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Rev Diabet Stud, 2017, 14(4):334-353 DOI 10.1900/RDS.2017.14.334

Polymeric Scaffolds for Pancreatic Tissue Engineering: A Review

Nupur Kumar, Heer Joisher, Anasuya Ganguly

Department of Biological Sciences, BITS-Pilani, K.K Birla Goa Campus, Goa, India 403726
Address correspondence to: Anasuya Ganguly, e-mail: ganguly@goa.bits-pilani.ac.in

Manuscript submitted November 20, 2017; resubmitted January 24, 2018; accepted February 5, 2018.

Keywords: diabetes, transplant, scaffold, polymer, pancreas, islets, tissue engineering

Abstract

In recent years, there has been an alarming increase in the incidence of diabetes, with one in every eleven individuals worldwide suffering from this debilitating disease. As the available treatment options fail to reduce disease progression, novel avenues such as the bioartificial pancreas are being given serious consideration. In the past decade, the research focus has shifted towards the field of tissue engineering, which helps to design biological substitutes for repair and replacement of non-functional or damaged organs. Scaffolds constitute an integral part of tissue engineering; they have been shown to mimic the native extracellular matrix, thereby supporting cell viability and proliferation. This review offers a novel compilation of the recent advances in polymeric scaffolds, which are used for pancreatic tissue engineering. Furthermore, in this article, the design strategies for bioartificial pancreatic constructs and their future applications in cell-based therapy are discussed.

Abbreviations: ANCOVA - analysis of covariance; 2D - two-dimensional; 3D - three-dimensional; AG-CHNP - agarose chitosan-based nanocomposite; BM-MSC - bone marrow-derived mesenchymal stem cell; ECM - extracellular matrix; EC - endothelial cell; EDC - 1-ethyl-3-(3-dimethylaminopropyl)-carbodiimide; EGF - epidermal growth factor; FDA - US Food and Drug Administration; FGF - fibroblast growth factor; GD - gestational diabetes; GLP-1 - glucagon-like factor 1; HEK - human embryonic kidney; HeLa - cervical cancer cell line (Henrietta Lacks); IDDM - insulin-dependent diabetes mellitus; IDE1 - inducer of definitive endoderm 1; IDF - International Diabetes Federation; IKVAV - isoleucine-lysine-valine-alanine-valine; INS-1 - insulinoma cell line; IPN - interpenetrating polymer network; iPSC - induced pluripotent stem cell; Mia PaCa-2 - human pancreatic carcinoma cell line; MODY - maturity onset diabetes of the young; MSC - mesenchymal stem cell; NCD - non-communicable diseases; NHS - N-hydroxysuccinimide; NKX2.2 - NK2 homeobox protein 2; PAA - polyacrylic acid; PCL - polycaprolactone; PDMS - polydimethylsiloxane; PDX1 - pancreatic and duodenal homeobox 1; PEG - polyethylene glycol; PGA - polyglycolic acid; PLA - polylactic acid; PLG - polylactide-co-glycolide; PLGA - polylactic-co-glycolic acid; PU - polyurethane; PVP - polyvinylpyrrolidone; RGD - arginylglycylaspartic acid; RIN-5 - rat insulinoma cell; ROS - reactive oxygen species; SC β-cell - stem cell-derived β-cell; STZ - streptozotocin; T1D - type 1 diabetes; T2D - type 2 diabetes; VEGF - vascular endothelial growth factor; WHO - World Health Organization; YIGSR - tyrosine-isoleucine-glycine-serine-arginine

1. Introduction

With changing economic development patterns, the world has experienced a steep increase in the number of patients with lifestyle diseases. Diseases associated with lifestyle imbalance include diabetes, hypertension, cardiovascular diseases, and certain types of cancers. Such diseases are associated with a lack of physical activity, unfavorable occupational habits, and increased obesity. Diet and lifestyle play an important role in maintaining physical and mental health [1]. For centuries, infectious diseases have been considered as the main killer around the world. But with non-communicable diseases (NCDs) taking the front seat, it is estimated that by the year 2020 NCDs will cause seven out of ten deaths in developing nations [2]. Diabetes, one of the four priority NCDs, is currently the eighth leading cause of death in both sexes [3, 4]. Initially labeled as a disease of rich countries, diabetes has shown a tremendous increase in the past few years, even in middle income nations. According to the WHO’s global diabetes report 2016, a total of 422 million people across the world are currently suffering from diabetes [5]. Its global prevalence increased from 4.7% in 1980 to 8.5% in 2014 (Figure 1) [5]. As per the International Diabetes Federation (IDF) report (2015), China ranks first in relation to the number of diabetic patients (between the age of 20 and 79 years) followed by India and the USA (Figure 2) [6].

Figure 1. Prevalence of diabetes worldwide. Worldwide increase in the occurrence of diabetes in the past two decades. Source: IDF Diabetes Atlas; http://www.diabetesatlas.org.

 

Figure 2. Prevalence of diabetes in India. Increasing similar to worldwide trend. Source: IDF Diabetes Atlas, http://www.diabetesatlas.org.

 

Diabetes mellitus is classified as one of the metabolic disorders characterized by a chronic hyperglycemic condition. This state is mainly attributed to defects in insulin secretion or to the action of insulin in cells or both. Most cases of diabetes are one of two types: type 1 diabetes (T1D) and type 2 diabetes (T2D). There are additional types such as gestational diabetes (GD) and maturity onset diabetes of the young (MODY). T1D, also known as insulin-dependent diabetes mellitus (IDDM), is an autoimmune condition where the body attacks its own β-cells, destroying them, and rendering them unfit to produce insulin, thereby increasing blood glucose levels [7, 8].

Currently, treatments for diabetes include insulin therapy, drugs such as biguanides, sulfonylureas, megalitinides, thiazolidinediones, alfa gluocosidase inhibitors, and others, whole pancreas transplantation, islet transplantation, and bariatric surgery. Even after the award-winning discovery of insulin therapy in 1921, diabetes treatment has not come a long way [9]. Over the years, significant research has focused on the development of substitute routes for insulin administration like nasal, rectal, and oral [10]. Several insulin release devices such as insulin pumps, pen injectors, and inhalation patches have been engineered to enhance patient amenability [11]. For controlled and tunable release of insulin, carriers made of hydrogels, microspheres, and nanoparticles have been formulated [12].

Although multiple modifications in the source, structure, and delivery mode of insulin have been made to improve the management of diabetics, millions of patients still continue to inject themselves with insulin several times a day. Apart from numerous injections, prolonged usage of insulin can cause diabetic retinopathy, ketoacidosis, weight gain, etc. All these treatment options have serious disadvantages which have led to a search for better options. In recent years, tissue engineering has shown promise in the treatment of various conditions.

