Developments towards an artificial kidney

This article reviews the present state of renal failure and its treatment in the industrialized world. Novel and experimental therapies for the treatment of renal failure are covered, with special emphasis on a hybrid bioartificial kidney currently undergoing clinical trials in the USA. Preclinical data, results from human trials and work on miniaturization of the bioartificial kidney for implantation are presented. Research on microfluidics and nanotechnology applied to dialysis is ongoing in many academic centers, and several promising approaches are discussed. After 10 years of incremental improvements in end-stage renal disease care, several revolutionary technologies are on the horizon and approaching the marketplace.

[The wearable artificial kidney: a promise for the future?]

Portable or wearable dialysis devices could increase treatment flexibility and dialysis patients' independence. Current renal replacement therapies such as intermittent haemodialysis extend life but are a burden, are time-consuming and immobilize patients. An additional disadvantage is the discontinuous nature of the treatment. Peritoneal dialysis is a good alternative, but is associated with relatively limited toxin clearance and a need for high glucose concentrations in the dialysate. Portable dialysis devices could be used as a replacement or to support existing dialysis techniques. At the moment several initiatives, including some started in the Netherlands, aim at the development of a portable device. Some of them are so far into development that they are at a preclinical phase, but as yet none has been approved for regular use in patients. To achieve the ultimate goal, an implantable artificial kidney, a lot of hurdles still have to be surmounted.

The artificial kidney: past, present, and future

World War II can be taken as a turning point after which the introduction and development of several new diagnostic and therapeutic discoveries have revolutionized medicine and improved the expectancy of life for millions. Notable amongst those technological therapeutic achievements is that of the artificial kidney, first used successfully in the closing years of the war. As a result of the improvements that followed, the kidney was the first solid organ whose function could be replaced, at least partially, by a machine. What started then as exploratory efforts to sustain life evolved over the next few decades into life saving replacement therapy for millions worldwide. Chronic maintenance hemodialysis has certainly changed the prognosis of the otherwise fatal end stage kidney disease that had afflicted humans theretofore. Unfortunately, many of the challenges and problems that had to be overcome in making artificial kidney treatment available continue to plague end-stage kidney disease patients on maintenance hemodialysis. Concerted investigative efforts are currently underway to improve the replacement of kidney function with artificial kidneys that better mimic kidney function. This article reviews the beginnings, evolution, and current challenges of the artificial kidney.

Dialysis and nanotechnology: now, 10 years, or never?

Nanotechnology, defined as the science of material features between 10(-9) and 10(-7) of a meter, has received extensive attention in the popular press as proof-of-concept experiments in the laboratory are published. The inevitable delay between feature articles and clinical endpoints has led to unwarranted skepticism about the applicability of the technology to current medical therapy. The theoretic advantages of micro- and nanometer scale engineering to renal replacement include the manufacture of high-hydraulic permeability membranes with implanted sensing and control structures. Recent data in membrane design and testing is presented, with a review of the challenges remaining in implementation of this technology.

Bionic kidney

Bionic Kidney is a project still in progress which aims at replacing all renal functions, which has been carried out in an ideal attempt to improve the overall results of Renal Replacement Therapy. It contains all the requisites for a complete rehabilitation from Uremia. As a futuristic mini-device implanted in the body, it should be a reliable support to Transplantation performance, considering the scarcity of kidney donors.

Artificial kidney: status of the art and new perspectives

Extracorporeal dialysis was first performed in 1943 and has become a routine for End Stage Renal Patients from the early sixties. In the last 30 years researchers have focused on biocompatibility of artificial materials and optimisation of removal of uremic toxins by the membrane as in the long term treatment many complications like amylodosis heart and bone lesions, accelerated amyloidosis and immune system failure can occur. From this point of view high flux dialytic membranes are currently considered more biocompatible therefore being able to prevent such diseases.

Tissue engineering the kidney

The means by which kidney function can be replaced in humans include dialysis and renal allotransplantation. Dialytic therapies are lifesaving, but often poorly tolerated. Transplantation of human kidneys is limited by the availability of donor organs. During the past decades, a number of different approaches have been applied toward tissue engineering the kidney as a means to replace renal function. The goals of one or another of them included the recapitulation of renal filtration, reabsorptive and secretory functions, and replacement of endocrine/metabolic activities. This review will delineate the progress to date recorded for five approaches: (1) integration of new nephrons into the kidney; (2) growing new kidneys in situ; (3) use of stem cells; (4) generation of histocompatible tissues using nuclear transplantation; and (5) bioengineering of an artificial kidney. All five approaches utilize cellular therapy. The first four employ transplantation as well, and the fifth uses dialysis.

Metabolic replacement of kidney function in uremic animals with a bioartificial kidney containing human cells

Current renal substitution therapy with hemodialysis or hemofiltration has been an important life-sustaining technology, but it still has suboptimal clinical outcomes in patients with end-stage renal disease or acute renal failure. This therapy replaces the small solute clearance function of the glomerulus but does not replace the metabolic and endocrinologic functions of the tubular cells. This article shows that the combination of a synthetic hemofiltration cartridge and a renal tubule cell assist device (RAD) containing human cells in an extracorporeal circuit replaces filtration, metabolic, and endocrinologic functions in acutely uremic dogs. The RAD maintained excellent performance and durability characteristics for 24 hours of continuous use in the uremic animals. The RAD increased ammonia excretion, glutathione metabolism, and 1,25-dihydroxyvitamin D3 production. Cardiovascular stability in the animals was documented in these studies during this extracorporeal treatment. With these results, clinical evaluation of this device in the treatment of severely ill patients with acute renal failure in an intensive care unit has been initiated.

