34 IEEE Spectrum | February 2005 | NA

STE

VE

MA

RS

EL

S

+Aging

TECHNOLOGY

REPLACEMENT PART: Biohybrid

livers made of layers of micro-

fluidic channels and live liver

cells could one day save lives.

Fourth in a series of reports on biomedical engineering innovations

Meet Bob.Bob might be your next-door neighbor. He’s an affa-

ble, 60-something retired engineer who took a buyoutbefore the Internet bubble burst. He golfs, enjoys his grand-children, and likes to travel with his wife of 35 years. Theonly problem is that he’s getting old—and he knows it.He’s bald on top, his skin is starting to sag in places, hiseyesight isn’t what it used to be, and he sometimes can’thear what his dinner companions are saying. Even worse,the diabetes that he was diagnosed with in his 40s is get-ting more difficult to control: his blood sugar seems likeit is up one minute, down the next. And it seems as thoughall his friends have had at least one heart attack.

Bob is worried about getting old.As an engineer, Bob knows that the body is just one big

system that runs by chemical gradients and electricalimpulses. So why is it so difficult to come up with replace-ment parts when bodies like his start to break down?

Luckily for Bob—and especially for those of us whoare younger and can wait a few more years—engineers anddoctors, mostly in the United States, have stepped up theirsearch for ways to build replacement parts.

The current effort is focused on so-called biohybridor bioartificial organs, which combine living cells withmaterials such as silicon and polymers. The hybrid organsget their structure from the inorganic material while rely-ing on living tissue, grown from cadavers, animals, or, oneday, from the patient’s own body, to do the complex tasksthey do best, such as processing biochemicals and fil-tering blood. Although biohybrids are being testedoutside the body and are at least five years away fromreaching the market, ultimately they are being designedas implants—seamless replacement parts.

If I’d known I was going to live this long, I’d have taken better care of myself.

—Ragtime musician James Herbert

(“Eubie”) Blake, at age 100, in 1983

February 2005 | IEEE Spectrum | NA 35

B DYSHOPS

PART HUMAN, PART MACHINE,

REPLACEMENT ORGANS

MAY ONE DAY EXTEND

YOUR LIFE

BY CAROL EZZELL WEBB

THE

Pump

Heat exchanger

RAD cartridge

Processed ultrafiltrate(urine)

Ultrafiltrate

Heatexchanger

Pressuremonitor

Dialysis machine

Pump

PumpVenousblood

ALL ILLUSTRATIONS: BRYAN CHRISTIE

AN UNCONVENTIONAL KIDNEY: The renal assist

device (RAD), under development at Nephros

Therapeutics, is a bioartificial organ being tested

as an improvement on conventional kidney dialysis.

In early trials, just a day or less of treatment with

the RAD extended the life of critically ill

patients by weeks.

Blood is pumped from a patient’s arm through a

kidney dialysis machine, which filters waste mate-

rial from the blood. Unfortunately, some useful

substances are also filtered out. So the filtrate and

blood are fed through the RAD. The RAD contains

renal proximal tubule cells that take the useful sub-

stances from the filtrate and put them back into the

blood, which is then pumped back into the patient.

Construction of replacement organs got off to a rocky start, whenthe first two companies to come out with a product, living skin,went bankrupt nearly two years ago [see “Synthetic Skin,” IEEESpectrum, December 2002]. But tissue engineers have gone back tothe lab. And nowadays it’s a semiconductor lab.

A good portion of the newest innovations involve growing cellsin specifically ordered arrays on chips made of semiconductor mate-rial. Electrical impulses pattern the cells as they would be in a nat-ural organ and stimulate them appropriately so they develop into theneeded tissues. Researchers have taken a page from the books ofmicroelectromechanical systems (MEMS) developers and have builttiny systems using semiconductor fabrication techniques to addressdiseases that are now either marginally treatable or untreatable.

