By Kathleen M. Wong

Another day dawns in the emergency room, and patients are coming in at a rapid pace. An elderly man complains of chest pain and trouble breathing; his skin is ashen, his face sweaty. A woman who was in a car accident has sustained deep arm lacerations and cannot move or feel her fingers. Moments later, an ambulance delivers a little girl with severe burns on her face and body.

In the ER of today, options for these patients are limited. Tests reveal the man has blocked coronary arteries but no healthy vessels in his legs or chest that could be used as replacements. The woman can’t use her hand because the nerves leading to her spinal cord have been severed. Without the protection of skin, the girl risks dehydration and infection and faces multiple surgeries to remove stiff and disfiguring scar tissue.

Song Li, UC Berkeley associate professor of bioengineering, is working to improve the options for such patients. A leader in the fast-growing field of tissue engineering—a fusion of cell biology, materials science and engineering—Li is working with his graduate students to develop replacement arteries, nerve grafts and wound healing technologies that work in concert with the body’s own natural repair systems. Three of his students are now combining their bioengineering know-how with business savvy to form startup companies that will bring these technologies to the clinical setting in the next five to 10 years.

“We’re trying to make biomimetic or bioinspired materials based on structures already in our tissues,” Li says. Key to his lab’s innovative products is the high-tech synthetic scaffolding they are built on. Using long fibers of polyesters (the bioabsorbable material surgical sutures are made of), the researchers can fashion membranes endowed with remarkable properties. To the naked eye, the membranes resemble shiny sheets of white tissue. But under the microscope, their surfaces reveal a nanoscale topography of grooves, divots and dimples that point cells in the direction they should grow and provide cargo space for stem cells, growth factors and other biomolecules that speed healing.

Li ascribes his lab’s success to his talented, creative students, who build on one another’s achievements by working as a team and identifying promising leads. The work comes not a moment too soon for patients in the ER of tomorrow.

Stem cells in training

Stem cells, the body’s most versatile building materials, have the potential to mature into virtually any type of tissue. Li and his team—to ensure that their scaffolds would function seamlessly when implanted—planned to coat them with a living surface derived from a patient’s own stem cells.

As a graduate student at UC San Diego, Li learned that the forces cells experience—the tug of gravity, an artery’s expansion and contraction with each heartbeat—influence their fates. He suspected that mechanical stresses might also affect how stem cells develop.

“Our hypothesis is that specific microenvironmental factors, such as mechanical stresses and chemical factors, can promote cells to differentiate into specific types,” he says.

In a groundbreaking experiment, Li’s former graduate student Jennifer Park (Ph.D.’06 BioE) used mechanical force to coax stem cells to differentiate into smooth muscle. She seeded the cells on a stretchy silicone membrane, then used a device to stretch the cells and their rubbery matrix along one axis, like a child stretching Silly Putty, for several hours. The pace and direction of the stretching were designed to mimic the swelling action that vascular muscle cells experience in an artery. Within hours, gene activity indicated that the cells had partially transformed into the smooth muscle that surrounds arteries.

Building on Park’s work, Li’s Ph.D. student Kyle Kurpinski took the experiment one step further. He increased the duration of the stretching to once per second over a period of days and added nanoscale grooves to the membrane to recreate the arrangement of collagen fibers in blood vessels. When the grooves were oriented in the same direction as the stretching, as in nature, the cells aligned themselves the same way. This orientation enhanced the differentiation of stem cells into vascular cells, a process that could prove useful for generating a renewable supply of arterial muscle cells for the more than 500,000 Americans who undergo coronary bypass surgery every year.

Repairing a broken heart

Coronary bypass surgery replaces one or more of the arteries that normally supply blood and oxygen to the heart but have become clogged with plaque. The operation can be a lifesaver, staving off the imminent threat of heart attack. Surgeons prefer to use vessels harvested from a patient’s own leg or chest, but in some patients these vessels are unusable, damaged by atherosclerosis or diabetes.

Current synthetic vessels don’t fit the bill, says YiQuian Zhu, a neurosurgeon and student in the UCSF & UCB Joint Graduate Group in Bioengineering. When synthetic vessels are fashioned in a caliber narrow enough to replace coronary arteries, these soon become occluded by clots.

Li and graduate student Craig Hashi set out to design a better graft. They seeded a mat of nanofibers with the body’s own universal replacement parts: stem cells harvested from bone marrow. The cells, they hoped, would smooth the surface of the graft and reduce the risk of aneurysms and clots. In time, the scaffolding would dissolve, to be replaced by the body’s own cells. Zhu, an expert in microsurgery, implanted the tiny grafts into rats.

“In the very first animal we tested, the vessel came out clean,” Hashi says. “And we just started rolling from there.”

