Rebuilding the Body
By Eileen McCluskey
George Pins, left, and Kristen Billiar place a sample on a device Billiar developed to stretch tissue in multiple axes.
A diabetes patient loses a toe to a wound that won't heal, but his physician reassures him the digit will grow back, thanks to a new ointment of progenitor cells. A firefighter suffers severe burns, but instead of enduring multiple skin grafts and painful reconstructive surgeries, her skin heals—quickly, like new—with the help of bioengineered tissue. A soldier returns from war without an arm. Yet he feels hopeful: he has seen others use prosthetic arms naturally, nearly effortlessly.
Researchers at WPI, some working in a new federally funded center for regenerative biology, work to realize these visions of repairing, aiding, and restoring the body.
As George Pins experiments on the intricacies of artificial skin, the associate professor of biomedical engineering says he finds it exciting "to play a role in developing a new generation of implantable materials that will make significant improvements in health care."
He began researching artificial skin when he worked at Shriners Hospital for Children in Boston, during his late-1990s postdoctoral studies. "Working in close proximity to children who suffered from burns provided a constant reminder about how important this research is."
To grow artificial skin, Pins uses scaffolds made to order at Massachusetts General Hospital's BioMicroElectroMechanical Systems (BioMEMS) Resource Center. Pins and his team seed these scaffolds with epithelial cells, which make up the majority of the epidermis, the protective barrier on the skin's surface, as well as the lining of organs like the lungs.
Pins focuses on replicating the basal lamina—a thin membrane consisting of connective tissue that underlies the epidermis—"to see how closely we can get the engineered membrane to mimic the natural basal lamina found in skin and organs," he says. Among its critical jobs, the basal lamina controls cell migration and differentiation when organs form and while wounds heal.Pins's novel approach includes copying, in his scaffolds, the topographical peaks and valleys of natural basal lamina associated with skin. These mountain range–like constructions seem designed, says Pins, to provide structural stability to the epidermis, and to supply nutrients to tissues.
"It's fascinating to try to emulate this complex structure," he says. It's also time-consuming. "This is a labor-intensive project with limited means for doing rapid analyses. The quantitative markers for assessing the functional outcomes produced by the engineered skin are very subtle."
Pins believes the payoff will be worth every calculation. "This research will provide bioengineered skin that's mechanically stable, and that structurally matches natural skin," he says. With this advancement, burn victims will no longer need multiple skin grafts. Rather, "one application will do the job, saving patients a lot of pain and saving the health care system significant treatment dollars and other resources." He expects clinics to start using the new tissues in five to 10 years.
In a nearby WPI lab, Kristen Billiar tackles another of bioengineered skin's primary problems. "One of the main criticisms of engineered skin is that it doesn't feel like real skin," says the assistant professor of biomedical engineering. "We need to provide more of the density and strength of natural skin."
Working toward this goal, Billiar investigates how the body's mechanical environment affects the formation and repair of skin tissue. This research will also improve artificial heart valves, since they share a similar makeup with skin.
"When I studied mechanical engineering in college," says Billiar, "I came to see the body as the most amazing machine. It's incredibly efficient, and can change itself."
Billiar harnesses the body's remodeling ability by studying how living cells respond to their mechanical environment. Artificial replacement parts, such as heart valves, deteriorate over time, he says. "In healthy heart valves, the leaflets are continually repaired by cells. I want to recreate this ability in bioengineered tissues.
Kristen Billiar studies how living cells respond to mechanical stimulation. Left: a custom-designed device that inflates lab-grown tissue and measures the strength of the remodeled tissue. Right: a device designed by a student team that stretches tissue in two directions simultaneously to characterize its subfailure properties.
Click here for a larger version.
"We've done the first multi-axial stretch on models of healing wounds," Billiar explains. These replicas—which look like clear Jell-O in petri dishes—consist of collagen and fibrin gels, "which are not as complex as skin, but similar." The tissue is stretched in all directions while cells remodel the matrix. "We're finding that the artificial skin becomes much more dense and is 10 times stronger than engineered skin that is not stretched multi-axially. It's very encouraging."
Billiar also studies ways to stop the scarring process before it goes too far, as it often does in diseased or damaged tissue. He hypothesizes that a dual dynamic of stretching and stiffness plays a role in pushing fibroblasts—cells that produce connective tissue—into becoming highly contractile myofibroblasts, a cell type implicated in excessive scarring. "If there are too many myofibroblasts, you get too many muscle-like cells," Billiar explains. "We're working to get them to revert to a quieter state at the right time. Controlling cell activity in time and space is the Holy Grail of tissue engineering, whether for heart valves, skin, or other tissue."
Billiar envisions engineered tissues that, when used in surgical and other clinical settings, would encourage regeneration of heart, lung, and other tissues damaged by disease, and in skin as it heals from burns and other wounds. Because it would meld so well with living bodies, this superior tissue would also change with patients. "Within a decade," he estimates, "we may see heart valves for children that grow with the child."
Ted Clancy explains to graduate student John David Quartararo the intricacies of a sensor that measures the electrical signals produced by muscles.
