Artificial kidneys are getting closer to becoming a clinical reality, thanks to a range of advances

Medical technology: Artificial kidneys are getting closer to becoming a clinical reality, thanks to a range of advances

Jun 1st 2013 |From the print edition

AN ARTIFICIAL kidney these days still means a refrigerator-sized dialysis machine. Such devices mimic the way real kidneys cleanse blood and eject impurities and surplus water as urine. But they are nothing like as efficient, and can cause bleeding, clotting and infection—not to mention inconvenience for patients, who typically need to be hooked up to one three times a week for hours at a time. Still, for 2m people around the world who suffer from chronic kidney failure, dialysis is the best option—unless they qualify for one of the 76,000 or so kidney transplants performed each year. Even those lucky few endure a lifetime on drugs to stop their bodies rejecting the foreign tissue.

A world in which new kidneys are grown using a patient’s own cells remains some way off. In the meantime, human patients are likely to be offered tiny versions of the dialysis machine. None is available yet, but last year America’s Food and Drug Administration (FDA) picked a couple of candidates to participate in a fast-track process called Innovation Pathway 2.0, which aims to bring promising medical gizmos to market more promptly.The first is the Wearable Artificial Kidney (WAK), which was designed by Victor Gura of the Cedar Sinai Medical Centre, in Los Angeles, and is being developed by a firm called Blood Purification Technologies. Dr Gura’s device squeezes the old dialysis machine’s clunky warren of tubes and filters into something that weighs about 5kg and can be worn on a belt around the waist.

The WAK works by sucking blood from a patient’s vein and pumping it through a membrane that consists of hollow, semi-permeable fibres a few microns across. As the blood flows through the insides of these fibres, a waste-absorbing fluid, called a dialysate, flows the other way past their outsides. Since the concentration of metabolic waste is higher in the blood than in the dialysate, molecules smaller than the fibres’ pores diffuse from the blood and into the dialysate.

Manipulating the pressure of the blood and the dialysate allows molecules of different sizes to be extracted from the blood. These are removed from the dialysate by running it through a substance called a sorbent, which soaks up the waste and is binned once a day. Thus cleansed, the dialysate can be recycled, so the WAK needs less than half a litre of it, rather than the 120 litres used—and flushed down the drain—in old-fashioned dialysis.

A few years ago an early version of the WAK was tested on eight patients in London for up to eight hours at a time. Later this year a small trial at the University of Washington, in Seattle, will attempt to extend this to 24 hours, before proceeding to larger trials in hospitals, and ultimately unsupervised ones in patients’ homes.

The WAK does, however, suffer from a problem inherent in dialysis. Along with the bad substances it removes from the blood, some good stuff vanishes too. The FDA’s second pick, the implantable Renal Assist Device (iRAD), tries to avoid this by harnessing natural kidney cells’ ability to distinguish between wanted and unwanted molecules.

The iRAD, which is the size of a coffee cup, consists of two parts: a filter and a bioreactor, where the cells are stored. Though its layout is different from that of a natural kidney, it works in basically the same way. In a real kidney, part of the aqueous fraction of blood is squeezed out of it in filters called glomeruli, leaving blood corpuscles, platelets and dissolved proteins behind to be returned to the bloodstream. This aqueous liquid then passes through tubes whose cells absorb valuable chemicals, such as salt and glucose, and also some of the water. These, too, are returned to the bloodstream. What remains drains into the bladder as urine.

In the iRAD, a silicon filter stands in for the glomeruli. The silicon is organised into layers, into each of which millions of slit-shaped pores have been etched using techniques similar to those used to make computer chips. The pores’ clever engineering and tiny size (just a few billionths of a metre across) allowed the iRAD’s designers—William Fissell of Vanderbilt University, in Nashville, Shuvo Roy of the University of California, San Francisco, and H. David Humes of the University of Michigan—to dispense with an internal pump. Instead, the iRAD uses natural blood pressure supplied by the heart.

Once they have been separated, both fluid and filtrate are passed through the bioreactor. This consists of a series of silicon plates similar to those in the filter. In its case, though, some of the plates’ surfaces are covered with kidney cells (taken from human kidneys deemed unsuitable for transplant) of the type that line the urine-generating tubes.

Alternate spaces between the silicon plates carry fluid and filtrate. The cells, which line the spaces carrying the fluid but not those carrying the filtrate, extract glucose, salts and other desirables from the fluid and transfer them through the pores to the filtrate along with some of the water, just as would happen in a real kidney. What emerges, in different streams, is cleansed blood and urine.

Tests in both the laboratory and in pigs suggest the iRAD should work, but it is further away from clinical reality than the WAK is. Both devices would be expensive to buy, but cheap to run—and offer hope to the 95% of patients who die while awaiting a transplant.