2. Introduction of tissue engineering

Tissue engineering comprises several disciplinary fields including engineering, material science, and life sciences; it aims to produce biologically viable substitutes for tissue and organ regeneration [13]. Tissue engineering provides a means to synthesize substitutes for repair or replacement of tissues or organs damaged due to pathology, trauma, or injury. It provides an alternative to bridge the ever-growing gap between demand and supply of organs for transplantation [14].

Major research efforts have focused on an in situ tissue engineering approach, with the aim of leveraging the innate regenerative potential of the human body to enable tissue regeneration at the site of injury using bioactive molecule-based cues [15]. The tissue engineering triad comprises a combination of cells, scaffolds, and biologically active molecules or growth factors [16]. Scaffolds are three-dimensional constructs with the prime function of being able to mimic the physico-chemical properties of the natural extracellular matrix (ECM) [16]. For successful application in the field of tissue engineering, scaffolds need to be able to provide structural and mechanical support to the cells and to promote regeneration by effectual delivery of therapeutic molecules [17].

Polymeric scaffolds have been used widely for tissue engineering applications. Biodegradable polymers provide the advantages of enhanced inflammatory tolerance, high biocompatibility, and nontoxic enzymatic degradation in vivo [16]. Numerous approaches have been implemented for the fabrication of these biomimetic scaffolds, including electro-spinning, phase-separation, solvent casting, freeze-drying, and self-assembly (Figure 3) [18]. Electro-spun nanofibers from biomaterials form structures analogous to the fibrous native ECM, and possess beneficial mechanical properties and enhanced cellular infiltration [19, 20]. Pre-vascularized tissue constructs possessing enhanced cell proliferation have been developed by pre-seeding endothelial cells and fibroblasts on hydrogels [13].

Figure 3. Fabrication techniques for scaffold preparation. Over the last two decades, various approaches have been implemented in the fabrication of scaffolds for tissue engineering, including electro-spinning, phase-separation, solvent casting, freeze-drying, and self-assembly. The selection of the appropriate scaffolding approach depends on the scaffold requirements and tissue-specific considerations.

 

An alternative is provided by scaffold-less tissue engineering. It has been developed by virtue of 3D bioprinting using self-assembling multicellular units as bioink particles, and has been used to realize self-organizing vascular constructs [21, 22]. The latest strategy of 4D bioprinting, which involves time as the additional dimension, has enabled the development of smart biomaterials which can evolve their shapes as a function of time in response to exogenous cues like pH and temperature [23]. Organ decellularization is another recent avenue where organs, decellularized by detergents, retain ECM and vascularization, and hence can be transplanted after in vitro recellularization [24]. Hydrogels made from natural and synthetic polymers have been used for encapsulation and to protect the transplanted cells from the host immune system. The permeability of the encapsulating matrix is fine-tuned to block the passage of antibodies and T cells, and at the same time, to allow the inflow and outflow of bioactive signaling molecules, thus aiming to avoid the usage and eventual side-effects of immunosuppressive agents [25, 26].

3. Pancreatic tissue engineering

A surgical cure for diabetes has been proposed by pancreatic transplantation, which is accompanied by long-term immunosuppressive therapy [27]. To reduce the extent of surgical intervention and the risk involved in pancreatic transplantation, new strategies have been developed for islet transplantation [28]. It has been found that immunoisolation of islets, using tissue engineering techniques like encapsulation and coating with semi-permeable and biocompatible biomaterial membranes, minimizes the need for long-term immunosuppression [29].

Cell-based (HEK293) microencapsulation of islets has also been tested which showed sustained release of insulin [30]. Although this technique requires further improvement, islet surface modifications with growth factors such as vascular endothelial growth factor (VEGF) and peptides such as arginylglycylaspartic acid (RGD), isoleucine-lysine-valine-alanine-valine (IKVAV), and tyrosine-isoleucine-glycine-serine-arginine (YIGSR) have already been shown to enhance islet engraftment and reduce immunogenicity in pancreatic islet transplantation [11].

To increase donor tissue sources, transplantation of β-cells derived from stem cells differentiated in vitro has become a new focus in diabetes research [31]. Hydrogels and microspheres made of polymers like alginate, polyethylene glycol (PEG), agarose, and chitosan-gelatin among others have been used for β-cell encapsulation. These encapsulated β-cells have been shown to have enhanced viability, cell survival, and insulin-secretory potential. Major efforts have been directed to mimic the islet niche and native interactions in the capsules to improve the efficacy of islet transplantation [11]. Natural and synthetic polymers have been widely vetted as a means of transplantation to enhance the efficacy of islet survival [32].

4. Polymers

Polymers can be categorized as natural or synthetic polymers depending on their origin. Naturally occurring polymers like polysaccharides (chitosan, alginate, hyaluronic acid), inorganic polymers (hydroxyapatite), and natural proteins (collagen, fibrin, silk) exhibit several benefits such as low toxicity, biocompatibility, and enzymatic degradation [16, 33]. Natural polymers also contain bioactive motifs, which help to establish cell-scaffold interactions, thus enhancing tissue functionality [34]. The downsides associated with natural polymers include temperature sensitivity, immunogenicity, and source-dependent heterogeneity (Table 1) [17].

Table 1. Advantages and disadvantages of natural polymers for tissue engineering applications

 
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The second family of polymers, synthetic polymers, includes alpha-hydroxy acids such as polylactic acid (PLA), polyglycolic acid (PGA), polylactic-co-glycolic acid (PLGA) copolymers, and polycaprolactone (PCL) [33, 35]. Synthetic polymers are widely applied in the field of tissue engineering because of their tunable physico-chemical properties. The polyester family of synthetic polymers provides controllable and reproducible material properties, including elasticity and degradability, which are very useful in tailoring matrices with desired functions (Table 2) [34]. The lower possibility of infections and risk of immunogenicity give synthetic polymers an edge over natural ones [36].

Table 2. Advantages and disadvantages of synthetic polymers for tissue engineering applications

 
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Legend: PCL - polycaprolactone, PDMS - polydimethylsiloxane, PEG - polyethylene glycol, PGA - polyglycolic acid, PLA - polylactic acid, PLGA - polylactic-co-glycolic acid.

 

Using the advantages of both classes of materials, recent work has focused on synthesizing hybrid scaffolds with both natural and synthetic components. Although there have been several prior studies focusing on the selection of the ideal polymer for encapsulation of islets, there has been no review to date which deals comprehensively with the optimal choice of scaffold materials for islets or pancreatic tissue engineering. The present review offers such a comprehensive insight into the application of natural and synthetic polymer-based scaffolds. This review also analyzes critically the problems associated with the construction of the bioartificial pancreas and discusses different design strategies.