Many, if not most, of the new biohybrid research projects tar-get some of the primary disorders of aging: kidney failure, emphy-sema, and heart disease—all among the top 10 killers of Americansolder than 65. In 2001, heart disease alone killed one-third of every-one older than 65 who died in the United States. Kidney disease alsotakes a significant toll: more than 72 000 people in the United Statesdie each year from end-stage renal disease. And current, stopgaptherapies are costly: the 350 000 Americans who need kidney dialy-sis require a total annual expenditure of US $25 billion.

“THE NUMBER ONE ORGAN needed right now is the kidney,” saysAnthony Atala, director of the new Institute for RegenerativeMedicine at Wake Forest University Baptist Medical Center, in

Winston-Salem, N.C. Roughly 15 000 kidney transplants are per-formed in the United States every year, but 58 000 Americans andabout 40 000 Europeans are waiting for kidneys at any given time.(Global numbers are difficult to come by, because few countries keepsuch statistics.) The dire shortage of kidneys—and the relative sim-plicity of their blood-filtering function—makes them likely to bethe first biohybrid organs available to patients.

Diabetes and hypertension are the top two reasons for needinga kidney transplant, and the organ-damaging effects of those twodiseases escalate with age. There are two basic types of kidneyfailure. Acute failure is reversible and typically caused by a severeinfection or as a consequence of coronary bypass surgery. Chronicfailure means a person’s kidneys have essentially ceased function-ing. And that means frequent dialysis: a time-consuming treat-ment that keeps fewer than 20 percent of patients over 65 alivefor as long as five years. “The prognosis for a dialysis patient isfrightening,” says Jeffrey T. Borenstein, director of the biomedicalengineering center at the Charles Stark Draper Laboratory Inc., inCambridge, Mass., who is developing a replacement kidney.

Developers of biohybrid kidneys are targeting acute kidney fail-ure first, because those patients are hospitalized and can be treatedwith a device that functions outside the body. Later, the developershope to use what they learn to devise implantable devices that mightextend the lives of those with chronic kidney failure.

If our friend Bob developed acute kidney failure today, perhapsfollowing a coronary bypass, the most advanced treatment he could

Blood

Proximaltubule cells

Membrane

receive might include an experimental renal assist device (RAD)developed by H. David Humes, a professor of internal medicineat the University of Michigan, in Ann Arbor. The RAD is now inits second of three phases of clinical trials for use as an adjunctin treating acute kidney failure.

The RAD is a device that can be hooked up to a dialysis machineat a patient’s bedside [see illustration, “An Unconventional Kidney”].It’s a biohybrid because it contains human proximal renal tubulecells, kidney cells that normally recover useful chemicals from thefluid that remains after the blood has been strained through theglomeruli, the kidney’s first line of filters. The glomeruli act con-servatively but usually filter out some beneficial compounds alongwith the bad. To counteract that, cells in the renal proximal tubulesscavenge glucose and salts, in particular, from the filtered fluid andreturn them to the blood. Thecells also appear to help theimmune system fight off sys-temic infections, a major causeof death from acute renal fail-ure. The RAD, containing prox-imal renal tubule cells takenfrom kidneys that couldn’t beused for transplant, is de-signed to do the same. It filtersfluid that has already gonethrough a dialysis machine,which also removes some use-ful substances.

In its Phase I clinical trial,the RAD was used in combi-nation with kidney dialysis totreat 10 patients with acute kid-ney failure for up to 24 hours.All but one of the patients wasexpected to die within days;instead, six survived for weeks.While it was hooked to apatient, the device boosted kid-ney function and urine outputand seemed to help the patient’sown kidneys recover, Humessays. He founded NephrosTherapeutics Inc., Lincoln, R.I.,to develop the device.

Ultimately, Humes wouldlike to transform the RAD intopart of an implant for treating chronic renal failure. A nephrologistwho collaborates with Humes, William H. Fissell, has begun to growrenal proximal tubule cells on silicon chips to create such a device.One of the toughest challenges will be making something smallenough to be implanted in someone’s body but large enough to fil-ter the roughly 180 liters of blood processed each day by a per-son’s two kidneys.

