# Design of Antibody-Functionalized Polymeric Membranes for the Immunoisolation of Pancreatic Islets
## 1 Introduction  
Type 1 diabetes is a worldwide debilitating autoimmune disease, in which \(\beta\) cells within pancreatic islets are selectively destroyed by autoimmune responses against \(\beta\) cells [1]. Among available therapies, islet transplantation can treat the most severe cases of type 1 diabetes, although the shortage of isletdonors and the requirement of a chronic immunosuppression to prevent rejection and recurrence of autoimmunity limit its applicability [2]. In this context, tissue engineering and regenerative medicine offer new strategies to circumvent these problems by properly modifying the graft before transplantation, in order to achieve immunoexclusion and, thus, limit the need for immunosuppressive treatments [3; 4; 5]. Modifications are focused on improving viability, functionality, and protection of the graft through the creation of a suitable microenvironment for the implant, thus, favoring the engraftment of the islets [6; 7]. Immuno encapsulation is usually achieved by means of approaches that permit exchange of oxygen, nutrients, insulin, and waste, and, at the same time, protect those islets from the immune response [3; 8]. The employment of thin polymeric layers reduces the size of the coating and the invasiveness of the process with respect to other approaches, such as the development of macrodevices and the microencapsulation of islet cells [3; 9; 10]. Indeed, the main drawbacks of these approaches are related to the size of the coating (500 to \(>\)1000 \(\upmu\)m) that limits oxygen diffusion to the core of the capsule, thus favoring the survival of the islets that are closed to the surface [11; 12]. On the other hand, conformal coating facilitates oxygen and nutrient diffusion to support islet survival and function after implantation, and enables a physiologic release of insulin into the bloodstream compared to microcapsule [13; 14]. Among conformal coating studies, the use of polyethylene glycol (PEG) hydrogels obtained by photopolymerization for islet immunoisolation showed great promise in rodents [15; 16; 17]. Indeed, PEG hydrogels display soft tissue-like properties, non-immunogenicity, the intrinsic resistance to protein adsorption, and the suitability for being covalently linked to multiple functional moieties [18; 19]. However, one of the fundamental deficiencies with current encapsulation technology is the evidence that passive physical barriers, while preventing T-cell infiltration and cell contact-mediated destruction of grafted islets, cannot protect islets from exposure to local cytokines and other small, diffusible cytotoxic molecules produced by activated immune cells [20; 21]. To this aim, PEG was exploited as a substrate to conjugate immunomodulatory antibodies or bioactive molecules, potentially able to enhance the immunoprecipitation activity and drive targeted immunosuppression. Indeed, we propose a PEG-antibody functionalization protocol that preserves the antigen-recognition site, namely a site-directed functionalization. Non-specific physical adsorption or chemical crosslinking results in the immobilization of the biomolecules in a random orientation. Site-specific covalent attachment, on the other hand, leads to molecules being arranged in a definite and oriented way [22]. The proposed functionalization strategy exploits the selective, oriented, and reversible immobilization of a recombinant protein (that is the antigen) bearing a polyhistidine-tag (His-tag) to a transition metal terminated nitrilotriacetate group derivatized support [23; 24]. Then, the site-oriented immobilization of the antibody to the antigen enables the functionalization of the constant region of the antibody with PEG, followed by the recovery of the antibody-PEG complex upon breaking of the antigen-antibody bond. Therefore, the recovered PEG-functionalized antibody displays the binding site, being available to exert its bio-protection activity upon coating of the pancreatic islets.