5. Natural polymers

5.1 Collagen

Collagen is a structural basement membrane protein and a widely used biomaterial for cell adhesion and proliferation [37]. Being a part of the extracellular matrix, collagen has found wide application in tissue engineering. It has been used for engineering heart valves [38], lung [39], bone [40], and other organs. Several reports have indicated the use of collagen for pancreatic tissue engineering. Jalili et al. incorporated fibroblast in type-1 collagen gels. Before solidification, islets were also embedded in the collagen gel. Collagen provides the ECM for islet growth, and fibroblasts maintain matrix integrity. This scaffold showed improved cell survival and insulin secretion. Importantly, incorporation of fibroblasts reduced the number of islets required to reverse diabetes through transplantation [41]. In another study, basement membrane proteins (laminin and heparin sulfate proteoglycan) were combined along with collagen to form gels in which islets were embedded. These cells showed better proliferation, attributed to the reduced caspase-3 expression, and improved cell survival [42].

To carry neonatal porcine islets Ellise et al. used scaffolds containing the following constituents:

  • Rat tail collagen cross linked with 1-ethyl-3-(3-dimethylaminopropyl)-carbodiimide (EDC) N-hydroxysuccinimide (NHS)
  • A combination of chondroitin-6-sulfate, chitosan, and mouse laminin

The islets survived up to 28 days indicated by positive insulin and glucagon staining. Also, the matrix did not show any signs of inflammation, and the scaffold could maintain its shape and size for over 28 days [43]. Another study illustrated early restoration of euglycemia post-transplantation (from 17 to 3 days) relative to controls using PLG scaffolds coated with collagen-IV, laminin, and fibronectin [44]. The collagen IV-modified scaffolds showed improved islet survival, enhanced islet metabolism, and better glucose-induced insulin secretion.

Collagen alone does not provide the mechanical strength required for pancreatic tissue architecture. Hence, a combination of other polymers such as chitosan, chondroitin-6-sulfate, and laminin or crosslinking has been used to improve the scaffold [43]. Almost all the studies mentioned above showed that incorporating collagen with other basement membrane proteins tended to improve islet survival and function.

5.2 Gelatin

Gelatin, a natural product generated from hydrolysis of collagen, has offered great potential as scaffolding material [45]. Being a natural polymer and having the beneficial properties of biocompatibility, biodegradability, and lack of antigenicity and immunogenicity, gelatin-based scaffolds have shown promising results for tissue engineering of cartilage [46], bone [47], skin [48], and other tissues. Various research groups have shown effective application of gelatin and its blends for engineering islets. Collagen is a component of the basement membrane of ECM in the adult human pancreas, thereby providing gelatin with an advantage over other polymers. One of the major properties required for pancreatic tissue engineering is good mechanical strength. Gelatin alone does not fulfill this criterion. Therefore, various blends of gelatin with other polymers have been used.

Gelatin has been used for encapsulating rat pancreatic islets grown on polyglycolic acid scaffolds. These engineered islets were transplanted into streptozotocin-induced (STZ-induced) diabetic nude mice. The diabetic mice maintained normal glycemia until 120 days of transplantation, with the islets showing potential to secrete exogenous insulin [49]. Muthyala et al. used gelatin to synthesize 3D porous interpenetrating polymer network (IPN) scaffolds along with polyvinylpyrrolidone (PVP) using the cross-linkers glutaraldehyde and EDC hydrochloride by freeze-drying [50]. IPN scaffolds displayed ideal properties for tissue engineering with good mechanical strength. Out of the many scaffolds synthesized, one of them (gelatin-PVP-semi-IPN) showed good growth of viable β-cells even up to 30 days [50]. Moreover, the authors showed that a combination approach, consisting of mouse islets grown on the gelatin-PVP semi-IPN scaffold encapsulated in a PU-PVP semi-IPN microcapsule, (a capsule made up of polyurethane extrusion grade Tecoflex 60D (TFPU) and PVP coated with semi-IPN solution), reversed diabetes in rat models for up to 90 days [51].

Previous studies have used gelatin in combination with dextran to produce three scaffolds (DEXGEL). Sodium meta-periodate was used to incorporate the aldehyde group in dextran which could link with the amine group of gelatin, thereby negating the use of additional cross-linkers. DEXGEL served as a platform for differentiation of adipose stem cells into islet-like clusters. These islets provided higher levels of insulin secretion than 2D culture systems [52].

5.3 Fibrin

Fibrin is a protein involved in blood clotting. It has been used widely for tissue engineering applications because of properties such as self-assembly and soft elasticity [53]. Fibrin hydrogel has shown various impressive properties such as low toxicity to cells, good cell anchorage, proliferation, and migration [54, 55]. Fibrin gels have been used to differentiate chemically human endometrial stem cells into pancreatic β-cells using activin A, nicotinamide, fibroblast growth factor (FGF) and epidermal growth factor (EGF) [56]. Insulin secretion was found to be higher in 3D fibrin gels enclosed with differentiated cells than in their 2D counterparts. Khorsandi et al. have shown differentiation of bone marrow-derived mesenchymal stem cells (BM-MSCs) into insulin-producing cells using 3D culture and fibrin glue [57]. Previously, long-term proliferation of a rat insulinoma cell line (INS-1) on fibrin gel had shown increased insulin secretion in response to glucose stimulation [58].

Fibrin has also been shown to significantly improve insulin secretion in diabetic mice which were transplanted with fibrin-cultured islets. These mice had shown highly vascularized islets along with improved viability [59]. This shows the importance of fibrin in maintaining islet cell viability and angiogenesis. Although the use of fibrin for islet proliferation has brought about much improvement, one of the major drawbacks associated with it is the risk of immune response in vivo [53]. Furthermore, the potential application and risks associated with fibrin for islets proliferation and transplantation are not yet fully elucidated.

5.4 Agarose

Agarose, a naturally occurring polysaccharide, is one of the most widely used polymers in the field of tissue engineering. Its favorable properties include biodegradability, soft tissue-like mechanical abilities, and strong and rapid gelling capacity, and make it an ideal candidate for soft tissue engineering [60]. Agarose gel has been used as gene delivery vehicle [60], scaffold for implantation surgery [61], cartilage tissue engineering [62], liver tissue engineering [63], and other applications.

Recently, our group formulated an agarose chitosan-based nanocomposite (AG-CHNP) using a freeze-drying technique. Our scaffold showed good biocompatibility with various cell lines, including HEK, Mia PaCa-2, and HeLa, hemocompatibility, and antibacterial activity. These scaffolds showed continuous increased growth of HeLa for a period of 16 days [14]. We further used AG-CHNP scaffolds for chemical differentiation of BM-MSCs into insulin-producing cells. The differentiated cells showed positive results for the pancreatic markers PDX1 and NKX2.2. The differentiated cells secreted insulin confirmed by western blot (unpublished data). These preliminary results suggest that such agarose-based scaffolds are suitable for pancreatic tissue engineering.

