The Future Human
More than ever, engineering is informing medicine. Breakthroughs that range from synthetic skin to artificial hands are restoring people to health and challenging the very concept of disability.
This special section features some of the work being done by Michigan Tech researchers at the junction of healing and engineering.
- An artificial leg that mimics our innate gait
- Cardiac vessels made from stem cells
- A new nano-surface that could slash the failure rate of titanium implants, from hip replacements to new teeth
Science fiction is replete with cyborgs, seriously injured souls rebuilt to have amazing powers. Today’s scientists and engineers are accomplishing something more extraordinary: re-creating the natural intricacy of a healthy human body.
Stepping out in style
Walking is tricky business, as any toddler knows. And while most artificial feet and limbs do a pretty good job restoring mobility to people who have lost a leg, they have a ways to go before they equal the intricacy of a natural gait. As a result, over half of all amputees take a fall every year, compared to about one-third of people over the age of sixty-five.
In cooperation with a Mayo Clinic scientist, researchers at Michigan Tech are taking a giant step toward solving the problem. They are making a bionic foot that could make an amputee’s walk in the park feel, well, like a walk in the park.
The secret lies in the ankle. Mo Rastgaar, an assistant professor of mechanical engineering–engineering mechanics, and PhD student Evandro Ficanha are working on a microprocessor-controlled ankle-foot prosthesis that comes close to achieving the innate range of motion of this highly complex joint.
These computerized artificial legs have pressure-sensitive sensors on the bottom of the foot that detect how the amputee is walking. The sensors instantaneously send signals to a microprocessor, which in turn adjusts the prosthesis to make walking more natural.
The microprocessor-controlled prostheses on the market can move an artificial foot in only one direction, toe up and toe down, which is fine if you are marking time on a treadmill, said Rastgaar. “But in reality, we never walk in a straight line for any length of time,” he said. “When you walk and reach an obstacle, you have to turn, and there’s always something in our way.”
So, Rastgaar and Ficanha designed an ankle-foot that can move on two axes, incorporating a side-to-side roll as well as raising the toe up and down. And they moved the power and control mechanism up and away from the leg using a cable-driven mechanism. That lightens the prosthesis, making it much more comfortable and easy to use.
The cable that moves the prosthetic ankle-foot is similar to those used in bicycle brakes. It runs from the control box to the ankle mechanism and can turn the foot in almost any direction.
As part of their study, the team designed and built a large circular treadmill on which the robotic foot “walks” in circles. In tests, the prosthetic was able to copy the angles of a human ankle walking in a straight line and turning.
Kenton R. Kaufman, director of the Biomechanics/Motion Analysis Laboratory at the Mayo Clinic in Rochester, Minnesota, is collaborating in the effort to refine the prosthesis and make it available to amputees, especially wounded warriors.
“Artificial limbs tend to evolve from wars because of the increased awareness of the problems faced by amputees,” said Kaufman. A primary focus is improving safety.
“Amputees have lots of problems with falling; 64 percent of above-the-knee amputees fall every year, compared to 33 percent of older adults,” he said.
The latest generation of microprocessor-controlled prosthetics is a step in the right direction. “They provide active control of the joint and improve safety and function,” Kaufman said. “But the advantage of Mo’s foot is that it is biomimetic—it mimics biology—so it allows a more natural walking pattern to occur, which should result in a better gait and fewer falls.”
The researchers expect to begin refining their design at the Mayo Clinic this summer.
Arteries in aisle 9
Stems cells are a body’s newborns, bright, malleable, and full of promise. Feng Zhao is guiding them toward a future as life-saving blood vessels.
In the United States alone, hundreds of thousands of people a year undergo surgery to bypass a blocked artery and restore blood flow. Typically, surgeons first cut a vein from the patient’s leg or chest. Then they use it to create a detour around a blockage in the artery, restoring circulation.
The technique works well—if the patient has a good vein and suffers no complications. However, many thousands of patients do not have healthy veins available. They must settle for grafts made from synthetic materials, which are prone to clots and blockages, or veins taken from animals, which can provoke a dangerous immune response.
New research is leading to the development of bioengineered vessels grown from human stem cells, but so far, the vessels have been on the large side. Some cardiac bypasses require smaller grafts, with an interior diameter of six millimeters or less. Scientists are growing smaller blood vessels on a scaffold, but the materials in that scaffold can set off the body’s immune system.
