Hydrogel-Based Cell Therapies for Kidney Regeneration: Current Trends in Biofabrication and In Vivo Repair

Abstract

Facing the problems of limited renal regeneration capacity and the persistent shortage of donor kidneys, dialysis remains the only treatment option for many end-stage renal disease patients. Unfortunately, dialysis is only a medium-term solution because large and protein-bound uremic solutes are not efficiently cleared from the body and lead to disease progression over time. Current strategies for improved renal replacement therapies (RRTs) range from whole organ engineering to biofabrication of renal assist devices and biological injectables for in vivo regeneration. Notably, all approaches coincide with the incorporation of cellular components and biomimetic micro-environments. Concerning the latter, hydrogels form promising materials as scaffolds and cell carrier systems due to the demonstrated biocompatibility of most natural hydrogels, tunable biochemical and mechanical properties, and various application possibilities. In this review, the potential of hydrogel-based cell therapies for kidney regeneration is discussed. First, we provide an overview of current trends in the development of RRTs and in vivo regeneration options, before examining the possible roles of hydrogels within these fields. We discuss major application-specific hydrogel design criteria and, subsequently, assess the potential of emergent biofabrication technologies, such as micromolding, microfluidics and electrodeposition for the development of new RRTs and injectable stem cell therapies.

Keywords: Proximal tubules; hydrogels; injectable formulations; renal assist devices; stem cells; uremic toxin secretion.

Fig. (1)

Schematic overview of current strategies for renal replacement therapies development. (A) Whole kidney engineering aims for a lab-grown replication of the organ as transplant. (B) Renal assist devices are biotechnological approaches to complement conventional dialysis, with extracorporeal and implantable applications. (C) Biological injections of therapeutics promote in vivo regeneration via direct growth factor delivery and/or paracrine cellular effects.

Fig. (2)

Degradable hydrogels for biomedical applications. Cell-laden hydrogels can be prepared from isolated cells and/or growth factors plus gel precursors in the preferred medium. By crosslinking the reactive hydrophilic polymer (gel precursors), a certain 3D structure can be obtained. This scaffold can slowly degrade via hydrolysis or enzymatic degradation, while cells produce the surrounding ECM until a native-like tissue is generated.

Fig. (3)

Hydrogel-based RADs could improve cell function. Mechanical and biochemical cues by various ECM COMPOUNDS are shown to improve cell organization and function; an ECM-mimicking hydrogel layer (depicted in blue) could, therefore, enhance proximal tubule cell characteristics like solute secretion and electrolyte reabsorption. (The color version of the figure is available in the electronic copy of the article).

Fig. (4)

Schematic overview of biofabrication techniques relevant to RRTs. (A) Bulk emulsification: the disperse phase of aqueous reactive polymer (depicted in blue) is stirred into the continuous oil phase (depicted in orange). After crosslinking, the oil phase is washed away with organic solvents. (B) Photolithography: the aqueous reactive polymer is spread onto a plate, and a pre-cut photolithographic mask is placed above. After crosslinking, the mask can be removed and the hydrogel particles can be collected. (C) Micromolding: a pre-fabricated mold is filled with aqueous reactive polymer. After crosslinking, the mold can be removed and the patterned hydrogels can be collected. (D) Microfluidics: a micron-sized channel is used to drip or jet the disperse phase of aqueous reactive polymer into the continuous phase (depicted in orange). After crosslinking, specific hydrogels are formed, e.g. micron-sized spheres (depicted in dark blue). (E) Electrodeposition: pressure-driven flow of aqueous reactive polymer is pumped through an electrically charged conductive nozzle towards an oppositely charged or grounded collector plate. Depending on the voltage and other factors, the polymer stream can drip, spin or spray from the nozzle to form several types of hydrogel geometries. A bath filled with a continuous phase can be used to collect the polymer for subsequent crosslinking. (The color version of the figure is available in the electronic copy of the article).

Fig. (5)

Free-standing hollow tubule of MDCK cells. Channels were formed in an ECM gel using a retractable needle, and filled with MDCK cells; after 5-10 days, cells without contact to the ECM underwent apoptosis, leading to a tubule-like system (left and middle panel, scale bar = 200 µm). After 10 days, MDCK-based tubules could be released from the ECM mold, leading to a free-standing hollow tubule (right panel, scale bar = 500 µm). Adapted with permission from [92].

Fig. (6)

Fluorescence microscopy images of GelMA microgels (light red) with encapsulated MDCK cells (green) and corresponding 3D spheroid reconstructions below. In vitro cultured cell-laden microgels show cell growth and cyst formation over time. This suggests potential of these gels as injectables for in vivo regeneration. Scale bar = 100 µm. Adapted with permission from [127]. (The color version of the figure is available in the electronic copy of the article).

Fig. (7)

Hollow gelatin–hydroxyphenylpropionic acid hydrogel fibers seeded with MDCK cells. Optical micrograph (left) and cryosectional image (right) of fibers formed through coaxial stream-based microfluidics using an inner H₂O₂ and an outer PBS flow with a middle flow of hydrogel precursor and MDCK cells. Adapted with permission from [158].