For islet engineering, agarose-agarose islet macrobeads were used to encapsulate porcine islets. These macrobeads were xenotransplanted into pancreatectomized dogs. This, along with anti-inflammatory pravastatin therapy, showed prolonged functionality and biocompatibility of the islets [64]. Luan et al. transplanted islets into a prevascularized subcutaneous space. Induction of blood vessels was performed using freeze-dried agarose rods comprising basic FGF (bFGF) and heparin. 1500 islets were transplanted into the prevascularized subcutaneous tissue without using any immunosuppressive regimen. This therapy reverted hyperglycemia, and showed long-term allogeneic islet graft survival and function [65].

One report used agarose microwells made up of polydimethylsiloxane (PDMS) molds for the formation of primary islet aggregates, which are pseudoislets with pre-defined proportions [66]. Dissociated islets, when aggregated in a controlled environment, led to a change in the core mantle arrangement of α- and β-cells, which underwent modification after implantation under the kidney capsule. After transplantation, these islets behaved almost like native islets. This observation demonstrated the importance of cell-to-matrix interaction, and the necessity for the islets to have the appropriate size and shape for the maintenance of their structure and function in vivo [66].

Apart from this finding, Ichihara et al. used size-controlled pseudoislets from rat pancreas on agarose gel-based microwells. The micromolds were synthesized using soft lithography of different diameters (100, 300, 500 μm). These small islet aggregates showed better insulin secretion and cell survival than medium-sized and large aggregates. Also, native tissue-like cell organization was observed in both small- and medium-sized islet aggregates [67]. This study highlighted the role of size in islet transplantation. Recently, a novel approach of combining agarose gel scaffolding with BM-MSCs showed improved insulin secretion compared to controls where islets were grown on agarose gel only. This study highlighted the role of BM-MSCs and agarose gel in improving the overall functionality of islets. It is suggested that BM-MSCs provide growth factors and paracrine signaling, and agarose gel allows cells to absorb nutrients in an unlimited manner which improves islet function [68].

5.5 Alginate

As mentioned above, microencapsulation is a process of entrapping cells or tissues within a polymeric membrane that acts as immunosuppressive barrier [69]. Currently, much research has been done in microencapsulating β-cell grafts to allow easy transplantation, immunoprotection, and the use of non-human islets [70]. A commonly used polymer for microencapsulation is alginate, a polysaccharide isolated from brown sea weed [71]. It has gained tremendous popularity after Lim et al. used islet-encapsulated alginate beads as artificial pancreas [72]. From then on, alginate has been widely used.

Alginate hydrogels have found application as beads, delayed gelation systems, macroporous scaffolds, 3D printed scaffolds, etc. [73]. Although alginate beads enable a rapid and non-toxic encapsulation of cells, their property of limited cell adhesion is a huge disadvantage for their wide application in tissue engineering. Therefore, in the field of islet tissue engineering, alginate has been mainly used for microencapsulation only. A breakthrough in alginate-based encapsulation techniques was made in 2010 when Opara et al. suggested a multi-layer model of bioartificial pancreas containing two alginate layers separated by a semi-permeable membrane made up of poly-l ornithine. The inner layer was used to encapsulate the islets, and the outer layer for the adjunct of angiogenic proteins. These microcapsules were implanted into the omental pouch of rats. The authors reported that the use of such alginate beads enabled controlled delivery of growth factors and initiation of blood vessel formation, thereby improving graft viability and function [74].

Gelation of alginate takes place in the presence of ions (Ca2+ or Ba2+). But an ionically bound alginate hydrogel of this kind may not be able to withstand the mechanical stress associated with implantation. Therefore, alginate was modified by incorporating a carboxylic group into alginate backbone, and covalent linking to modified PEG (phosphine group at the end) using Staudinger ligation. This hydrogel had better stability and cell attachment than the alginate controls [75]. The microencapsulation system proposed by Opara et al. was improved with a thick and cross-linked outer alginate layer. This procedure helped to maintain the stability of the system for a longer period; the microcapsule remained intact even after 90 days of transplantation. This work also suggested the omental pouch as a potential implantation site for islet transplantation [76].

Richardson et al. demonstrated a stage-wise directed differentiation of alginate-encapsulated human embryonic stem cells into islet-like cells. Clear viable colonies were evident after differentiation and maturation. Encapsulated cell differentiation resulted in strong maturation marker expression and improved hormone secretion as compared to their 2D counterparts [77]. Additionally, 3D bioplotting has been used to formulate alginate-gelatin porous scaffolds which can be used as extrahepatic islet-delivery systems (bioplotting is a technique that causes extrusion polymers to create custom-engineered scaffolds). When islets were removed from the hydrogel, they showed full functionality [78]. This study is one of the most recent reports on the use of 3D bioplotting for islet engineering.

Another recent study proved the benefits of a modified form of alginate, triazole-thiomorpholine dioxide (TMTD) alginate, for islet implantation [79]. Emphasizing that the size of the microspheres affects the immunological response to the implants, human embryonic stem cell-derived β-cells (SC β-cells) were encapsulated in 1.5 mm TMTD alginate spheres, which showed better glycemic control than the conventionally used 500 µm alginate spheres. This was the first study to report long-term glycemic control in immune-competent mice containing SC β-cells [79]. This report highlights the role of alginate and its derivatives as an immuno-isolatory device in a xenotransplantation setting.

Alginate-encapsulated islets have also been used for clinical applications in patients with type 1 diabetes by various groups. They have shown long-term stability of the capsule in vivo with continuous reduction of exogenous insulin [70]. However, a perfect site of implantation, which overcomes all the disadvantages, is yet to be found [69].

5.6 Silk

Silk protein is commonly used in the textile industry; it is produced by silk worms and spiders. The fibrous protein in it native form consists of a component (sericin) which can elicit an inflammatory response [80]. However, this component can be removed by the process of alkali- or enzyme-based "degumming".

Apart from textiles, silk is also applied in tissue engineering and drug delivery. It offers various outstanding properties which amplify its role as a biomaterial. One of these beneficial properties for tissue engineering is its excellent mechanical strength, which is higher than that of Kevlar, a synthetic fiber used as a reference point in fiber technology [81]. Apart from this advantage, silk has further beneficial properties that simplify handling, including good biocompatibility, water-based processing, chemical modifiability, and biodegradability [81, 82]. Silk can be molded into any form such as films, electro-spun fibers, hydrogels, scaffolds, and particles.

