Globally, around 2.6 million people receive renal replacement therapy (RRT), and a further 4.9–9.7 million people need, but do not have access to, RRT. The next generation RRT devices will certainly be in demand due to the increasing occurrence of diabetes, atherosclerosis and the growing population of older citizens. As such, a new engineering perspective on the future of artificial kidneys continues to draw research interests. Based on our literature review, there are two main approaches: cell-based and non-cell-based. The cell-based artificial kidneys represent a long-term, complete solution but it can take years of development due to the limitations of current cell seeding technology, viability and complicated behaviour control. Alternatively, there is much potential for near and medium term solutions with the development of non-cell based wearable and implantable devices to support current therapies. Based on recent fundamental advances in microfluidics, membranes and related research, it may be possible to integrate these technologies to enable implantable artificial kidneys (iAK) in the near future. The above figure shows the schematic timeline for the development of artificial kidney, where the vertical axis gives the qualitative idea of the "Technology Development Level" and the horizontal axis represents the time.
The non-cell based devices can be thought of as a near-term incremental extensions (or miniaturizations) of the traditional hemodialysis (HD) which utilizes synthetic semi-permeable membrane to replicate the blood-filtering function of the natural kidneys. In general, the driving concept is to shrink the large-sized dialysis machine into a small and portable devices known as Wearable Artificial Kidney (WAK). This requires the state-of-the-art from the fields of microfluidics, membrane systems and integration of the related technologies. The road map to the realization of this device is illustrated in the following figure.
The main challenges to this undertaking include the requirements of: (1) Small size comparable to the natural kidney, (2) Ultrafiltration rate to reach ~120 mL/min, (3) A self-cleaning dialyzer/membrane, (4) Biocompatibility and (5) Dialysate-free system.
At large scale, geometrical spacers are frequently added to the membrane structure to improve the filtration performance by directing the flow and inducing turbulent mixing. However, the benefits of adding spacers on the microscale remains largely unknown due to the different fluid behavior at low Reynolds number. At smaller microscale, viscous forces dominate over inertial forces and the initiation of mixing is increasingly difficult, without which the resulting concentration polarization and membrane fouling can severely limit filtration efficiency.
Our research group investigated three complex 3D-printed microspacer designs (with feature sizes in the range of 100~400 µm), incorporated into narrow channels to consider their enhancement effects for microfiltration and ultrafiltration applications. These structures (shown in the following figure) included two herringbone designs and one triply periodic minimal surface, e.g. a ‘gyroid’ spacer. Experiments and simulations found that the gyroid design achieved the highest membrane flux enhancement (i.e. 81 and 93% above a plain channel for blood mimicking and plasma mimicking solution tests, respectively). This was significantly better than the enhancement by herringbone designs. All of the spacers added back-pressure, with gyroid incurring a 23% higher pressure drop than the plain channel, which was considered as an acceptable performance trade-off. Based upon this work, 3D-printed microspacers were shown to enhance mixing and improve membrane filtration by reducing concentration polarization and fouling. Further, this study indicates that 3D printing can enable a promising new class of efficient, small-format devices for filtration processes. Dang et al. (2020) "Can 3D-printed Spacers improve filtration at the microscale?"