Borenstein, from the Draper Labs, is working on the scale-upproblem. He recently reported initial results from his prototypekidney system, a microfluidic device that has an area of 15 squarecentimeters and is about 1 millimeter thick. The device mimics thestructure of the glomeruli, which filter the blood before it passesthrough the renal proximal tubules in the kidney, and consistsof two planar sheets of plastic etched with microchannels andseparated by a special polymer-based filtration membrane. As bloodcourses through the microchannels in the top sheet, waste prod-ucts pass through the membrane into the microchannels of the bot-

tom sheet. “You scale up by adding layers,” says Borenstein, whobegan his career as a physicist in the semiconductor industry. Theultimate device might have hundreds or thousands of layers, hepredicts. In the tests, he demonstrated that the MEMS could removecreatinine and urea from blood samples—two of the three majorwaste products filtered by the kidneys.

As a next step, Borenstein plans to incorporate renal proximaltubule cells into the MEMS. He foresees creating a device attachedto the circulatory system—perhaps at the wrist—that would bedriven by the body’s own blood pressure.

IF BOB EVER HAD VIRAL HEPATITIS, he might eventually need a livertransplant, because the damage wreaked by hepatitis viruses oftenleads to liver failure years later. Most researchers agree that the liver’s

complex digestive, hormonal,and metabolic functions willmake it tough to replicate. Butsince the demand for livertransplants is so high—atroughly 17 500 in the UnitedStates, second only to theneed for kidneys—several aca-demic labs are trying anyway.They are pursuing bio-artificial livers by building tinychannels in MEMS to mimicthe liver’s microvasculature.

One notable laboratory inthe search is led by Joseph P.Vacanti, a professor of surgeryat Massachusetts GeneralHospital, in Boston. Vacanti’sgroup uses patterned silicon asa master mold to produce thinsheets made of biodegradablepolymers such as polylacticglycolic acid (PLGA), the samematerial from which dissolv-able sutures are made [seeillustration, “Layers of Liver”and photo, “Replacement Part”].“You pour [the PLGA] on thesurface of the silicon, let it dry,and then peel it off,” Vacantisays. The end result is a micro-patterned substrate with tiny

channels that mimic the microvasculature feeding the liver or thetracery of bile ducts that lead to the digestive tract.

Each liver module consists of two layers of PLGA. One is seededwith liver cells. The other is honeycombed with channels for bloodflow that are lined with endothelial cells, the primary constituentof blood vessel walls. Separating the two is a thin polymer mem-brane through which proteins and toxic substances can pass to bebroken down or detoxified. The support layers are biodegradable,because once the bioartificial liver matures, the hope is that it willbe able to support itself without the PLGA.

The resulting biohybrid liver module overcomes an early stum-bling block of tissue engineers: you need lots of cells in order tosimulate an organ, but the more tightly you pack them, the fartherthey get from a blood supply, so they run out of oxygen and starveto death. In Vacanti’s devices, “the cells are never more than onemembrane away from their blood supply,” he says.

Vacanti reports that his group is working to stack the liver

February 2005 | IEEE Spectrum | NA 37

The polymer is peeled off the silicon.

Photolithography defines a pattern

of microscopic blood vessels and

other structures on a silicon wafer

that will act as a mold.

A biodegradable polymer is spread

over the mold and allowed to set.

Polymer layers are seeded with the

various types of liver cells needed,

and then stacked and sealed to form

a complete biohybrid liver.

1

2

4

3LAYERS OF LIVER:

An experimental scheme con-

structs a bioartificial organ out

of multiple layers of patterned

biodegradable polymer.

Photolithography beam

Silicon wafer

Polymer

38 IEEE Spectrum | February 2005 | NA

modules in sufficient quantities that they could take over for anadult liver. So far, their top number is 10, however, and the jobmight require as much as 100 times that. The group is evaluat-ing modules implanted in animals, and it plans to report itsresults within a few months. If the studies pan out, Vacanti says,the next step might be to test the modules as a means of keep-ing patients alive while they wait for traditional liver transplants.Eventually, he says, he hopes the devices might substitute fortransplants. “We see this technology evolving into implantablebiohybrid devices and, ultimately, into organs made entirely fromtissue,” he says.