In the field of tissue engineering, silk (alone and in combination with other polymers and nanostructured fibers) has been used for wound healing [83] as well as the regeneration and reconstruction of bones [84], tendons and ligaments [85], urethra [86], cartilage [81, 87], and other tissues. Silk has also been widely used for pancreatic tissue engineering.

Silk hydrogels have been used to encapsulate mice islets. These hydrogels provided a 3D environment in which the islets could maintain their viability and functionality. In the normal pancreas, islets are surrounded by ECM-containing collagen, laminin, and fibronectin which help in cell adhesion and proliferation. To mimic a similar environment, extracellular proteins and secondary stromal cells were incorporated in silk hydrogel which showed enhanced islet function [88]. Do et al. showed that oral ingestion of silk fibroin hydrolysates helps in maintaining pancreatic β-cell integrity, and improves insulin secretion by increasing β-cell mass in hyperglycemic mice [89].

Co-encapsulation of β-cells and mesenchymal stem cells (MSCs) using silk hydrogels has also been explored. Though silk is a magnificent biomaterial, it may still stimulate host inflammatory responses which harm islet growth. However, the presence of MSCs reduces this effect because of their immunomodulatory and angiogenic properties. This multi-dimensional approach has proved successful in terms of graft functionality and revascularization, with an undesirable drawback of bone differentiation [90]. Recently, Kumar et al. microencapsulated silk scaffold with alginate and agarose. This scaffold showed sustained growth for rat insulinoma cells (RIN-5). Rat β-cells also showed better growth on the 3D scaffold as compared to its 2D counterpart which was confirmed by expression of primary pancreatic genes [91].

6. Synthetic polymers

6.1 Polyglycolic acid

Polyglycolic acid (PGA) is a biocompatible polymer approved by the US Food and Drug Administration (FDA). It is obtained by ring cleavage polymerization of glycolide. PGA hydrolyses in vivo to give glycolic acid, which is a metabolite in the citric acid cycle, thus resulting in low toxicity [92, 93]. PGA has a wide range of applications in the field of tissue engineering due to its tunable degradation rate and intrinsic tendency to form stable 3D structures [33]. However, PGA undergoes rapid absorption in vivo, causing failure of the scaffold. Also, inflammatory responses are provoked because of increased release of acidic degradation products. Combination of PGA with several copolymers such as PLGA or PEG has been shown to enhance its beneficial physical and mechanical properties [33]. PGA has been widely used to make bioresorbable sutures and cartilage regeneration [16].

A hybrid scaffold of collagen and PGA with basic fibroblast growth factor has been developed to promote wound healing in type 2 diabetic mice. This hybrid matrix has enhanced compression strength, thus suppressing wound contraction, while also inducing angiogenesis and granulation tissue formation [94]. A study by Chun et al. showed that the islet cells grown on PGA scaffolds functionalized with a layer of poly-l-lysine enhanced the surface activity and adhesion capacity of PGA scaffold, and promoted cell proliferation. The PGA scaffold was also shown to provide superior nutrient absorption and metabolite excretion to the cultured islets, providing an appropriate microenvironment for their growth and survival. The cultured islets exhibited enhanced viability, improved morphology, and increased glucose-stimulated insulin secretion [93].

The viability of PGA islet grafts transplanted into the leg muscles of rats with STZ-induced diabetes has also been investigated. This scaffold provided a compatible 3D microenvironment with visible adhesive growth of islets on the scaffold and an adequate supply of blood and nutrients. The results showed increased insulin secretion and significantly decreased blood glucose concentration in rats transplanted with PGA islet grafts as compared to controls [95].

Recently Li et al. used PGA scaffolds for increasing the efficacy of islet coating by endothelial cells (ECs). Coating islets with ECs has been shown to improve revascularization and to reduce initial inflammatory response. Due to the presence of PGA scaffolds, enhanced coating efficiency of ECs on the islets was observed. Islet functionality was also improved with enhanced glucose-stimulated insulin release. The authors thus recommended the use of PGA scaffolds in pre-transplant culturing of islet cells and ECs [96].

6.2 Polylactic acid

Polylactic acid (PLA) is also approved by the FDA. It is an aliphatic polymer widely applied in the field of biomedical devices and tissue engineering [97]. PLA hydrolyses in vivo to release lactic acid, which becomes incorporated into the citric acid cycle and is naturally excreted, thus making PLA biocompatible and biodegradable in nature [98]. However, numerous surface treatments need to be implemented to hydrophobic PLA to impart enhanced biomimetic and cell adhesion properties [99]. PLA has tunable and versatile physical and chemical properties, and can be molded to take on a myriad of shapes, including microspheres, scaffolds, sutures, and nanoparticles [100]. Taking advantage of its long half-life, PLA has been extensively used in fabrication of long-term implantable devices for therapeutic applications [98]. PLA and its copolymers are extensively applied in tissue engineering, including skin grafting and the regeneration and reconstruction of bone, spinal cord and nerve tissue [100].

In the field of pancreatic tissue engineering, the potential therapeutic application of PLA microspheres has been studied in the treatment of diabetic periodontitis. The microspheres, loaded with 25-hydroxyvitamin D3, were shown to prevent inflammatory responses and bone loss in rats with diabetic periodontitis [101].

PLA-PEG-based nanoparticles have also been used as a means for subcutaneous delivery of insulin. Nanoparticles, loaded with 50 IU of insulin per kg, were shown to control blood glucose levels, thereby restoring normoglycemia in diabetic rats. These biodegradable nanoparticles proved to be non-toxic in nature; they are thus qualified as potential candidates for parenteral insulin therapy [102].

Kasujo et al. described the application of PLA-based porous capsules to obtain a vascularized microenvironment for extrahepatic islet transplantation. The bioartificial cavity showed numerous vessels and guided infiltration of the host’s connective tissue cells and vascular endothelial cells with no significant infiltration by inflammatory cells, providing a favorable microenvironment for islet transplantation [103]. A 3D delivery system has been developed which can be used for encapsulation and implantation of pancreatic cells. The PLA-based nanogland provided support to islet-like aggregates derived from differentiation of human MSCs, enhancing their viability and maintaining their function in vitro. The nanogland provided steady secretion of insulin, demonstrating potential benefits for diabetic cell therapy [98]. Recently, a 3D printed encapsulation system has been formulated using polylactic acid for subcutaneous implantation of pancreatic islets. After surface treatment was employed to functionalize the system, it was implanted with VEGF-enriched platelet gel to enhance vascularization. This system enabled transcutaneous refillability and potential retrievability of the graft [99].