The researchers are now investigating the use of scaffolding without the stem cells. Patients often need surgery immediately, while harvesting and culturing the cells requires extra care, time and expense. “Ideally, you could take the vessel off the shelf and it would be ready to go right into the patient,” says Hashi, who, with Li, Zhu and several other graduate students, is now testing other molecules that might possibly lure stem cells to the site.

The concept won first place in two national invention competitions and at the 2007 Global Life Sciences Competition and caught the eye of outside investors interested in licensing the technology for commercial development. Hashi expects to be testing the grafts in humans within two to four years. When he graduates next month, he’ll assume the post of chief scientific officer for NanoVasc, a biotechnology startup, and begin the process of developing the grafts for clinical use.

Better healing skin deep

Li and his team realized that micropatterned guidance of cells could solve another serious health problem: wound healing. In a deep cut, Kurpinski says, “the cells don’t really know where to go. There’s no structure left. That’s one reason you get scar formation. If you have a big wound, the cells will randomly put down matrix and collagen, and it gets very disordered.”

To prove his theory, Kurpinski laid down a nanofibrous scaffolding material between a gap and seeded both sides with cells. When the fibers weren’t aligned, relatively few cells traveled into the space, but when the fibers led into the gap, cells followed like trains on a track, closing up the space. Laying such nanotextured sheets over the edges of a gaping wound could facilitate healing.

“We could guide new cells into the area of missing tissue and, we hope, improve soft tissue regeneration,” Kurpinski says. “That way, we don’t need a cell source. Instead, the body’s cells will be able to feel this new patch and migrate in the right direction to form healthy tissue.”

As they funnel cells into injured areas, the sheets could deliver molecules that encourage tissues to mend. Just as the nooks and crannies in an English muffin hold extra drops of butter, the scaffolding’s nanotextured surface can hold surprisingly large quantities of biomolecules. “Skin is like a storage depot for growth factors and matrix proteins,” Li says. “We can load up our scaffolding with chemical factors found in native tissue” to further accelerate healing.

Already, the technology has been licensed to a startup company, EscharaX, that will make products to promote wound healing. Kurpinski will spearhead the company’s research and development when he graduates this year.

Paving the way for neurons

Nerves, like skin cells, are notoriously fickle about regenerating after injury. Each year in the United States, accidents and surgeries leave several hundred thousand people with trauma to the nerves that give feeling and movement to their arms or legs. Such peripheral neurons do have the capacity to heal themselves, sending new axons across an injured site and retracing their paths to muscles and sensory receptors, but only if the gap is no more than a few millimeters wide. Meanwhile, other tissues can fill the space, blocking the path to recovery. These patients face a lifetime of disability from irreversible paralysis.

To repair these connections, Li says, nerves “just need guidance.” Scaffolding with nanoscale patterns, he suspected, might show severed neurons the way to reestablish their connections.

Li’s recent graduate student Shyam Patel (Ph.D.’07 BioE) set out to demonstrate the idea. He compared nerve tissue cultured on membranes with randomly directed versus aligned fibers. Neurons on the unaligned sheets sent out axons every which way, splitting their efforts so that no single axon traveled far. The neurons on the aligned fibers were a different story. “It was like a fast track; the axons just followed and grew very fast,” Li says.

In neuron repair, time is of the essence; chances of surgically restoring nerve function diminish within months to a year. To hurry regeneration along, Patel doped the scaffolding with molecules that encourage neurons to extend. He found that neurons on coated, aligned nanofiber membranes grew five times longer than randomly oriented membranes without coatings. On the enhanced scaffolding, the neurons extended almost four millimeters in just five days—a growth rate comparable to the gold standard in neural repair, a section of nerve harvested from elsewhere in the body.

“We showed that you could use this topographical guidance and, combined with chemical guidance, make a new kind of scaffold. You could not only direct the extension of the axons but enhance their growth,” Patel says. In order to translate the research to a clinically viable product, Patel has developed technology to fabricate tubular grafts composed of aligned nanofibers.

The nerve graft, named one of the top micro/nano technologies of 2007 by R&D Magazine, is now licensed by new startup company NanoNerve, which aims to shepherd the device to the clinic. Patel, the company’s chief scientific officer, has just begun animal trials; he plans to begin human trials of aligned nanofiber grafts by year-end and trials of bioenhanced grafts by 2009.

“Since we’ve begun working on the technology, people come up to us at conferences and competitions, people who can’t move their feet or arms anymore,” Patel says. “We started this work from a purely scientific standpoint, but it’s very compelling to witness people who suffer from these injuries and know that this technology could some day improve their quality of life.”

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Kathleen M. Wong is a science writer and editor based in Oakland. She writes Science Matters @ Berkeley, the online news journal of UC Berkeley’s College of Letters & Sciences.