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Conventional dialysis, in which a patient’s blood is pumped through an external filter to drain out accumulating toxins, is far from ideal for the 1.4 million people with kidney disease worldwide whose lives depend on it. The common regimen of three half-day blood-cleansing sessions per week removes, on average, just 17 percent of the toxins that a healthy kidney would clear, so that only one-third of all dialysis patients survive more than five years of treatment.

Nanotechnology could offer an alternative, according to nephrologist William Fissell at the University of Michigan. He and colleagues are working on nano-pore membranes that could enable dialysis to be miniaturized into implantable devices that provide round-the-clock clearance of toxins, untethering dialysis patients from bulky pumps and clinics. “This is a fundamentally liberating technology,” says Fissell.

Fissell and colleague Shuvo Roy, a biomedical engineer at the Cleveland Clinic Foundation, claim to have solved half of the challenge: engineering nano-membranes that are efficient enough to support a compact, low-power implant. The team secured a patent for the concept earlier this year. However, engineering pores with the required selectivity–pores that drain away the worst toxins without robbing the body of critical proteins such as albumin, blood clotting factors, and antibodies–is proving to be tougher than expected.

As currently practiced, dialysis is a crude procedure. Patients are hooked up intravenously to a powerful pump that circulates their blood through a cartridge of porous plastic fibers. Fluids, dissolved toxins, and salts pass through the fibers and are discarded, while the proteins and blood cells caught in the sieve are supplemented with electrolyte before returning to the patient. The filter’s poor fluid dynamics are a function of their imprecision: filter manufacturing produces a wide range of pores, so to avoid having too many large pores, which would suck out valuable proteins, the fibers must be manufactured with a preponderance of very small pores. The machine’s pump makes up the difference, forcing blood through these inefficient sieves.

In contrast, Fissell and Roy etch pores into ultrathin wafers of silicon with lithographic precision. The result is a homogenous array of pores, each capable of flow rates several orders of magnitude higher than the average pore in a conventional filter. The pores mimic the exquisitely precise yet efficient diaphragms that filter blood in a human kidney, resembling a panel of Venetian blinds, says Fissell.

Current prototypes contain roughly 10,000 pores per square millimeter, according to Fissell. Next-generation membranes, now being engineered, will have more than 100,000 pores or slits per square millimeter and provide more than 10 times the flow. An implanted device carrying several hundred square centimeters of this next-generation membrane should, Fissell estimates, filter at least 30 milliliters of blood per minute at average blood pressures–about one-third of normal kidney function. The implant would be tucked under the skin; small fluid bags worn externally could receive the ultrafiltrate and supply replacement electrolytes.

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Tagged: Biomedicine

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