6.3 Polylactic-co-glycolic acid

Polylactic-co-glycolic acid (PLGA) is an FDA approved copolymer obtained by ring-opening copolymerization of lactide and glycolide [104]. PLGA has been widely used in varied forms such as films, porous scaffolds, hydrogels, and microspheres for biomedical tissue engineering and drug delivery purposes due to its high biocompatibility and non-toxic biodegradation [36]. An additional advantage of the physico-chemical properties of PLGA is the tunable mechanical strength and biodegradation rate achievable by altering the PLA:PGA ratio [35]. However, PLGA has adverse surface characters such as hydrophilicity, protein absorbance, and poor cell affinity [105]. Numerous surface modulation strategies like surface immobilization, physical adsorption of bioactive molecules, plasma treatment, and incorporation of other biocompatible materials into the PLGA matrix have been tested to make the interface between PLGA and its environment more biomimetic which improved cell affinity [36].

Recently, biocompatible PLGA scaffolds have been produced using 3D printing for use in tissue engineering [106]. Electrospun PLGA-based hybrid nano-fibrous membranes and scaffolds have been widely used for skin, bone, nerve, and soft tissue engineering applications [105].

In the field of pancreatic tissue engineering, micro-porous, biodegradable PLGA has been successfully utilized as a platform for islet transplantation in mouse models [107]. Salvay et al. explored the effects of PLGA scaffolds with adsorbed ECM components on the survival of transplanted islets. It appeared that adsorption of these proteins by the scaffold enhanced the efficacy of islet grafts and significantly decreased the time needed for the reversal of diabetes in mice [108]. The effects of integrated ECM components on long-term maintenance of human pancreatic islets cultured in a micro-fabricated PLGA scaffolds have also been investigated in vitro. The PLGA scaffold provided a viable niche, with the in-vitro-cultured islets displaying insulin release profiles characteristic of native islets [109].

Kheradamand et al. demonstrated the use of PLGA scaffolds as an extra-hepatic site for islet transplantation [110]. The addition of ethylcarbodiiminde-fixed (ECDI-fixed) donor splenocyte infusions to the PLGA scaffolds enhanced the efficacy of tolerance induction in vivo, and indefinite normoglycemia was maintained in diabetic mice models [110]. Bioresorbable PLGA microspheres have been designed for encapsulation and sustained administration of β-cell-proliferative compounds to intact mouse islets in culture [111]. The improved bioavailability of the mitogen to β-cells in vivo may lead to increased β-cell proliferation, and may thus be regarded as a therapeutic application in the restoration of normoglycemia in diabetic patients [111].

Recently, Liu et al. investigated the fabrication of artificial islet tissues using a fibroblast-modified PLGA membrane for differentiating pancreatic stem cells into insulin-producing cells. This construct secreted insulin and was shown to reduce blood glucose levels in diabetic nude mice. The modified PLGA membrane showed higher compatibility, improved proliferation, and increased viability of pancreatic stem cells compared with the unmodified membrane. Also, it had an enhanced histocompatibility with nude mice [112].

6.4 Polycaprolactone

Polycaprolactone (PCL) is a hydrophobic, biodegradable FDA-approved polymer prepared by ring-opening polymerization of ε-caprolactone in the presence of SnO2 and heat [16]. PCL has gained an edge in the field of biomedical research because of its low melting point, remarkable blend compatibility, and viscoelastic properties. PCL has been widely used in drug delivery systems as surgical sutures and scaffolding material for tissue engineering because of its tunable degradation rates and beneficial mechanical properties [113]. Drawbacks associated with PCL include hydrophobicity, limited bio-regulatory activity, and susceptibility to bacteria-mediated degradation [33]. To enhance favorable cellular responses, various functional groups have been incorporated into the polymer, making it more hydrophilic and biocompatible [113]. PCL and its copolymers such as PCL-PEG and PCL-PLA have various applications in cartilage, bone, and peripheral nerve regeneration [16].

Nano-fibrous PCL scaffolds have been used for differentiation of human induced pluripotent stem cells (iPSCs) into definitive endoderm cells using inducer for definitive endoderm 1 (IDE1). Electrospun PCL scaffolds exhibited more pores, decreased toxicity, and reduced thickness of the nanofibers, enabling more surface space for cellular proliferation and attachment [114]. A composite hydrogel made from polycaprolactone (PCL) and polyacrylic acid (PAA) is applied in oral delivery of the drug gliclazide, which is used in the treatment of type 2 diabetes. The balance of hydrophobic PCL with hydrophilic PAA provided the property of the hydrogel that controls swelling. The PCL/PAA hydrogel offered a controlled release of the drug and was shown to enhance its bioavailability, resulting in reduced glucose levels [115].

PCL is also applied in diabetic wound healing. Gholipour-Kanani et al. blended PCL with chitosan to avoid the use of chemical cross-linkers and achieve a nano-fibrous scaffold with sustainable integrity in aqueous media. This poly(caprolactone)-chitosan-poly(vinyl alcohol) (PCL:Cs:PVA) scaffold was found to promote diabetic wound healing because of its biocompatibility and structural similarity to native ECM [116]. Ranjbu-Mohammadi et al. showed the application of curcumin-loaded poly(ε-caprolactone) (PCL)/gum tragacanth (GT) (PCL/GT/Cur) nanofibers in the field of wound healing. The antibacterial nano-fibrous membranes enhanced the healing process by simulation of native ECM, presence of curcumin and GT, and improved mechanical stability of the scaffolds because of the presence of PCL.

Tissue-engineered scaffolds were also shown to decrease blood glucose levels in rat models [117]. A current finding highlighted the use of heparanized ring-shaped PCL scaffolds functionalized with VEGF for carrying islets in an alginate core. Vascularization was successfully induced throughout the scaffold by the presence of immobilized VEGF. The embedded islets were shown to maintain their viability and functionality, responding normally to glucose stimulations, and at the same time, possessing obvious immune protection properties. The scaffold demonstrated improved revascularization; it may thus be used as potential vessel for subcutaneous islet transplantation [118].

Recently, Smink et al. demonstrated the use of a modified PCL, poly (D,L-lactide-co-ε-caprolactone) (PDLLCL) to create a scaffold which acted as an artificial and retrievable subcutaneous transplantation site for pancreatic islets. PDLLCL was shown to be compatible with islet viability and functionality. Also, islets cultured on PDLLCL exhibited comparatively more insulin granules and lower release of immune system-provoking double-stranded DNA, suggesting PDLLCL as a suitable scaffold with potential for application in the treatment of type 1 diabetes [119].