Although not implantable, a biohybrid liver-assist device hasalready made it out of the lab. But its producer, Circe BiomedicalInc., in Lexington, Mass., recently went out of business. Circe’sdevices were composed of plastic pipes packed with straws madeof an artificial membrane material and filled with a type of humanliver cells, hepatocytes. Patients were hooked up to external pumpsthat ran their blood around the straws to be cleansed by the hepa-tocytes, after which the blood was returned to the body. Circe’shuman trials showed, however, that the device worked only in thesmall subset of people with fulminant liver failure, a conditionthat kills hundreds of people each year and is characterized bybrain swelling. When the U.S. Food and Drug Administration(FDA) demanded additional trials, the added expense knockedCirce out of business.

“A lot of things that seemed to work haven’t made it throughthe FDA, because small companies just don’t have the money orthe skill sets to get them approved,” says Michael J. Lysaght, direc-tor of the Center for Biomedical Engineering at Brown University,in Providence, R.I.

BOB SMOKED FOR 30 YEARS before finally quitting a decade ago,so he’s at risk for developing some type of chronic obstructivepulmonary disease (COPD), such as emphysema, which killsroughly 100 000 Americans each year. Along with severe infec-tions, it is a primary disorder requiring lung transplants, and bothconditions are more likely among the elderly. The demand for

lung transplants tends to be a third of that for livers, but it willundoubtedly grow as the world population ages.

Bob might benefit from an implantable biohybrid lung beingdeveloped by associate professor William J. Federspiel and grad-uate student Kristie Henchir, both at the McGowan Institute forRegenerative Medicine at the University of Pittsburgh. The bio-hybrid attempts to emulate the gas-exchange function of the lungs,in which oxygen is absorbed into the blood and carbon dioxide isreleased. Lungs are so efficient at their job because inhaled air endsup in tiny sacs called alveoli, which are coated with a fine net of cap-illaries. Accordingly, blood and air are separated by only a thinmembrane, through which gases can diffuse.

The biohybrid lung is based on a MEMS device, about the sizeof a thick credit card, that duplicates the function of the alveoliby bringing air and blood into close contact [see illustration,“Breathing Chip”]. The MEMS is laced with microchannels con-taining either air or blood, which are separated by thin membranesthat mimic the alveolar wall. To prevent coagulation, the blood-containing microchannels are lined with endothelial cells, pastwhich the blood has evolved to flow smoothly without clotting.Federspiel and his collaborators use endothelial cells from dis-carded umbilical cords, but they eventually plan to use cells takenfrom a patient’s own fat tissue.

“We’ve been able to grow the cells in the channels, and we’renow looking to see what level of blood flow we can use and still keepthe endothelial cells attached to the microchannels,” Federspiel says.“Once we get these little units perfected, we could integrate theminto larger-scale modules that would be either implanted or wornoutside the body.”

THE ULTIMATE BIOHYBRID-ORGAN design problem might be the heart,because it must beat steadily and continuously throughout a per-son’s entire lifetime—about 100 000 beats each day. Fabricatinga complete heart made entirely from human cells turned out tobe more difficult than expected. Instead, tissue engineers are nowlooking for ways to replace pieces of the heart.

Robert S. Langer, a professor at the Massachusetts Institute of

BREATHING CHIP: A prototype bioartificial lung [left] under development at the McGowan Institute for

Regenerative Medicine consists of alternating layers of microfluidic channels through which blood and air flow.

The layers are separated by a gas-permeable membrane, and the channels that carry blood are lined with

endothelial cells, the principal constituent of blood vessels.

MEMs chip

Air

Blood Blood channels

Endothelialcells

Gas-permeablemembrane

2 cm

Airchannel

The polymer patch is then

seeded with the three major

types of heart cells.

To simulate the conditions

in a living heart and

coax the cells to form

functioning heart muscle,

the patch is stimulated

with electric current.

Once a way is found to increase the

thickness of the experimental

patches, they will be strong enough

to be grafted onto human hearts.

