In clinical islet transplantation, allogeneic islets of Langerhans are transplanted into the portal vein of patients with type 1 diabetes, enabling the restoration of normoglycemia. After intra-hepatic transplantation several factors are involved in the decay in islet mass and function mainly caused by an immediate blood mediated inflammatory response, lack of vascularization, and allo- and autoimmunity. Bioengineered scaffolds can potentially provide an alternative extra-hepatic transplantation site for islets by improving nutrient diffusion and blood supply to the scaffold. This would ultimately result in enhanced islet viability and functionality compared to conventional intra portal transplantation. In this regard, the biomaterial choice, the three-dimensional (3D) shape and scaffold porosity are key parameters for an optimal construct design and, ultimately, transplantation outcome. We used 3D bioplotting for the fabrication of a 3D alginate-based porous scaffold as an extra-hepatic islet delivery system. In 3D-plotted alginate scaffolds the surface to volume ratio, and thus oxygen and nutrient transport, is increased compared to conventional bulk hydrogels. Several alginate mixtures have been tested for INS1E β-cell viability. Alginate/gelatin mixtures resulted in high plotting performances, and satisfactory handling properties. INS1E β-cells, human and mouse islets were successfully embedded in 3D-plotted constructs without affecting their morphology and viability, while preventing their aggregation. 3D plotted scaffolds could help in creating an alternative extra-hepatic transplantation site. In contrast to microcapsule embedding, in 3D plotted scaffold islets are confined in one location and blood vessels can grow into the pores of the construct, in closer contact to the embedded tissue. Once revascularization has occurred, the functionality is fully restored upon degradation of the scaffold.

Surface wettability and topography are recognized as critical factors influencing cell behavior on biomaterials. So far only few works have reported cell responses on surfaces exhibiting extreme wettability in combination with surface topography. The goal of this work is to study whether cell behavior on superhydrophobic surfaces is influenced by surface topography and polymer type. Biomimetic superhydrophobic rough surfaces of polystyrene and poly(l-lactic acid) with different micro/nanotopographies were obtained from smooth surfaces using a simple phase-separation based method. Total protein was quantified and showed a less adsorption of bovine serum albumin onto rough surfaces as compared to smooth surfaces of the same material. The mouse osteoblastic MC3T3-E1 cell line and primary bovine articular chondrocytes were used to study cell attachment and proliferation. Cells attached and proliferate better in the smooth surfaces. The superhydrophobic surfaces allowed cells to adhere but inhibited their proliferation. This study indicates that surface wettability, rather than polymer type or the topography of the superhydrophobic surfaces, is a critical factor in determining cell behavior.

Although regarded as a promising treatment for type 1 diabetes, clinical islet transplantation in the portal vein is still hindered by a low transplantation outcome. Alternative transplantation sites have been proposed, but the survival of extra-hepatically transplanted islets of Langerhans critically depends on quick revascularization after engraftment. This study aims at developing a new 3D scaffold platform that can actively boost vascularization and may find an application for extra-hepatic islet transplantation. The construct consists of a 3D ring-shaped polycaprolactone (PCL) scaffold with heparinized surface to electrostatically bind vascular endothelial growth factor (VEGF), surrounding a hydrogel core for islets encapsulation. Heparin immobilization improves the amount of VEGF retained by the construct, up to 3.6 fold, compared to untreated PCL scaffolds. In a chicken chorioallanthoic membrane model, VEGF immobilized on the construct enhances angiogenesis in close proximity and on the surface of the scaffolds. After 7 days, islets encapsulated in the alginate core show functional response to glucose stimuli comparable to free-floating islets. Thus, the developed platform has the potential to support rapid vascularization and islet endocrine function.

The structure of calcium-saturated alginate hydrogels has been studied by combining rheological determinations and relaxometry measurements. The mechanical spectroscopy analyses performed on alginate gel cylinders at different polysaccharide concentration allowed estimating their main structural features such as the average mesh size. The calculation was based on the introduction of a front factor in the classical rubber elasticity approach which was correlated to the average length of the Guluronic acid blocks along the polysaccharide chain. Transverse relaxation time (T(2)) determinations performed on the cylinders revealed the presence of two relaxation rates of the water entrapped within the hydrogel network. The cross-correlation of the latter data with the rheological measurements allowed estimating the mesh size distribution of the hydrogel network. The results obtained for the hydrogel cylinders were found to be consistent with the relaxometric analysis performed on the alginate microbeads where, however, only one type of water bound into the network structure was detected. A good correlation was found in the average mesh size determined by means of relaxometric measurements on alginate microbeads and by a statistical analysis performed on TEM micrographs. Finally, the addition of a solution containing calcium ions allowed investigating further the different water relaxation modes within alginate hydrogels.

Islets of Langerhans need to maintain their round morphology and to be fast revascularized after transplantation to preserve functional insulin secretion in response to glucose stimulation. For this purpose, a non-cell-adhesive environment is preferable for their embedding. Conversely, nutrient and oxygen supply to islets is guaranteed by capillary ingrowth within the construct and this can only be achieved in a matrix that provides adhesion cues for cells. In this study, two different approaches are explored, which are both based on a layered architecture, in order to combine these two opposite requirements. A non-adhesive islet encapsulation layer is based on polyethyleneglycole diacrylate (PEGDA). This first layer is combined with a second hydrogel based on thiolated-gelatin, thiolated-heparin and thiolated-hyaluronic acid providing cues for endothelial cell adhesion and acting as a growth factor releasing matrix. In an alternative approach, a conformal PEGDA coating is covalently applied on the surface of the islets. The coated islets are subsequently embedded in the previously mentioned hydrogel containing thiolated glycosaminoglycans. The suitability of this approach as a matrix for controlled growth factor release has been demonstrated by studying the controlled release of VEGF and bFGF for 14 days. Preliminary tube formation has been quantified on the growth factor loaded hydrogels. This approach should facilitate blood vessel ingrowth towards the embedded islets and maintain islet round morphology and functionality upon implantation.Graphical abstract