The endocrine pancreas is one of the most functionally complex organs in the human body, and its selective loss — particularly that of the β-cells within the islets of Langerhans — constitutes the pathogenic basis of type 1 diabetes mellitus and advanced forms of type 2 diabetes. The most ambitious therapeutic approach for these conditions remains pancreatic islet transplantation, clinically standardized through the Edmonton Protocol beginning in 2000. However, despite advances in islet isolation procedures and immunosuppressive therapy, long-term survival of transplanted tissue remains difficult to achieve: the loss of extracellular matrix (ECM) during islet isolation, the absence of immediate vascularization, and the host inflammatory response significantly impair graft function in a substantial proportion of cases.
It is within this critical context that tissue engineering — and particularly the use of biocompatible polymeric scaffolds — has gained a central role in translational research. Among natural protein-based biomaterials, silk fibroin (SF) has emerged over the past two decades as one of the most promising candidates for constructing three-dimensional architectures capable of hosting, protecting, and supporting pancreatic cells. The intrinsic properties of this polymer — biocompatibility, mechanical stability, controllable biodegradability, and processing versatility — make it particularly suitable for addressing the specific requirements of the pancreatic microenvironment.
Physicochemical properties of silk fibroin relevant to pancreatic applications
Silk fibroin is a high-molecular-weight fibrous protein. Its structure is characterized by repetitive amino acid sequences — predominantly glycine, alanine, and serine — which organize into antiparallel β-sheet configurations, conferring exceptional mechanical strength to the material. This structural organization can be modulated during processing, allowing the production of scaffolds with different rheological properties depending on the intended application.
From the perspective of pancreatic tissue engineering, two properties are especially relevant. The first is controlled enzymatic degradability: SF is degraded by tissue proteases such as proteinase K and collagenase, with kinetics that can be tuned by varying the degree of crosslinking and β-sheet content, ensuring progressive scaffold resorption as newly formed tissue replaces it. The second is intrinsic biocompatibility: unlike many synthetic polymers, SF does not induce significant local inflammatory responses, a critical advantage in a setting where peritransplant inflammation is already one of the primary causes of graft failure.
SF scaffolds and pancreatic islets. Evidence on cell encapsulation
The problem of pancreatic islet encapsulation is particularly complex because it requires an environment capable of protecting β-cells from immune recognition while preserving their viability and metabolic functionality, and at the same time allowing rapid diffusion of glucose, oxygen, and insulin through the matrix. Several studies have demonstrated that SF hydrogels effectively meet these requirements.
Davis and colleagues conducted some of the most widely cited studies in this field, comparing the encapsulation of pancreatic islets and mesenchymal stromal cells (MSCs) in SF matrices versus other systems. In vitro models showed that the hybrid islet+MSC system embedded in SF hydrogels improved insulin responsiveness, correlated with increased gene expression of insulin I, insulin II, glucagon, and PDX-1 (Pancreatic and Duodenal Homeobox 1), a transcription factor critical for β-cell differentiation and maintenance. In a subsequent in vivo study using a streptozotocin-induced diabetic mouse model, marginal islets and “pelletized” islets co-encapsulated with MSCs in an SF matrix demonstrated glycemic control within 4 and 15 days respectively, while the combined system normalized blood glucose levels within 9 days. The system also induced VEGF (Vascular Endothelial Growth Factor) expression, promoting survival and functionality of the hydrogels under physiological conditions.
Particularly significant was the observation that SF preconditions islets by increasing surface expression of the GLUT2 transporter. Comparative studies evaluating SF, alginate, and standard culture medium demonstrated that GLUT2 expression remained elevated even under cytokine-induced inflammatory stress. This finding suggests a specific molecular interaction between fibroin and β-cells that extends beyond simple mechanical support, raising the possibility that the biomaterial actively modulates cellular responses.
Heparin-functionalized macroporous scaffolds
One of the major limitations in islet transplantation is the loss of the specialized capillary network that, in the native pancreas, ensures rapid glucose sensing and appropriately timed insulin secretion. A particularly innovative strategy has involved the functionalization of SF scaffolds with heparin (H-SF), exploiting the glycosaminoglycan’s ability to bind and locally release proangiogenic growth factors.