6.5 Polydimethylsiloxane

Polydimethylsiloxane (PDMS), a silicon based organic polymeric compound, has been commonly used as surfactant, stamp resin for soft photolithography, and other applications. Its superior properties make it a better choice for tissue engineering than other synthetic equivalents. PDMS has high biocompatibility, biostability, and oxygen solubility, which makes it a perfect candidate for implantation [120]. PDMS was used to construct a macroporous scaffold via solvent casting and a particulate leaching method. PDMS has a hydrophobic surface which is ideal for slow release of compound, but does not support cell adhesion. Therefore, fibronectin was added to the surface of PDMS scaffolds to make it hydrophilic.

Islets loaded onto PDMS scaffolds and implanted into the omental pouch showed good islet retention and long-term normoglycemia. Interestingly, islets on the scaffold showed enhanced viability and function under low oxygen tension compared to 2D controls [121]. Another study group seeded a fibrin platelet-derived growth factor hydrogel loaded with islets onto the PDMS scaffold and transplanted it into mice. This system helped to reduce the time required for attaining normoglycemia, and enabled increased vessel branching [122].

Recently, PDMS scaffolds have been used for delivery of anti-inflammatory agents such as dexamethasone and fingolimod [123, 124]. Dexamethasone was added to PDMS scaffolds in various quantities. Low concentration of dexamethasone showed improved islets engraftment, but higher concentrations were found to be detrimental as they alter glucose-induced insulin secretion by suppressed activation of the PLC/protein kinase C signaling system [123, 124]. Fingolimod exhibited persistent release, but at a very low concentration (0.1% w/w), which does not have any significant effect on the islets [125].

6.6 Polyethylene glycol

Polyethylene glycol (PEG) is one of the most popular synthetic polymers used for tissue engineering applications. PEG is a low-immunogenic material, has tissue-like elasticity, and its chemistry is well-defined, which provides an advantage over other polymers for islets engineering [126]. PEG has been used in the form of scaffolds and encapsulating agents for islet transplantation. As PEG is biologically inert, it does not support any form of cell growth. Therefore, for use as a scaffold, it must be augmented with another co-polymer.

Mason et al. used collagen fibrils in PEG hydrogels, and studied their effect on encapsulated embryonic pancreatic precursor cells. The cells on these scaffolds showed high glucose responsiveness, and had an improved level of insulin gene expression [127]. PEG scaffolds have also been supplemented with fibrin ribbons, which were used for co-culturing endothelial cells and islets [128]. Endothelial cells were encapsulated within the fibrin ribbons, and islets were implemented in the PEG hydrogel. The results suggested an optimum growth for both cell types, penetration of endothelial cells into the hydrogel, and improved vascularization.

A major problem associated with islet transplantation is the requirement for large numbers of islets. To overcome this problem, surface modifications of islets are currently being tested. The main objective of this technique is to reduce the number of islets required for transplantation [126]. Glucagon-like peptide 1 (GLP-1) is produced by the L-cells of the distal ileum, and is an insulinotropic ligand. Kizilel et al. directly immobilized GLP-1 on the surface of islets by layer-by-layer assembly of biotin-PEG-NHS, streptavidin, and biotin-PEG-GLP-1. Coated islets showed better insulin secretion in response to high glucose than control islets, which proved the efficiency of this technique. This study also addressed the issue of donor shortage as it required lower numbers of transplanted islets to achieve normoglycemia [129]. Another study using PEG as an encapsulating agent developed a device which had rat islets growing on an acellular scaffold and encapsulated in a PEG/VA semi-permeable membrane. This device was implanted in diabetic rats, and showed a reduction in insulin requirement for at least 2 weeks, restoring partial insulin secretion. To achieve complete euglycemia, the question of the optimal islet number to be transplanted needs further investigation [130]. PEG-based hydrogel microwells have also been developed using photolithography. MIN6 β-cells were seeded on the microwells, and maintained for 5 days preceding retrieval and encapsulation. This PEG-based microwell consistently demonstrated successful formation of MIN6 aggregates. Also, the encapsulated MIN6 aggregates showed better insulin secretion and positive expression of the intracellular binding protein E-cadherin as compared to single cell encapsulations [131].

One of the major causes of islet loss after transplantation is hypoxia which affects the longevity of the implant. Therefore, to facilitate short oxygen supply to the islets, PEG-stabilized hemoglobin had been used as an artificial oxygen carrier [132]. But this system does not support long-term use because of continuous conversion of hemoglobin to methemoglobin by autoxidation and free radical damage, which is deleterious to the cells. Therefore, PEG-based hemoglobin conjugates cross-linked with antioxidant enzymes (superoxide dismutase and catalase) have been used [133], and demonstrated excellent protection against free radicals and oxygen-induced stress in RINm5F cell line. The viability of RINm5F cells was higher and the generation of reactive oxygen species (ROS) was reduced for cells treated with the conjugates. The results also showed sustained or increased insulin release from the treated islets under partial oxygen pressure situations. This study provided insight into the value of PEG-based conjugates for preventing hypoxia-induced graft failure. Another attempt to prevent post-transplantation islet loss was made by Golab et al., where islets were immunoprotected by coating them with Treg cells conjugated with biotin-PEG-SVA (succinimidyl valeric acid ester). This approach was found to be favorable compared to that using biotin-PEG-NHS for coating pancreatic cells with Treg cells, and showed slightly improved insulin secretion [134].

PEG hydrogels have also been used for encapsulation of islets in combination with BM-MSCs, GLP-1, and ECM-based cell adhesion ligands [135]. Insulin secretion could be increased 7-fold by the synergistic effects compared with islets alone, islet functionality and viability could be improved, and MSCs have shown immunomodulatory effects. As mentioned previously, angiogenesis plays a major part in maintaining islets functionality. Pancreatic islets comprise only 1-2% of the total pancreatic cell population, but require as much as 15-20% of the total pancreatic blood supply [136]. Therefore, maintaining similar angiogenic effects post-transplantation is absolutely required. Phelps et al. created PEG hydrogels with mild maleimide-thiol cross-linking. These scaffolds were further modified by the addition of RGD motif for cell adhesion and VEGF for vascularization [137]. This study highlighted the use of mesentery as a transplantation site which is far less invasive than hepatic portal transplantation. This scaffold and the delivery strategy showed beneficial outcomes in terms of vascular invasion and insulin secretion. This research also negated injection of islets into the blood stream which can cause immediate inflammatory reaction. Recently, a blend of synthetic PDMS and natural PEG polymer was used for transplantation of islets into epididymal fat pad [138]. Islets were mixed with PEG and then included in photo-linked PDMS molds. As the islets were encapsulated in PEG and PDMS, they were protected from immediate inflammatory attack by the immune system. An interesting finding of this study was that glucose tolerance test revealed normoglycemia within 90 minutes of transplantation.

