An Implantable Artificial Kidney Requires Kidney-Scale Architecture

Miniaturisation of artificial organs to an implantable form factor is one of the great challenges in biomedical engineering. Most patients with end-stage kidney failure depend on dialysis, yet dialysis remains burdensome. An implantable artificial kidney would be a paradigm shift that would largely replace the need for dialysis. By coupling the joint constraints on hemocompatibility, hemodynamics, and epithelial transport to boundary conditions, such as the pressure and shear stress imposed by the human body, we show that: (i) blood compatibility requires centimetre-scale blood-distribution manifolds and preferably endothelialised channels; (ii) concentration polarisation and hemoconcentration limit filtration performance, causing diminishing returns from higher membrane permeability; (iii) hemodynamics and packing constraints drive local channels toward capillary-scale dimensions; and (iv) proximal-tubule-only architectures cannot satisfy sodium, potassium, phosphate, and water balance without shifting a large homeostatic burden to the patient. This necessitates multiple epithelial modules requiring ∼ 1.15 m2 of active surface area and ∼ 250 layers and makes near-kidney proximal-tubule transport capacity a prerequisite for geometric feasibility. Together, our results show that the minimally viable artificial kidney is still constrained to a kidney-scale, macro-to-micro architecture. This conclusion applies not only to synthetic scaffold-based designs, but also to biological and hybrid strategies in which cells participate in architecture formation, since all such routes must satisfy the same coupled requirements.