the film patterns.”
The
researchers further showed that the virus assembly process could be
used in biomedical applications. They genetically engineered the virus
to express specific peptides, which influence the growth of soft and
hard tissue. They used the resulting viral films as tissue-guiding
templates for the biomineralization of calcium phosphate, forming a
tooth-enamel like composite that in the future could be applied as a
regenerative tissue material.
The
simplicity of the technique bodes well for adapting it for use in
manufacturing, the researchers said. Once the parameters are set, it is
possible to step aside and let the self-assembly process take place.
“We
let this run overnight, and by the next morning there were trillions of
viral filaments arranged in patterns on our substrate,” said Lee. “One
of the most important aspects of our work is that we have started to
understand nature’s approach to creating such complex structures, and we
have developed an easy way to mimic and even extend it.”
Funding
from the National Science Foundation, the National Institute of Dental
and Craniofacial Research and the Defense Advanced Research Projects
Agency helped support this work.
