Complex structures, achieved by application of free-form fabrication methods using light-induced cross-linking of polymers. (A and B) Scanning electron micrographs of branched tubular structure, generated by two-photon polymerization of photo-polymerizable α,ω-polytetrahydrofuranether-diacrylate (PTHF-DA) resins. The height of the tubular structures is ∼160 μm, where the inner diameter and wall thickness are ∼18 and 3 μm, respectively. (Reproduced from Meyer et al., [49] with the permission of the authors.) (C and D) SEM images of a highly porous CAD–CAM structure out of a cytocompatible photopolymer (from [75]), fabricated by microstereolithography applying a UV laser. (Reproduced from Baudis et al. [38] with the permission of IOP Publishing and the authors.) In both cases, the materials proved to be cytocompatible, but do not yet meet the required physicochemical properties for artificial blood vessels. (E) Graphical illustration of sophisticated material design. Top: Commonly used hydrogels based on polyethylene glycol diacrylates (PEGDAs) with a high cross-link density along the polymer backbone are sensitive to cracks as they cannot dissipate energy by inter-chain gliding modes. Middle: The addition of reactive diluents based on methacrylate loosens the network as the cross-link density is reduced. Bottom: The application of chain transfer agents was tested in order to change the polymer architecture and to provide relief to the brittle material properties that seem to be an intrinsic property of the poly(acrylate) back bone. (F) Tubular structures fabricated from the new formulations based on (E) using digital light processing (DLP). (C and D) reproduced from Baudis et al. [38] with the permission of IOP Publishing and the authors). MA: monoacrylate; CTA: chain transfer agent.