E. The kidney-on-chip implantable devices

Realizing an artificial kidney involves the regeneration of multiple functionalities of the kidney. Hence, the biomimetic kidney must be physiologically relevant, which carries the cell–cell interactions that exist between tubular and glomerular vascular endothelial cells and transcellular osmotic pressure and electrochemical gradient. This can be achieved by immortalized cell lines in microphysiological systems. The common cell types used in developing kidney-on-chip devices are MDCK (Madin-Darby Canine Kidney Epithelial Cells),163,164163. N. Ferrell et al., “A microfluidic bioreactor with integrated transepithelial electrical resistance (TEER) measurement electrodes for evaluation of renal epithelial cells,” Biotechnol. Bioeng. 107(4), 707–716 (2010). https://doi.org/10.1002/bit.22835164. S. Kim et al., “Pharmacokinetic profile that reduces nephrotoxicity of gentamicin in a perfused kidney-on-a-chip,” Biofabrication 8(1), 015021 (2016). https://doi.org/10.1088/1758-5090/8/1/015021 pro crane,165165. N. Ozgen et al., “Evaluation of long-term transport ability of a bioartificial renal tubule device using LLC-PK1 cells,” Nephrol. Dial. Transpl. 19(9), 2198–2207 (2004). https://doi.org/10.1093/ndt/gfh399 opossum,166,167166. N. Ferrell et al., “Albumin handling by renal tubular epithelial cells in a microfluidic bioreactor,” Biotechnol. Bioeng. 109(3), 797–803 (2012). https://doi.org/10.1002/bit.24339167. C. Shen et al., “Fabrication of collagen gel hollow fibers by covalent cross-linking for construction of bioengineering renal tubules,” ACS Appl. Mater. Interfaces 7(35), 19789–19797 (2015). https://doi.org/10.1021/acsami.5b05809 and hPTECs (Human proximal tubular epithelial cells).168168. M. Adler et al., “A quantitative approach to screen for nephrotoxic compounds in vitro,” J. Am. Soc. Nephrol. 27(4), 1015–1028 (2016). https://doi.org/10.1681/ASN.2015010060 Unfortunately, no cell types that are mentioned above regenerate the primary phenotype and functional differentiation.169169. S. E. Jenkinson et al., “The limitations of renal epithelial cell line HK-2 as a model of drug transporter expression and function in the proximal tubule,” Pflügers Arch. Eur. J. Physiol. 464(6), 601–611 (2012). https://doi.org/10.1007/s00424-012-1163-2 Out of these, hPTEC proves to be comparatively better and widely used in the in vitro devices.170170. K. Narayanan et al., “Human embryonic stem cells differentiate into functional renal proximal tubular–like cells,” Kidney Int. 83(4), 593–603 (2013). https://doi.org/10.1038/ki.2012.442 However, the cells also come with problems of a limited number of population doubling, reliability, and storage difficulties. All these cell types are used in developing various parts of the nephron, such as glomerulus, proximal tubule, distal tubule, and collecting duct. The individual part has its specific cell phenotype, and all are replicated in different compartments to later integrate them together.

1. Glomerulus-on-chip

The efferent arteriole carries the blood into the nephron glomerulus. The glomerulus is the fundamental functional unit in a nephron, where the ultrafiltration of impure blood happens. The first successful replication of glomerulus-on-chip is performed by musk171171. S. Musah et al., “Mature induced-pluripotent-stem-cell-derived human podocytes reconstitute kidney glomerular-capillary-wall function on a chip,” Nat. Biomed. Eng. 1(5), 1–12 (2017). https://doi.org/10.1038/s41551-017-0069 by utilizing terminally differentiated podocytes from human iPSCs (induced pluripotent stem cells). They showed that iPSCs derived podocytes can produce glomerular membranes and can regenerate the tissue–tissue interaction of the glomerulus. They reported that their glomerulus-on-chip mix adriamycin albuminuria, differential clearance of albumin and insulin, and podocytes injury. Furthermore, a microfluidic chip has been proposed to be equipped with anti-podocyte autoantibodies. The device showcased albuminuria which is proportional to the patient's proteinuria which was not observed with primary podocytes. Around 2000 chips were analyzed, which showed high reproducibility and the system for high throughput in screening pharmacological compounds.172,173172. S. Musah et al., “Directed differentiation of human induced pluripotent stem cells into mature kidney podocytes and establishment of a glomerulus chip,” Nat. Protoc. 13(7), 1662–1685 (2018). https://doi.org/10.1038/s41596-018-0007-8173. A. Petrosyan et al., “A glomerulus-on-a-chip to recapitulate the human glomerular filtration barrier,” Nat. Commun. 