Kidney failure is common and devastating at the individual level due to the crushing burden of treatment and at the public level due to the extraordinary cost of dialysis. The best treatment, a transplant, is generally not available on demand due to the profound scarcity of donor organs. The ever-growing mismatch between demand and supply, as well as the incremental and marginal advances in dialysis technology, have stimulated interest in xenotransplantation, stem cell therapies, repopulation of matrix scaffolds, and our own work advancing hybrid solutions.
Many of the dozen functions of the kidney can be replaced with pharmacotherapy: vitamin D hydroxylation, erythropoesis, acid-base balance, and even the concentrations of most cationic electrolytes can be controlled medically. Serum osmolality can be autologously regulated by thirst. From an engineering perspective, it is a distraction to focus on any function other than the essential. Kidneys are essential to excrete nitrogenous wastes and maintain neutral extracellular fluid volume balance. It is these two homeostatic processes we must reproduce if we are to engineer renal therapy.
Blood is a uniquely active and hostile biological battlefield. The interfacial boundary between blood and tissue, the endothelium and its glycocalyx, is the killing zone where two exponentially amplifying cascades interact: the coagulation system and the complement system. In each of these, the endothelium governs a dynamic equilibrium poised between activation and degradation that is very strongly biased towards runaway activation. It is small surprise then that the majority of renal diseases that culminate in kidney failure are diseases of the natural filter, the glomerulus. The delicate terminally differentiated non-renewing glomerular podocytes that are thought to constitute the filtration barrier have an exquisitely fragile phenotype that has never been reproduced in an engineered environment.
Instead of chasing this delicate and incompletely understood biology, our team at The Kidney Project chose a biohybrid approach to tissue engineering. We developed a unique and novel nanostructured hemofiltration membrane that bears a monodisperse array of slit pores reminiscent of the glomerular slit diaphragm. The silicon nanopore membrane retains albumin and larger molecules and it has such high permeability that it functions by cardiac perfusion pressure alone, without any mechanical pumps. Careful attention to fluid dynamics and surface chemistry culminated in a series of implanted hemofiltration cartridges that continuously process flowing blood without anticoagulants like warfarin or heparin. We avoid the complexity of reverse-engineering the coagulation and complement cascades through highly hydrated biomimetic polymers that do not interact with blood as an endothelium, either autologous, allogeneic, or xenogeneic, would.
Kidneys excrete a significant mass of wastes - for example, about 14 mg/min of urea nitrogen - while maintaining wastes at low concentrations in the body - for example, blood urea nitrogen at 14 mg/dL. Consequently, kidneys filter a very large volume of blood water to continuously scavenge dilute wastes, and then concentrate those wastes into the precise volume needed to achieve neutral fluid balance. In the kidney, a specialized array of epithelial cells performs this concentrating function. Renal proximal tubule cells evolved to actively transport salts and allow passive movement of water, while maintaining a barrier to wastes. In a bioartificial organ, tubule cells are an ideal solution to the problem as they evolved to separate wastes and nutrients, and they do so using the power of cellular metabolism via basolateral sodium-potassium ATPase. There is no heavy, bulky dialysate, and no sorbent cartridges to carry and exchange. In recent years, we have developed protocols that unlock differentiated tubule cell function, such as active transport, in vitro.
One of the challenges in xenotransplantation and allotransplantation is the need for pharmacologic immune suppression. Cancer is a contraindication to transplant, and new cancers and infections claim the lives of transplant recipients, as the COVID-19 pandemic brutally demonstrated. A bioengineered Universal Donor Kidney cannot require systemic immune suppression if it is to be truly universal. The cellular and molecular effectors of the innate and acquired immune system are orders of magnitude larger than sugars, electrolytes, and amino acids. The nanostructured membranes we originally developed as hemofilters might also serve as a mechanical immunoisolation barrier to protect the tubule cells from the host in an immune sanctuary, but as importantly, prevent donor-derived viruses from passing from cultured cells to host, as seems to have happened in a cardiac xenograft. Inspired by pioneering work of my mentor, H. David Humes, we repurposed our hemofilter membrane as a cell bioreactor scaffold to be placed in series with the hemofilter membrane. In this paper, we test and prove our hypothesis that xenograft cells are protected by the silicon nanopore membrane scaffold.
Projects such as (Re)Building a Kidney and the Kidney Precision Medicine Project have the potential to someday stem the tide of 130,000 annual new cases of kidney failure that burden patients, caregivers, and payors alike in the United States. In contrast, we decided at the outset of our project that the urgent need for a new and better solution to kidney failure demanded the Gantt chart of development rather than the uncertain calendar of discovery research. The Kidney Project is a pragmatic goal-oriented engineering enterprise to build a mass-produced medical combination product that delivers enough waste removal and adequate fluid balance to meet the needs that patients themselves desire: freedom to travel, relief from the exhaustion of recovering from a dialysis session every other day, and the ability to eat and drink like anyone else.