“There’s not a perfect solution on the market,” says Zhao, an assistant professor of biomedical engineering. “So we’re trying to make a completely biological vessel. We use just the stem cells. We let them do all the work.”
The stem cells used in her research are harvested from bone marrow and fatty tissue. Like all stem cells, they have a unique advantage. They can be extracted from a donor and then transplanted into someone else without triggering an immune response. If all goes well in their new host, they become naturalized citizens, differentiating into a specific cell type and blending in with the natives.
However, a blob of stem cells is no substitute for a working blood vessel. So Zhao’s team has coaxed them into forming proto-blood vessels in the lab. She grows them in a nutrient-rich, low-oxygen fluid that mimics conditions inside the body. “They are like silk worms; they build a little house for themselves out of proteins and carbohydrates,” she said.
Initial tests show promise; she has successfully transplanted these stem cell tubes in rats. “After two weeks, they look pretty good,” she said. “We see them differentiating into vascular cells and becoming denser and stronger.”
In a similar vein, Zhao is also using stem cells to grow the tiniest blood vessels of all. Unlike their larger cousins, these sheets of capillaries would not be used to treat blockages. Instead, they would serve as the plumbing system for artificial tissue.
Artificial tissue is engineered from cells and other materials to replace or repair living tissue that has been damaged by injury or disease. However, unless artificial tissue is very thin, it may not get sufficient nutrition after it is implanted, which results in tissue death. That is where Zhao’s work comes in.
“We pre-vascularize the tissue, so it can hook up to the patient’s blood supply after it is implanted,” she said.
Her team has developed a technique for growing dense webs of the tiny vessels from stem cells, which can be layered on a sheet of artificial tissue. The tissue can then be rolled up, like a jellyroll, or stacked in alternating layers, like a club sandwich. In both cases, the capillaries assure that blood can reach the interior of a transplant of any size, providing life-giving nourishment throughout.
Next, Zhao hopes to begin animal studies with tissue embedded with capillaries and do long-term animal studies with her blood vessels. Plus she wants to speed up the time it takes to make a replacement artery. Success, she believes, is just a matter of time.
“The stem cell is quite magic,” says Zhao. “They are very smart cells.”
A joint venture
by Tony Fitzpatrick
Keat Ghee Ong has developed a sensor that one day could enable total knee replacement patients to move freely with “grace under pressure."
The key ingredient is a special coating applied to the artificial knee that can sense pressure while a person is moving through their daily routines—walking, bending, kneeling, twisting. Currently, there is no similar device available to monitor the new knee’s fitness under strain in real time after the surgery. All an orthopedic surgeon can do to test the knee replacement’s response to stress is to monitor it while a patient is still prone and under sedation.
The impact of bringing such a sensor to knee replacement technology would be major. In 2010 alone, there were over 500,000 such knees implanted in patients across the United States. While the vast majority of artificial knee replacements are successful and can last as long as twenty years, as much as five percent of such surgeries encounter functional problems due to stress on the artificial joints. With an aging Baby Boomer population of nearly 80 million Americans wearing out their natural joints every day, the technology could strengthen the state-of-the-art of artificial knee replacement practice--and benefit thousands of patients who might otherwise have their implants fail.
By providing highly detailed information on how artificial knees work in the body, the sensors would open the door to better implants. "Now, there is very little information doctors can get after the implant is inside the patient," said Ong, a professor of biomedical engineering at Michigan Tech. "Our technology is not just for individual patient care; it could also be used to improve the design of artificial knees, so they can last longer and be more comfortable."
Here’s how his technology would work. A typical artificial knee implant consists of the top part, the femoral replacement, and bottom part, the tibial component, usually made of titanium. In between is an insert made of a special, high-density plastic. It replaces the meniscus, the cushion of cartilage that in healthy knees keeps the bones from rubbing together. Ong’s sensor, made of iron-based, magnetoelastic materials, can be applied to the insert or embedded inside it.
When magnetoelastic material is exposed to mechanical energy (from walking, doing the tango, or shooting hoops), its magnetic properties change. Thus, a magnetoelastic sensor in an artificial knee could generate data revealing how much strain is buffeting the new plastic insert. Ultimately, the data could be used to map areas that are subject to stress in real time.