10(1), 1–17 (2019). https://doi.org/10.1039/C7LC00134G The glomerulus-on-chip model for diagnosis of glomerular disorders using notch activation that differentially results in renal progenitors' proliferation and differentiation. They concluded that the severity of the glomerular disease depends on the notch regulated balance between podocyte death and regeneration provided by the renal progenitors.174174. L. Lasagni et al., “Notch activation differentially regulates renal progenitors proliferation and differentiation toward the podocyte lineage in glomerular disorders,” Stem Cells 28(9), 1674–1685 (2010). https://doi.org/10.1002/stem.492 Subsequently, a microfluidic glomerulus-on-chip has been developed for understanding diabetic nephropathy.175175. L. Wang et al., “A disease model of diabetic nephropathy in a glomerulus-on-a-chip microdevice,” Lab Chip 17(10), 1749–1760 (2017). https://doi.org/10.1039/C7LC00134G The journey of microfluidics toward the realization of glomerulus-on-chip is presented in this review.176176. N. Ashammakhi, E. A. Elkhammas, and A. Hasan, “Glomerulus-on-a-chip. Life Up,” Transplantation 101(11), e343–e344 (2017). https://doi.org/10.1097/TP.0000000000001896

2. Proximal convoluted tubule-on-chip

The glomerulus filters the RBCs, WBCs, and platelets in the impure blood and leave out plasma. The plasma is collected in Bowman's capsule, which is then directed toward the tubule network. In the tubule network, the proximal convoluted tubule (PCT) carries a significant role. The PCT is responsible for 60% of filtered NaCl and water, along with the fact that it filters 90% of bicarbonate, glucose, and amino acids. Inside the PCT, the apical membranes are a little wider to facilitate reabsorption that is created by dense arrays of microvilli or brush borders. The cellular transport is highly dependent on the Na+ concentration gradient that is established by Na+ / K+ATPase. The solute transport too is linked to the Na+ gradient. The carbonic inhibitors present in the proximal tubule, such as acetazolamide (a class of weak diuretic agents), is useful for alkalizing the urine. In the lateral parts of PCT, the Cl− absorption is initiated by apical exchange of cellular format for higher luminal concentrations, and the glucose reabsorption is completed at the end of the PCT. A variety of organic acids, carboxylic ions, amino cations, and protein drugs are reabsorbed and separated in the PCT that were not filtered in glomerulus.177177. D. L. Longo, J. Larry Jameson, and D. Kaspe, Harrison's Principles of Internal Medicine (Macgraw-Hill, 2011), Vol. 2. Several investigations have been successfully done by culturing the proximal tubule cells in hollow fibers, providing immunoprotection during extracapillary blood flow. Another device with hPTEC cells seeded on the inner surface of fibers that are composed of fibrin-coated hydrogels; it is noted that the device has a satisfactory rate of transport potential of glucose.178178. C. P. Ng et al., “A fibrin-based tissue-engineered renal proximal tubule for bioartificial kidney devices: Development, characterization and in vitro transport study,” Int. J. Tissue Eng. 2013, 1–10 (2012). Jang et al. developed microchannels cultured with hPTECs that are composed of collision type-IV coated with polyester membranes.179179. K.-J. Jang et al., “Human kidney proximal tubule-on-a-chip for drug transport and nephrotoxicity assessment,” Integr. Biol. 5(9), 1119–1129 (2013). https://doi.org/10.1039/c3ib40049b They demonstrated that in the absence of fluid shear stress (FSS), the cells regained normal columnar shape, polarity, and primary cilia. Furthermore, they concluded that the cells displaced an increase in cellular uptake of albumin and cisplatin-induced damage recovery also increased. Another study published reports a huge number of metabolically active cells that were coated with fibronectin. The work utilized human renal progenitor cells (HPCs) upon polycarbonate porous membranes. They have used several coatings, such as laminin, fibronectin, and matrigel, but fibronectin has shown better outcomes. Furthermore, it was concluded that HPCs are polarized due to fluid shear stress.180180. A. G. Sciancalepore et al., “A bioartificial renal tubule device embedding human renal stem/progenitor cells,” PLoS One 9(1), e87496 (2014). https://doi.org/10.1371/journal.pone.0087496 Later, the biofunctionalized hollow fibers cultured with hPTECs to replicate renal tubules have been reported by Jansen et al.181181. J. Jansen et al., “Bioengineered kidney tubules efficiently excrete uremic toxins,” Sci. Rep. 6(1), 1–12 (2016). https://doi.org/10.1038/srep26715 The work has claimed