“It’s not just a foreign material you’re putting into the body. The
nanofabricated nature of the mesh creates a scaffold that cells can
easily penetrate and populate to recapitulate the body’s tissues.”
Nanofiber scaffolds also play a key role in speculative new
rotator cuff repair treatment. Using a $293,000 National Insti-
tutes of Health grant, researchers from Marshall University in
West Virginia are working on a way to replicate Mother Nature’s
tendon-shoulder bone attachment. Their initial design combines
a nanofiber scaffold with adipose-derived stem cells to promote
tissue repair after rotator cuff surgery, one of the most common
orthopedic procedures in the United States (roughly 300,000 are
performed annually, according to industry statistics).
Current rotator cuff repair methods have a failure rate that
ranges between 20 percent and 90 percent, due largely to the
manner in which tendons are reattached to the bone. Marshall
University researchers have found that controlling the miner-alization of electrospun nanofibers greatly enhances their mechanical properties (i.e., stiffness, ultimate tensile strength and
toughness), creating scaffolds with gradient in fiber organization
that better imitates collagen fibers at tendon-to-bone insertion
sites. The researchers hope to test their hybrid in rats within two
years and begin clinical trials in five to 10 years.
Nanocomposite scaffolds are becoming popular tools in knee
repair as well. Clinicians have successfully treated osteochondral knee defects with a biological scaffold of type I collagen and
nanostructured hydroxyapatite (HA). The tri-layered biological
implant consisted of a cartilage layer (100 percent type I collagen), a transition region ( 40 percent Nano-HA and 60 percent
type I collagen) and a bone region (70 percent Nano-HA and 30
percent type I collagen). This type of implant, researchers contend, may become an easier, less morbid, cell-free“off-the-shelf”
solution to focal defects of articular cartilage than either two-stage autologous chondrocyte engineering procedures or single-stage autograft mosaicplasty.
Electrospun scaffolds have shown promise in annulus fibrosus
engineering, but more work is needed to perfect the process. In-vestigators’first attempts to match the stiffness of native lamellar
tissue failed miserably, however they have had limited success by
using opposing collagen orientations of ± 30 degrees.
Although still in its infancy, nanotechnology perhaps shows
the most potential in bone repair. Nano-coated implants have
proven to be more conducive to osteoblast function, encouraging bone ingrowth, and engineered nanomaterials likely will be
stronger and lighter than the current crop of contenders. “For example, carbon nanotubes have the same stiffness as diamonds,
and they are a hundred times stronger than steel—but only a
sixth of its weight,” Baldini noted.
One of the major problems with man-made implants is their
nanometric surface smoothness, which tend to induce the growth of
fibrous tissue rather than bone. A nanotextured surface, on the other
hand, can encourage the function of osteoblasts and reduce that
of fibroblasts, experts claim. Materials like nanophase HA, nano-engineered titanium and cobalt-chromium-molybdenum promote
osteoblast adhesion more than their conventional counterparts.
While preventing fibrous growths certainly would improve
implant function, it single-handedly would not reduce the risk
of failure—a complication that affects up to 5 percent of all total
hip and knee replacement recipients. Researchers from the Massachusetts Institute of Technology (MIT), however, are coming to
the rescue with a high-tech adhesive that more securely bonds
implants to bone by promoting cell growth between natural and
artificial body parts.
In a study published last summer, the MIT team and its collaborators from several other institutions reported the implant
adhesive—a multi-layered coating of ceramic and nanolayers of
polymers infused with proteins—worked so well on lab rats that
it soon will be tested in humans.
The nanolayers, or super-thin sheets of material, hold therapies such as growth factors that attract and encourage the formation of bone cells, causing them to firmly attach to titanium
implants. The coated implants required significantly more force
to pull free than uncoated ones; indeed, the researchers said the
resulting bond is so strong that under stress, the bone would fracture first before the interface with the implant.
“If you have bonding that is so strong that you actually break
the bone, and you don’t get failure at the implant site, that would
be very significant,” Guillermo Ameer, a professor of biomedical
engineering at Northwestern University who was not involved in
the study, divulged to the Boston Globe. “It’s pretty exciting if this
is scaled up to humans.”
The implant coating works like a tiny, elegant machine. The
top coating consists of repeating layers, each impossibly thin,
that contain the bone growth factor BMP-2. The layers gradu-
ally break apart over a period of weeks, releasing BMP-2 into the
body. The factor then stimulates stem cells in bone marrow to
transform themselves into new bone cells.
The bottom part of the coating is made of a ceramic that mimics bone, thereby attracting bone cells to its surface. This side of
the coating is attached to the implant, and recently formed bone
cells tend to affix to this ceramic and grow outward, adhering like
“superglue” to attach the implant to the bone, said Nisarg Shah,
lead author of the MIT project.
The conventional approach to adhering implants uses a polymer called bone cement to attach them to bone. This cement can
fragment and loosen over time. Moreover, because the body recognizes the cement as a foreign material, it often surrounds it with
scar tissue, preventing bone from firmly attaching to the implant.
In contrast, most of the materials in the nanocoating either
exist in the body or mimic natural substances such as bone. And
most are already FDA-approved.
The nanocoating is flexible enough to use on surfaces other
than metal implants. By applying it to polymer scaffolds that mimic body parts,“this kind of technology can be adapted for replacing
bone,”said Paula Hammond, a professor at MI T who specializes in
materials design and molecule delivery at very small scales. “This
is another area where there is a lot of potential. There’s a range
of needs that are associated with dentistry, dental implants and
cranial-facial reconstruction.” v