Ted Clancy also studies natural body tissue, but the asso-ciate professor of electrical and computer engineering does so, in part, to improve powered artificial limbs. Using a densely spaced array of small electrodes developed in his lab, Clancy studies the complex electrical signals produced by muscles to create movement, examining which muscle groups control specific motions.
Clancy's interest in muscular control began through boyhood interests in sports and engineering, when he "became fascinated with the body, and wondered how engineers could help people." Then, while an undergrad-uate electrical engineering major at WPI, he discovered biomedical engineering.
Prosthetic limbs that move like their natural counterparts may be a decade or more away, says Clancy. "Up until a few years ago, prosthetic limbs weren't equipped with digital signaling capabilities." Indeed, microcomputer-outfitted prostheses "are just now capable of using the more advanced algorithms that researchers like myself have been developing." But even with the wait, he says, "it's exciting to know we're contributing to technologies that can help people."
Clancy, whose research has been applied to stroke patients to help them re-learn the basics of walking and other mechanical tasks, enjoys collaborating with a diverse team of scientists, engineers, and physicians across the globe. Among these colleagues are Paolo Bonato, director of the motion analysis laboratory at Boston's Spaulding Rehabilitation Hospital; Gary Kamen, professor of kinesiology at UMass Amherst; Roberto Merletti, director of the Laboratory for Engineering of the Neuromuscular System at Politecnico di Torino in Italy; Dario Farina, associate professor at Aalbory University in Denmark; and Denis Rancort, professor at the Université de Sherbrooke in Canada.
Physicians insert needles to evaluate the electrical activity of muscles when diagnosing various neuromuscular disorders. Ted Clancy and his students have been working on an alternative: a noninvasive array of surface sensors. A key challenge is to make the sensors more selective, so as to focus on the activity in one portion of a muscle without interference from other areas. The graph shows the difference a more selective technique makes when two signals (left and right in the graph) move through the same muscle. In the dotted line, obtained with a less selective method, both signals appear.
Click here for a larger version.
While the results of Clancy's research work their way into better prosthetic limbs, two other WPI-affiliated scientists attempt to make it possible for people to regrow fingers, toes, and even limbs lost to disease or traumatic injury. Working as research assistant professors in WPI's Biology and Biotechnology Department, Tanja Dominko, president and chief scientific officer of the Worcester-based biotech firm CellThera, and Ray Page, CellThera's research director, are part of a multi-institution team that received a one-year, $3.9 million award from DARPA (Defense Advanced Research Projects Agency), the R&D arm of the U.S. Department of Defense. Other institutions in this collaboration are Tulane University, the University of California, Irvine, and the University of Louisville.
In his research, Glenn Gaudette, assistant professor of biomedical engineering, is developing ways to use the tools of tissue engineering and stem cell therapy to repair heart muscle. It is a challenging task: myocytes, the beating muscle cells in hearts, must perform the unique and daunting job of contracting in a perfectly synchronized wave (about three billion times in a lifetime) to push blood efficiently throughout the body.
In recent editorials he co-authored in Circulation (the journal of the American Heart Association, which supports his research) and the Journal of Molecular and Cellular Cardiology (JMCC), Gaudette assesses the state of a promising new line of research aimed at using adult stem cells to repair damage caused by myocardial infarctions—blockages of coronary arteries that deprive heart muscle of oxygen and lead to cell death. Gaudette notes that a key factor in the success of such therapy will be whether the types of cells chosen can not only add mechanical strength to the damaged muscle, but properly connect and harmonize with the heart's electrical network. Cells that don't, the JMCC editorial notes, "can predispose patients to life-threatening consequences."
Dominko and Page say they also hope to encourage the connective tissue–producing fibroblast cells to morph into blastemas—undifferentiated cells that, like embryonic stem cells, give rise to new tissues.
"As mammals became more complex," says Dominko of the team's approach to regeneration, "skin began to heal by forming scars. In mammals, the first response to injury is to stop the bleeding and combat infection, which is, of course, helpful for basic survival. But that same response in amphibians, like the salamander, covers the wound with a type of skin cell layer that changes fibroblast cells into blastema cells. These cells then regenerate the entire lost structure."
Page and Dominko manipulate fibroblasts to make them behave like blastemas. Early results are encouraging. In their preliminary analyses, the scientists have seen round groupings of cells that look more like stem cells than the original stringy fibroblasts. When examined using dyes, they see these new clusters expressing what look to be nerve markers and muscle cell markers. Their collaborators in this ambitious program will conduct more detailed evaluations of the new cells.
Like most pursuits in rebuilding the body at WPI, "There's no road map for this process," says Page. "We're challenging the dogma that says cell lineage can't be changed."
This team envisions a day when doctors will apply a paste to the site of missing fingers and limbs. Given a little time, the healing process will produce new digits, new arms, new legs.
Like their colleagues at WPI, Page and Dominko feel hopeful as they look toward the not-so-distant future. "In four years, we should be able to regrow a finger, if not a limb," says Dominko. "It's ambitious, I know, but if you're going to make great progress, you have to take a leap."
"We just need to remind cells how to regenerate digits and limbs, instead of covering wounds in scar tissue," says Dominko. "It's exciting to realize we can turn the cell into anything we need."