“What I’m trying to address is the surface-to-surface contact information,” Ong says. “Doctors don’t have information on what the metal attached to the plastic cartilage is doing while the patient is jumping, twisting, or dancing, even. To my knowledge there’s no comparable technology out there that does that."
While his sensor is wireless, to make it work will require a fairly simple, inconspicuous battery-driven device worn outside the body atop the knee. This device would generate the magnetic field that converts the mechanical force the sensor gathers into magnetic signals that would be read by an external apparatus like a PC or laptop computer.
Besides the novelty of providing a real-time analysis and functioning wirelessly, the sensor has other advantages: it is biodegradable and biocompatible.
“It can be made to degrade over a period of time, if needed,” Ong says. “There’s no harm to the body. Because it’s biocompatible, you wouldn’t have to worry if the plastic insert were damaged and the sensor exposed.”
Another plus is that magnetic energy penetrates the body much more easily than ultrasound or acoustic energy, which can be disrupted by bones.
Ong and his collaborators have done preliminary tests on simple animal models, and the results have been promising. The next step would be to implant a sensor in large animals and eventually humans. He also aims to improve its performance; the data he has gathered with the sensor provide a simple stress mapping profile, but he is working on a denser profile with higher resolution and richer, more-complex data.
Ong believes the technology holds promise in other arenas. “While my major thrust is with knee implants, I’m also looking at using magnetoelastic materials to improve cardiovascular stents,” he said. Ultimately, he hopes to use their unique properties to improve medical implants throughout the body, from head to toe and all places in between.
Good to the bone
For Tolou Shokuhfar, developing better surgical implants had always been about engineering. Then an orthopedic surgeon approached her after she gave a talk on her work.
She recalls the conversation. “You are changing people’s lives,” he told her. “You never want to see a patient with an infected implant. It is so hard on them. When they come to see me, we cry together.”
Until that moment, Shokuhfar confesses, she hadn’t given the matter much thought. “I was happy to be solving a serious problem, but the actual patients in the hospital were so far from my reality in the lab. Now I tell my students, ‘Guys, remember, we are really helping people.’”
Titanium and its alloys have a leg up on all other materials used to make the orthopedic implants used by surgeons to repair damaged bones and joints. They are light, strong, and virtually inert. Most of the time, titanium implant surgeries are successful. But if an infection sets in, or if the bone simply fails to heal properly, the results can be agonizing.
Since she was a graduate student, Shokuhfar, an assistant professor of mechanical engineering–engineering mechanics, has been researching a new surface for titanium implants to help head off such disasters. Using a simple procedure she developed (“You could do it in your kitchen sink.”), she etches nanotubes into the titanium dioxide that naturally encases metallic titanium.
To bone cells, those nanotubes feel like home. In lab tests, osteoblasts have clung to them and proliferated far better than to plain titanium or even the roughened titanium used on some implants. This may be because the nanotube surface forms a regular lattice, not unlike that matrix that forms the basis of bone tissue.
Ironically, early tests show that bacteria are repelled by the nanotube surface. Now scientists at the University of Tennessee are conducting additional research into how two types of bacteria react to nanotube-coated titanium. One is MRSA, an antibiotic-resistant form of staphylococcus known for causing intractable infections. Another is one of the bacterial species that cause gingivitis.
Why gingivitis? Shokuhfar is also working with colleagues at the University of Chicago’s College of Dentistry to develop better dental implants. They are posts, usually made of titanium, that are surgically placed into the jawbone and topped with artificial teeth. Like other types of implants, they sometimes fail or become infected; the same nanotube coating that could improve knee replacements could also brighten somebody’s smile.
The nanotube coating has another attribute that Shokuhfar believes could reduce the failure rate in all types of titanium implants. It can serve as a drug-delivery system for antibiotics, anti-inflammatory drugs, or even silver nanoparticles. “Silver has antimicrobial properties, and we are capable of obtaining a dose that can kill microbes but would not hurt healthy cells and tissues,” she said.
On the horizon are animal tests and eventually clinical trials. Because the nanotubes are simply another form of titanium dioxide, Shokuhfar hopes the approval process will be short.
“We want to get to clinical stage as soon as possible, so we can get this out there to people who need it,” she said. “I hope that in the future, none of these patients will ever cry again.”