to produce collagen matrix, formation of tubular structures, epithelial differentiation, renal morphology, and self-polarization. The authors have also reported that for the first time the apical intake and basolateral solute transport. This work has demonstrated the capabilities of microchannels to develop physiologically relevant proximal tubule mimicking. A microphysiological model for proximal tubule realized on chip that allows the modeling of renal cells in different nephrotoxicity situations, which is depicted in Fig. 9.182182. E. J. Weber et al., “Development of a microphysiological model of human kidney proximal tubule function,” Kidney Int. 90(3), 627–637 (2016). https://doi.org/10.1016/j.kint.2016.06.011Microfluidic screening assay developed for renal drug transport interaction by conditionally combining proximal tubule cells overexpressing anion transporter 1 (CiPTEC-OATI). The authors described self-polarization, gene expression, organic cation transporter 2, P-glycoprotein, and multidrug resistance for matching the levels of 2D static cell cultures. The 3D MF PTEC model finds extensive uses in drug transporter interactions screening and offers multiplexing of outcomes for drug transporter interaction and toxicity.183183. J. Vriend et al., “Screening of drug-transporter interactions in a 3D microfluidic renal proximal tubule on a chip,” AAPS J. 20(5), 1–13 (2018). https://doi.org/10.1208/s12248-018-0247-0 A reusable microfluidic chip that regenerates the proximal tubule and glomerulus that exactly mimic the manifestation of renal disease states is developed. The model specifically aims to represent glomerular filtration rate, nephrolithiasis, hyperglycemia, and drug-induced nephrotoxicity. The outcomes represent varying stress related responses when comparing cells that were cultured and image immobilized in static and dynamic conditions, which indicates more realistic and sensitive predictions.184184. C. M. Sakolish, B. Philip, and G. J. Mahler, “A human proximal tubule-on-a-chip to study renal disease and toxicity,” Biomicrofluidics 13(1), 014107 (2019). https://doi.org/10.1063/1.5083138 The biological relevance and toxicity biomarkers evaluation critical endpoints from tissue developer laboratory-related case studies are reported.185185. C. Sakolish et al., “Technology transfer of the microphysiological systems: A case study of the human proximal tubule tissue chip,” Sci. Rep. 8(1), 1–14 (2018). https://doi.org/10.1038/s41598-018-33099-2 A 3D microfluidic device with immortalized PTEC under physiological FSS 2.0 dyne/cm2 is investigated by the authors. The cells were exposed to FSS for 9 days, and the albumin uptake and P-glycoprotein and multidrug resistance were determined. The authors concluded that albumin uptake was increased by FSS, P-glycoprotein and cell elongation have also shown similar effects.186186. J. Vriend et al., “Flow stimulates drug transport in a human kidney proximal tubule-on-a-chip independent of primary cilia,” Biochim. Biophys. Acta 1864(1), 129433 (2020). https://doi.org/10.1016/j.bbagen.2019.129433 A dual-channel Nortis chops shape was injected by human-derived renal proximal tubule cells for predicting drug-induced kidney injury. This work has claimed to provide kidney tubule physiological relevance over single-compartment models.187,188187. T. T. G. Nieskens et al., “A multicompartment human kidney proximal tubule-on-a-chip replicates cell polarization–dependent cisplatin toxicity,” Drug Metab. Dispos. 48(12), 1303–1311 (2020). https://doi.org/10.1124/dmd.120.000098188. K. R. Long et al., “Proximal tubule apical endocytosis is modulated by fluid shear stress via an mTOR-dependent pathway,” Mol. Biol. Cell 28(19), 2508–2517 (2017). https://doi.org/10.1091/mbc.e17-04-0211 The 3D microphysiological system with flow-directed human kidney proximal tubule is reported for regenerating PT function.189189. S. M. King et al., “3D proximal tubule tissues recapitulate key aspects of renal physiology to enable nephrotoxicity testing,” Front. Physiol. 8, 123 (2017). https://doi.org/10.3389/fphys.2017.00123The system is capable of mimicking PT polarity, can express relevant marker proteins, and is also able to exhibit synthetic and biochemical functions as shown Fig. 10. Furthermore, the mimicking of resorptive and secretive processes associated with PT was also attempted. The outcomes suggested that the proposed microphysiological system can be considered for ex vivo modeling of renal drug clearance.189189. S. M. King et al., “3D proximal tubule tissues recapitulate key aspects of renal physiology to enable nephrotoxicity testing,” Front. Physiol. 