3D Bioprinting of Functional Islets of Langerhans in an Alginate/Methylcellulose Hydrogel Blend

Abstract

seen great progress over the last decades which led to some successful preliminary studies in humans [3,4] but is still not routinely implemented in the clinics, largely due to a shortage of donor organs. [5] Most trials in islet transplantation concentrated on injecting allogenic islets into the portal vein [6] and preventing rejection of donor islets with immunosuppressants. Since these tended to impair islet function [7,8] and in the liver islets are exposed to detrimental components in the blood, [5] soon thoughts turned to the possibility of protecting islets from the immune system by a thin coating of hydrogels. [9] Furthermore, protection from an immunological response might enable even xenotransplantation, for instance of porcine islets, [10] which could solve the problem of donor shortage.To achieve immune-isolating encapsulation, many researchers focused on alginate and alginate based hydrogel blends as coating, since this polymer cannot be digested in the human body lacking the relevant enzymes and has been shown to be nonimmunogenic if sufficiently purified. [11][12][13] Alginate is a polysaccharide extracted from brown algae, which can be crosslinked in different strengths by the addition of divalent cations such as Ca 2+ , Sr 2+ , or Ba 2+[14] and has been successfully used for encapsulation of different cell types in the past. [15] For pancreatic islets, either macro-or microencapsulation in alginate were established; in both cases islets have been found to retain their viability and functionality for a certain time, [16] but capsule size and composition definitely have an impact on function of islets, mainly as a function of diffusion distances. [17,18] Microcapsules only contain a single islet per capsule, encased in a thin layer of alginate, which decreases the distance between islets and the Transplantation of pancreatic islets is a promising strategy to alleviate the unstable blood-glucose control that some patients with diabetes type 1 exhibit and has seen many advances over the years. Protection of transplanted islets from the immune system can be accomplished by encapsulation within a hydrogel, the most investigated of which is alginate. In this study, islet encapsulation is combined with 3D extrusion bioprinting, an additive manufacturing method which enables the fabrication of 3D structures with a precise geometry to produce macroporous hydrogel constructs with embedded islets. Using a plottable hydrogel blend consisting of clinically approved ultrapure alginate and methylcellulose (Alg/MC) enables encapsulating pancreatic islets in macroporous 3D hydrogel constructs of defined geometry while retaining their viability, morphology, and functionality. Diffusion of glucose and insulin in the Alg/MC hydrogel is comparable to diffusion in plain alginate; the embedded islets continuously produce insulin and glucagon throughout the observation and still react to glucose stimulation albeit to a lesser degree than control islets.

Methylcellulose – a versatile printing material that enables biofabrication of tissue equivalents with high shape fidelity

“…47 Duin et al encapsulated primary pancreatic islets from rats into an alg-mc blend and plotted the bioink into macroporous 3D constructs. 55 The pancreatic islets were homogenously distributed, and revealed a comparable viability to free control islets. Viability increased over time in bioprinted scaffolds.…”

“…Remarkably, the bioprinted islets demonstrated an ability to react to stimulation: release of insulin was low when cultivation was done with low glucose concentration (3.3 × 10 −3 M) and high in case of high glucose concentration (16.4 × 10 −3 M), confirming the functionality of the islets in the bioprinted constructs. 55 An ink of the same alginate : mc ratio but higher concentration overall (6% alginate and 18% mc) was used for successful fabrication of a volumetric spinal cord model by multichannel 3D bioprinting, involving induced pluripotent stem cell (iPSC)-derived spinal neuronal progenitor cells (sNPC) and mouse iPSC-derived oligodendrocyte progenitor cells (OPC). 56 These cells were printed in precise positions within 3D printed constructs, controlling the direction of axon growth throughout the scaffold.…”

Multicomponent polysaccharide alginate-based bioinks

“…In one of the reported studies, using a highly viscous blend (3.0% wt/vol alginate and 9.0% wt/vol methylcellulose), cross-linked with SrCl 2 , to obtain 3D printed constructs of viable and functional pancreatic islets from rats. 119 The printed cells kept their function and morphology and it was possible to recognize a and b pancreatic cells within the islets.…”

“…7). 79,119 However, relatively few research studies have been performed in this area and, therefore, this may be of interest for further investigation.…”

Recent Trends in Three-Dimensional Bioinks Based on Alginate for Biomedical Applications

“…Defense of these islets through the immune system might be achieved by incorporating a hydrogel, the majority of which is Alg. As an illustration in a vital investigation by Duin et al [ 124 ], islet incorporation coupled with 3D extrusion bioprinting employs Alg and methylcellulose, making it possible for loading pancreatic islets in 3D hydrogel constructs of described geometry though maintaining their viability, structure, and performance. The loaded islets regularly generate insulin and glucagon in the course of the treatment [ 124 ].…”