Problems and solutions for artificial kidney

The uremic syndrome is the prototype of a slowly progressive endogenous intoxication, when a detoxifying organ (in this case the kidney) fails. It is characterized by the gradual retention of a host of metabolites, which is in part corrected by dialysis, allowing survival with an acceptable quality of life. This paper reviews the main problems of hemodialysis today, and possible solutions. Adequacy of dialysis is estimated currently from the concentration of urea, which is used as a marker molecule. The problem is that urea is not really toxic by itself. Other markers with known toxicity, such as middle molecules (300-12,000 D) and protein bound compounds should be considered. The question then arises whether the classical dialytic concept based on diffusion should be modified. Adsorptive systems may be strong binders of protein bound solutes. Other concepts that are now arising, and that may add to toxin removal, are slow and daily dialysis. Another question that could be raised is whether it would not be possible to support toxin removal, by administering peroral sorbants. Dialysis patients are prone to vascular disease and die early from cardio-vascular complications. One of the solutions for this problem could be to bring the blood of dialyzed patients into contact with antioxidants (e.g. vitamin C or E). The risk for perdialytic hemodynamic instability is increased in many dialysis patients. The ideal solution would be to develop an "intelligent" dialysis system, whereby blood volume and plasma osmolality are sensed continuously, and ultrafiltration and dialysate sodium concentration are adapted in function of this evolution. An adequate vascular access is indispensable to perform adequate dialysis, but thrombotic/stenotic complications are frequent. This could be prevented by molecular biological modification of vascular grafts, whereby genetic information is entered into the cells, blocking the natural chain of events that otherwise unavoidably leads to neointimal hyperplasia and atherosclerosis. Another old dream is to develop a wearable artificial kidney, whereby patients can move around, and be treated 24 hours per 24 hours, in stead of being treated intermittently at a specific location by the dialysis machine. According to some authors, part of the natural renal function could be replaced by cultured renal tubular cells, which are brought in contact with the blood of the patients. It is concluded that thrilling improvements lie ahead in the future, but the following questions arise: 1) What is the cost of all these improvements? 2) Will it remain possible to reimburse all this? 3) What is going to happen in transplantation, mainly regarding improvements in immunosuppression and the development of xenotransplantation?

Bioartificial organ support for hepatic, renal, and hematologic failure

The current strategy to the treatment of SIRS and MODS uses a multidisciplinary approach that emphasizes supportive therapy. Herein, we have presented a futuristic approach that focuses on replacing the function of failed organs using bioartificial technology (Table 1). Bioartificial organ technology may allow the intensivist to provide physiologic organ replacement either as a bridge to transplantation or as a "time-buying" element until native organs that have become acutely dysfunctional or nonfunctional in a variety of clinical settings, can recover their function or regenerate their mass. As bioartificial organ technology matures, it is conceivable as an ultimate goal that non-immunogenic bioartificial organs would be miniaturized or redesigned and acutely placed within the intracorporeal space as replacement organs.

[The development of nephrology and dialysis in the past 40 years]

In the past 40 years the nephrology has been established in the GDR as a subspeciality of internal medicine and pediatrics. In the districts more than 60 nephrological centres were opened and modern diagnostical and therapeutical methods were introduced. More than 2000 patients were treated regularly by artificial kidney. Further detoxification methods including therapeutical plasmapheresis were introduced in the practice in the last years. The number of dialysis increase from 300,000 in 1989 to 500,000 in 1991.

[Hemodialysis in Japan--present and future]

On the base of official data of the Japanese Society of Dialysis Therapy in 1985 a survey of the current status of dialysis in Japan is presented. Furthermore, an outlook on the future of the development of dialysis therapy in Japan according to these data is given.

The current status and future of the artificial kidney

The use of the artificial kidney can presently be extended to almost all patients with end-stage renal failure. To reduce the cost of treatment, technological choices have to be made. These are always a compromise between cost and adequacy. The liberty obtained by technical improvements to perform a dialysis "à la carte," depending on patient and doctor wishes, is one of the main characteristics of the present status of hemodialysis.

[What is the current outlook for an implantable artificial kidney?]

Considering the homeostatic demands made by the organism on an artificial eliminating substitute kidney system, it seems that the haemofiltrative principle for transporting matter is the most suitable for simulating the natural glomerular kidney function in an implantable artificial kidney. Biophysical laws can be used to influence long-term filtrative efficiency and the blood-flow in the haemofilter system can be controlled by technical feedback regulation. Endogenically, the filtration process is, in terms of energy, driven by the aterial blood pressure (specifically, by the trans-membrane pressure thus generated). The definitive ultrafiltrate treatment takes place by intestinal reabsorption and elimination of the end products, whereby these functions may be assisted by the use of sorbents etc. or an adequate substitution therapy if reabsorption is insufficient. These processes can reasonably be triggered by neurohormonal feedback regulation if the homeostasis situation is greatly disturbed by the haemofiltration process. The implantable artificial kidney is thus a biochemical hybrid kidney.

Dialysate delivery: historical, theoretical, and practical aspect

Maintenance hemodialysis has progressed from a tentative therapy for a few patients in Seattle to a generally available treatment sustaining 100,000 patients around the world. The technical principles of dialysis are sufficiently understood to permit fabrication of disposable dialyzers in sheet, coil, or hollow-fiber configuration. Dialysate delivery systems can serve either single patients or groups of patients with high efficiency. Reduction in size of single-parent systems by incorporation of dialysate regeneration or miniaturization of components in a suitcase will promote patient mobility.