8, 123 (2017). https://doi.org/10.3389/fphys.2017.00123 Leveraging the organs proprietary 3D bioprinting platform, PT interstitial interface was developed in an in vitro model that comprises fibroblast, endothelial cells, and hPTECs for predicting the clinical outcomes at the tissue level. An in vitro model was developed comprising fully polarized renal proximal epithelial cells immobilized on a microfluidic platform. The experiment proceeded with 40 leak-tight tubules that were cultured to provide access to the epithelial cells' apical and basolateral sites. The proposed 3D functional tubule is equipped with advanced renal epithelial characteristics, which has potential applications in drug screening studies.190190. M. K. Vormann et al., “Nephrotoxicity and kidney transport assessment on 3D perfused proximal tubules,” AAPS J. 20(5), 1–11 (2018). https://doi.org/10.1208/s12248-018-0248-z Interestingly, there exists a system biomimetic kidney system utilizing herringbone architecture. The system is inspired by the self-cleaning mechanism of the kidney depending on the size of the particles. The system is shown in Fig. 11, and the reader may refer to the work for detailed system working and description.191191. H. Chen et al., “Hierarchically molecular imprinted porous particles for biomimetic kidney cleaning,” Adv. Mater. 32(52), 2005394 (2020). https://doi.org/10.1002/adma.202005394All the works presented so far utilize the human primary kidney cells. It is equally important to study the mechanosensory functions inside the tubules. It is experimentally elucidated that fluid shear stress (FSS) affects the cytoskeleton and cell phenotype. The epithelial cells inside the lining of the kidney tubule are continuously exposed to the apical FSS. This FSS has been calculated to be approximately 1 dyne/cm2 by Raghavan et al.192192. V. Raghavan et al., “Shear stress-dependent regulation of apical endocytosis in renal proximal tubule cells mediated by primary cilia,” Proc. Natl. Acad. Sci. U.S.A. 111(23), 8506–8511 (2014). https://doi.org/10.1073/pnas.1402195111 The kidney tubule cells sense this apical FSS through a soul organelle termed the primary cilium. The cilium mediates the signal transduction, which contributes to manifest differentiation of the renal tubule epithelial tissue.193193. H. A. Praetorius, “The primary cilium as sensor of fluid flow: New building blocks to the 640 model. A review in the theme: Cell signaling: Proteins, pathways and mechanisms,” Am. J. Physiol. Cell Physiol. 641, C198–C208 (2015). https://doi.org/10.1152/ajpcell.00336.2014 The cilium is a thin layer of about 0.25 μm generally projecting 2–3 μm from the apical membrane to the lumen. It consists of nine circularly arranged microtubules that are covered by the cell membrane.194,195194. L. Rodat-Despoix and P. Delmas, “Ciliar functions in the nephron,” Pflügers Arch. Eur. J. Physiol. 458(1), 179–187 (2009). https://doi.org/10.1007/s00424-008-0632-0195. J. A. Deane and S. D. Ricardo, “Emerging roles for renal primary cilia in epithelial repair,” Int. Rev. Cell Mol. Biol. 293, 169–193 (2012). https://doi.org/10.1016/B978-0-12-394304-0.00011-7 These cilia respond to the FSS acting on the apical membrane or by direct modulations of cultured MDCK cells.196196. H. A. Praetorius and K. R. Spring, “Bending the MDCK cell primary cilium increases intracellular calcium,” J. Membr. Biol. 184(1), 71–79 (2001). https://doi.org/10.1007/s00232-001-0075-4 It is determined that Ca2+ concentration inside the tubule varies vastly due to FSS. On the other hand, different transporters, such as ATPase and ATP hydrolysis, also mediate renal functions, such as secretion and reabsorption.197197. S. K. Nigam et al., “Handling of drugs, metabolites, and uremic toxins by kidney proximal tubule drug transporters,” Clin. J. Am. Soc. Nephrol. 10(11), 2039–2049 (2015). https://doi.org/10.2215/CJN.02440314 Furthermore, there are excellent reported works that demonstrate the on-chip realization of distal convoluted tubule198198. R. Baudoin et al., “Development of a renal microchip for in vitro distal tubule models,” Biotechnol. Prog. 23(5), 1245–1253 (2007). https://doi.org/10.1021/bp0603513 and collecting duct.199199. K.-J. Jang and K.-Y. Suh, “A multi-layer microfluidic device for efficient culture and analysis of renal tubular cells,” Lab Chip 10(1), 36–42 (2010). https://doi.org/10.1039/B907515A Very little concentration has been drawn by the tail parts of the tubule network as the PT itself does most of the necessary functions, yet all the parts of the nephron must be reconstructed for faithful regeneration of the whole kidney on a single microfluidic chip.