One study demonstrated that co-transplantation of H-SF scaffolds with pancreatic islets in murine models resulted in improved intra-islet vascular regeneration accompanied by VEGF upregulation. Using vegfr2-luc transgenic mice — in which VEGFR2 receptor expression can be monitored through bioluminescence imaging — the authors observed sustained signal enhancement following co-transplantation with H-SF scaffolds, directly implicating activation of the heparin-dependent VEGF/VEGFR2 pathway in promoting islet revascularization and endothelial proliferation. Beyond confirming the clinical feasibility of this approach, the study also demonstrated reduced post-transplant inflammatory reactions associated with H-SF scaffolds, a critical factor for long-term graft survival.
Hybrid SF/dECM scaffolds: replicating the native islet microenvironment
A complementary approach to chemical functionalization involves combining SF with decellularized extracellular matrix (dECM) derived from pancreatic tissue. dECM is rich in structural proteins such as collagen, elastin, fibronectin, and laminin, as well as glycosaminoglycans and proteoglycans, providing biochemical and mechanical cues that mimic the native environment in which islets develop and function. However, constructs based solely on dECM often exhibit insufficient mechanical properties to ensure in vivo structural stability.
Hybrid SF/dECM biomaterials provide an elegant solution to this problem: SF contributes the necessary mechanical strength, while dECM supplies essential cellular signaling. A study published in the Journal of Biomaterials Scienceproduced hybrid electrospun scaffolds by combining SF with porcine pancreatic dECM (P-dECM) to mimic in vivo islet ECM. Morphological evaluations by SEM confirmed well-defined hydrophilic fibrous architectures, while MTT cytotoxicity assays demonstrated the absence of toxicity. Functional analyses — including glucose-stimulated insulin secretion (GSIS) assays and qPCR evaluation of differentiation markers — confirmed that islets maintained greater viability and functionality on these hybrid scaffolds compared with controls.
Hydrolyzed fibroin and β-Cell regeneration
Alongside scaffold-based approaches, another line of research has explored the direct biological effects of hydrolyzed SF — peptide fragments generated through enzymatic digestion of native fibroin — on pancreatic cells. A landmark study in this area was conducted using C57BL/KsJ-Leprdb/db mice, a genetically obese and diabetic murine model widely used in type 2 diabetes research.
The results demonstrated that treatment with hydrolyzed SF induced expression of proliferating cell nuclear antigen (PCNA) while reducing the population of apoptotic cells within pancreatic islets. Even more significant were the molecular findings: treatment activated the expression of transcription factors involved in β-cell regeneration, including Neurogenin 3 (Ngn3) and NeuroD, two master regulators of pancreatic endocrine neogenesis, with documented increases at the protein level in pancreatic tissue. In parallel, small colonies of cells expressing insulin mRNA were observed within the pancreatic parenchyma of treated animals. Collectively, these findings suggest that hydrolyzed SF enhances proliferation and regeneration of pancreatic β-cells, indicating a mechanism of action that extends beyond structural support and directly influences the cellular biology of endocrine tissue.
Scaffold fabrication techniques and their impact on pancreatic engineering
The versatility of SF also derives from the wide range of processing techniques through which it can be fabricated, each generating structures with distinct architectural and functional properties highly relevant to pancreatic applications.
Electrospinning enables the production of fibrous membranes with diameters in the nanometer range, replicating the nanoarchitecture of native ECM. Electrospun SF scaffolds provide a high surface area that promotes cell adhesion and nutrient exchange, making them suitable for reconstructing islet-like microenvironments. SF hydrogels, by contrast, are hydrated three-dimensional structures with tunable rheological properties ideally suited for islet encapsulation, permitting bidirectional diffusion of glucose and insulin while protecting cells from mechanical and immunological insult. Finally, macroporous scaffolds — produced for example through freeze-drying and porogen-based techniques — generate sponge-like three-dimensional structures that promote cellular infiltration, vascularization, and tissue fluid exchange, representing the preferred platform for the heparin functionalization strategies described above.
Chemical crosslinking with agents such as genistein, EDC (1-ethyl-3-(3-dimethylaminopropyl)carbodiimide), or glutaraldehyde profoundly influences mechanical stability, degradation kinetics, and overall scaffold stiffness, all parameters that can be optimized according to the specific requirements of the implantation site and the intended duration of therapeutic support.