7. Generating a functional pancreatic construct

Diabetes is one of the leading causes of death in the world with an increasing global prevalence. The available treatment options have their own set of serious implications. Therefore, the focus of current research has shifted to the search for new and safe options which include the bioartificial pancreas. Research associated with the development of the artificial pancreas has seen numerous changes over the years. Various approaches include the use of polymer-based scaffolds, organ decellularization, scaffoldless tissue engineering, bioprinting, and encapsulation.

Polymeric tissue engineering involves the synthesis of an artificial scaffold using natural and/or synthetic polymers and a cross-linker. Various scaffolding approaches are available, including freeze drying, electrospinning, solvent casting, and others [18]. Both natural and synthetic polymers have their own set of advantages and disadvantages.

One of the major problems associated with pancreatic tissue engineering is the complexity of the organ itself. The pancreas contains both an exocrine section (including ductal and acinar cells) and an endocrine section (including the islets of Langerhans) [139]. Though β-cells are the major players in glucose metabolism, other small members of the islet cell family also play a critical part in the overall islet functioning [140]. Therefore, multiple cells types need to be considered in the process of islet graft engineering, which further complicates the work.

Another important factor is angiogenesis. Since islets require a much greater blood supply than any other pancreatic cell compartment [137], the choice of transplantation site becomes important to allow the transplanted construct receiving sufficient blood supply for survival. Various sites for transplantation have been examined, including the peritoneal cavity [122], hepatic portal vein [118], subcutaneous space [65], and subcapsular space of the kidney [41, 49]. Some sites such as omental pouch [76] and mesentery [137] have shown promising results, but the problem of hypoxia and shortage of blood supply is yet to be resolved.

Finally, the islets themselves are a cause of concern for pancreatic engineering since they have poor viability and stability in vitro [141]. The limited supply of islets intensifies this problem.

8. Conclusions

The construction of a bio-artificial pancreas is subject to a number of difficulties that need to be overcome. These difficulties include:

  • Choice of cell type
  • Culture environment
  • Site of implantation
  • Scaffolding approach
  • Requirement of encapsulation

Various categories of cell type have been tested for pancreatic tissue engineering and implantation, including allogeneic, xenogeneic, and alternative sources (embryonic stem cells, MSCs) [57, 64, 65, 79]. Although allogeneic and xenogeneic islets have shown promising results, limited availability and poor stability post-isolation have restricted their usability. Therefore, stem cells have received much attention in current research as they have shown promising differentiation potential [31]. While embryonic stem cells are banned in various countries because of the ethical issues, MSCs isolated from different sources have been widely studied [52, 57].

After selection of the cell type, it is important to choose the appropriate culture environment, i.e. whether to culture pancreatic cells individually or co-culture them with other cell types. Transplantation of β-cells alone has shown limited success. Co-culture with other cell types such as fibroblasts or MSCs have shown improved viability, functionality, and insulin secretion [41, 68, 90]. However, the major problem associated with islet transplantation is still substantial cell loss post-isolation and again post-transplantation due to hypoxia-induced apoptosis, loss of suitable microenvironment, and immune response [142-144]. Therefore, current research has been actively focusing on the use of encapsulation that is able to prevent the transplanted cells from direct crossfire from the host’s immune system. Moreover, growth factors and angiogenic factors encapsulated within the construct may help to maintain the viability and functionality of the islets [74].

There are many approaches that are under consideration for assembly of a functional bio-artificial pancreas (Figure 4). Considering the organ complexity and factor-dependent stability of the construct, research in the field of pancreatic tissue engineering has come a long way. Advances in tissue engineering and nanotechnology have provided tremendous insight into how we can improve the current approaches for creating a functional bio-artificial pancreas.

Figure 4. Advances in pancreatic tissue engineering. Various cell sources have been used for pancreatic tissue engineering, including allogeneic and xenogeneic islets (porcine and murine), mesenchymal and embryonic stem cells. Different growth factors have contributed to enhance the stability and proliferation of islet transplantation. Several scaffolding approaches have been employed to mimic the native microenvironment. Micro- and macro-encapsulation of transplanted islets has improved their overall viability and functionality.

 

We are still awaiting the creation of an appropriate scaffold that can act as a perfect environment for the growth of cells. Various polymers have been tried and tested for their application in scaffold designs. However, both natural and synthetic polymers fail to address all major requirements of an optimal scaffold for pancreatic tissue engineering. As the scaffold acts as a natural environment for the growth of cells, characteristics such as biocompatibility, biodegradability, vascularization, toxicity, and immunogenicity are critical [16]. One obstacle to finding the perfect polymer for pancreatic tissue engineering is that no single polymer has been studied intensively enough to learn whether it meets all the above-mentioned requirements.

As a result of our review, we found silk to be one of the most appropriate polymers for scaffold synthesis. Silk is well studied in the context of pancreatic tissue engineering, and found to be biocompatible and biodegradable [81, 82]. Apart from these advantages, the simplicity of chemical modification and its superior mechanical strength give it a small advantage over other natural polymers [81]. Silk hydrogels have been shown to provide a suitable environment for islets, and enable good islet viability and functionality [88]. Despite these favorable properties, its major drawbacks are its immunogenicity [90] and lack of evidence that silk scaffolds alone can induce angiogenesis. Several attempts have been made to overcome these shortcomings. While reduced immunogenicity and preliminary angiogenesis were observed when MSCs were co-cultured with β-cells, this approach led to unfavorable osteogenesis and chondrogenesis [90]. Additional work may focus on avoiding this co-lateral differentiation. A recent attempt to suppress immunogenicity was the macroencapsulation of silk scaffolds using alginate and agarose. This study showed positive results with respect to reduced immunogenicity [91]. Future research is necessary to improve the immunomodulatory effect of encapsulated silk scaffolds and to improve the overall viability and functionality of the transplanted islets.

As mentioned above, silk fulfills most of the criteria for application as a scaffold in engineering the bio-artificial pancreas. Since it is well studied and documented, silk may be a forerunner for scaffold design. However, future research on other polymers is also necessary since most have not been studied intensively and their complete capabilities have not been determined. It is thus impossible today to determine the optimal candidate. Future studies aimed at fully characterizing the available polymers and overcoming the existent limitations associated with scaffold fabrication may provide new avenues in the construction of the bio-artificial pancreas to prepare it for routine clinical application.

Funding: This review is supported by the Department of Science and Technology (SR/FT/LS-137/2011). NK would like to thank the Indian Council of Medical Research (ICMR) for her Senior Research Fellowship (BMS/FW/SCR/2015-20430/MAR-2015/06/GA/PVT).

Disclosures: The authors reported no conflict of interests.

Acknowledgments: The authors would like to thank Ms. Aastha Patel, Department of Electrical and Electronics Engineering, BITS-Pilani, K.K Birla Goa Campus for help in designing the schematic representation.

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