Development of photoswitchable ring-opening polymerization for 3D printing of synthetic polypeptides

A highly modular platform, origin from natural building blocks and unique properties give synthetic polypeptides promising potential for various biomedical applications. Ring-opening polymerization (ROP) of N-carboxyanhydrides (NCAs) provides a simple and expedient route for the preparation of high molecular weight polypeptides derived from natural and/or non-natural amino acids. As biocompatible and biodegradable materials, synthetic polypeptides are used for drug or gene delivery and tissue engineering. Recently, three-dimensional (3D) printing has emerged as a promising technique for producing scaffolds with good control over spatial chemistry for patient-specific requirements. In this process, the material is selectively deposited to form the object (extrusion and jetting), or the liquid photopolymer is selectively cured by light-activated polymerization, followed by removal of the unreacted liquid (vat photopolymerization). Polypeptides as feedstock for 3D printing applications have been poorly explored and there are few recent examples for inkjet and extrusion printing. However, inkjet and extrusion printing applications are limited to polypeptides with specific chemical compositions to produce polypeptide inks with suitable rheological behavior. For biomedical applications, a greater variety of polypeptides would be beneficial, as the diversity of chemical structures of amino acids would allow optimization of hydrophilicity and mechanical properties, as well as enhancement of cell adhesion, proliferation and differentiation. This could be achieved by vat photopolymerization, which has not yet been explored as the field of photoinduced ROP of NCAs is still in its infancy and lacks a fast-curing polymerization system, which is crucial for 3D printing by vat photopolymerization. Polymerization should only occur in the illuminated region, but the active species formed thereupon may diffuse into the dark region and initiate polymerization there as well, deteriorating spatial control and reducing printing resolution. Therefore, a photo ON switch that promotes fast polymerization kinetics is desirable for 3D printing, as well as the ability to switch the polymerization OFF. The objective of the proposed project is to develop a novel system for ON-OFF photoswitchable ROP of NCAs to produce synthetic polypeptides. We aim to achieve (i) temporal control over fast photoinduced ROP (on-demand controlling when the polymerization starts, stops and resumes, using light as a trigger), and (ii) spatial control by selective termination of ROP (controlling where polymerization occurs), which will allow the preparation of cross-linked polypeptide objects with predefined complex shapes without the use of molds. The results of the project will contribute to the underdeveloped field of NCA photopolymerization and provide the fundamental chemistry needed for the integration of synthetic polypeptides and 3D printing via vat photopolymerization, which is obviously a promising rapid prototyping technology for the production of intricate bioinspired scaffolds. In this project, we propose an acid-base system that allows photoinitiation and photoinhibition, thereby switching the ROP of NCAs ON and OFF. For this purpose, the reactive mixture will contain the NCA monomer, photobase and photoacid, which are inactive before illumination. Illumination of the mixture with the first wavelength will trigger the photobase to release a tertiary amine (ON switch). During the ROP, the photoacid can be triggered at any time and at any place by the light of the second wavelength. Thereupon, the released acid will protonate the active species to prevent further polymerization (OFF switch). The proposed project will pave the way for the development of a new generation of 3D-printed polypeptide scaffolds that can be used for patient-specific tissue engineering purposes.