MENDING A BROKEN HEART: Researchers at the Massachusetts

Institute of Technology are trying to engineer patches of heart

muscle to replace those areas damaged during heart attacks.

1 2

3

5

A thin square of a biodegradable

polymer is shot through with a laser

to form a fine network of channels

through which fluid can flow.

4When mature,

the patch beats

like a piece of

living heart.

ABOUT THE AUTHOR: CAROL EZZELL WEBB is a freelance journalist in Austin, Texas, and aformer editor at Scientific American. Her last article for IEEESpectrum ["Chip Shots," October 2004] described semiconductor-related drug delivery systems.

TO PROBE FURTHERNephros Therapeutics (http://www.nephrostherapeutics.com/) is commer-cializing H. David Humes's biohybrid kidney. Gordana Vunjak-Novakovic'smost recent cardiac muscle–patch research was published in December onthe Web site of the Proceedings of the National Academies of Science(http://www.pnas.org).

Laser

Polymer

Cells

Electrode Stimulator

Patch

Normal state Contracted state

February 2005 | IEEE Spectrum | NA 39

Technology, in Cambridge, was a pioneer in this area. He and MITprincipal research scientist Gordana Vunjak-Novakovic plan to beginanimal tests soon for cardiac tissue constructs that she calls “con-tractile patches.” A decade or so down the road, if Bob were to havea heart attack—the leading cause of death in industrialized coun-tries—he might turn to Vunjak-Novakovic and her colleagues fora living bandage. The MIT group is working to make a patch ofmuscle that would be surgically attached to a damaged part of theheart to take over its contractile duties.

The MIT researchers have been able to coax heart cells iso-lated from newborn rats to mature into cardiac muscle by placingthem on support scaffolds of biocompatible materials and expos-ing them to electrical stimulation and plenty of oxygen. Under theseconditions, the heart cells fuse into functional tissue and beginbeating synchronously. It works because the heart is essentiallyan electrical organ, crackling with current during each beat. “Wetrick the cells by using the same electrical stimulation they wouldget during development,” Vunjak-Novakovic says. The result istissue that looks and acts just like slices from a mature heart. “Whenwe compare the engineered tissue to native tissue, we see no essen-tial differences,” she says.

To produce their contractile patches, she and her colleagues takea biodegradable polymer invented at MIT, called biorubber, anduse a laser to pierce it with a fine network of channels [see illus-tration, “Mending a Broken Heart”]. Each piece is a rectangle roughly1 cm2 in area and up to 3 mm thick. Next, they seed the biorubberwith the three major cell types of a heart: cardiomyocytes, endothelial cells, and fibroblasts. (Human cells of these types canbe derived from so-called adult stem cells, found, for example, inleftovers from liposuction.) As the cardiomyocytes beat, theyadhere to and tug on one another, helping them to communicateelectrically and to secrete the growth hormones they need to survive.

To be strong enough to replace dead heart tissue in people whohave had heart attacks, however, the contractile patches must be atleast 5 mm thick. Once they can achieve that thickness, Vunjak-

Novakovic says, making patches of the needed size should be acinch. “It’s relatively easy to grow a larger rectangle,” she says.

OF COURSE, TECHNICAL HURDLES are only part of the biohybrid story.There are real concerns about how feasible biohybrid organ replace-ments will be as treatments for the disorders of aging. Right now,people older than 65 do not receive as large a proportion of organtransplants as one might expect, because policymakers have deemedthat such scarce resources should go to younger, healthier peoplewho have more years ahead of them.

Although that scenario could change if biohybrid organs ulti-mately become widely available, there are also the staggering coststo consider. Insurers are already reluctant to pay for organ trans-plants because they cost hundreds of thousands of dollars. Biohybridorgans could only be more costly because, unlike ordinary trans-plants, they must be manufactured.

With the intensive work being done on biohybrid organs, Bob—if he has the money and years more to live—might rest a bit easierknowing that replacement organs are on the horizon. But tissueengineers are wary of making bold predictions about how soonBob and his friends might benefit from their technology. As Vunjak-Novakovic says, “We are at the end of the beginning, but there is alot more to be done.” �