A definition of bioinks and their distinction from biomaterial inks

Biofabrication aims to fabricate biologically functional products through bioprinting or bioassembly (Groll et al 2016 Biofabrication 8 013001). In biofabrication processes, cells are positioned at defined coordinates in three-dimensional space using automated and computer controlled techniques (Moroni et al 2018 Trends Biotechnol. 36 384-402), usually with the aid of biomaterials that are either (i) directly processed with the cells as suspensions/dispersions, (ii) deposited simultaneously in a separate printing process, or (iii) used as a transient support material. Materials that are suited for biofabrication are often referred to as bioinks and have become an important area of research within the field. In view of this special issue on bioinks, we aim herein to briefly summarize the historic evolution of this term within the field of biofabrication. Furthermore, we propose a simple but general definition of bioinks, and clarify its distinction from biomaterial inks.

Three-dimensional laser micro- and nano-structuring of acrylated poly(ethylene glycol) materials and evaluation of their cytoxicity for tissue engineering applications

The natural cell environment is characterized by complex three-dimensional structures, which contain features at multiple length scales. Many in vitro studies of cell behavior in three dimensions rely on the availability of artificial scaffolds with controlled three-dimensional topologies. In this paper, we demonstrate fabrication of three-dimensional scaffolds for tissue engineering out of poly(ethylene glycol) diacrylate (PEGda) materials by means of two-photon polymerization (2PP). This laser nanostructuring approach offers unique possibilities for rapid manufacturing of three-dimensional structures with arbitrary geometries. The spatial resolution dependence on the applied irradiation parameters is investigated for two PEGda formulations, which are characterized by molecular weights of 302 and 742. We demonstrate that minimum feature sizes of 200nm are obtained in both materials. In addition, an extensive study of the cytotoxicity of the material formulations with respect to photoinitiator type and photoinitiator concentration is undertaken. Aqueous extracts from photopolymerized PEGda samples indicate the presence of water-soluble molecules, which are toxic to fibroblasts. It is shown that sample aging in aqueous medium reduces the cytotoxicity of these extracts; this mechanism provides a route for biomedical applications of structures generated by 2PP microfabrication and photopolymerization technologies in general. Finally, a fully biocompatible combination of PEGda and a photoinitiator is identified. Fabrication of reproducible scaffold structures is very important for systematic investigation of cellular processes in three dimensions and for better understanding of in vitro tissue formation. The results of this work suggest that 2PP may be used to polymerize poly(ethylene glycol)-based materials into three-dimensional structures with well-defined geometries that mimic the physical and biological properties of native cell environments.

Pulsed laser deposition of antimicrobial silver coating on Ormocer microneedles

One promising option for transdermal delivery of protein- and nucleic acid-based pharmacologic agents involves the use of microneedles. However, microneedle-generated pores may allow microorganisms to penetrate the stratum corneum layer of the epidermis and cause local or systemic infection. In this study, microneedles with antimicrobial functionality were fabricated using two-photon polymerization-micromolding and pulsed laser deposition.The antibacterial activity of the silver-coated organically modified ceramic (Ormocer)microneedles was demonstrated using an agar diffusion assay. Human epidermal keratinocyte viability on the Ormocer surfaces coated with silver was similar to that on uncoated Ormocer surfaces. This study indicates that coating microneedles with silver thin films using pulsed laser deposition is a useful and novel approach for creating microneedles with antimicrobial functionality.

Two photon induced polymerization of organic-inorganic hybrid biomaterials for microstructured medical devices

Three-dimensional microstructured medical devices, including microneedles and tissue engineering scaffolds, were fabricated by two photon induced polymerization of Ormocer organic-inorganic hybrid materials. Femtosecond laser pulses from a titanium:sapphire laser were used to break chemical bonds on Irgacure 369 photoinitiator within a small focal volume. The radicalized starter molecules reacted with Ormocer US-S4 monomers to create radicalized polymolecules. The desired structures are fabricated by moving the laser focus in three dimensions using a galvano-scanner and a micropositioning system. Ormocer surfaces fabricated using two photon induced polymerization demonstrated acceptable cell viability and cell growth profiles against B35 neuroblast-like cells and HT1080 epithelial-like cells. Lego-like interlocking tissue engineering scaffolds and microneedle arrays with unique geometries were created using two photon induced polymerization. These results suggest that two photon induced polymerization is able to create medical microdevices with a larger range of sizes, shapes, and materials than chemical isotropic etching, injection molding, reactive ion etching, surface micromachining, bulk micromachining, polysilicon micromolding, lithography-electroforming-replication, or other conventional microfabrication techniques.