Martin Schwentenwein

Toughening of photo-curable polymer networks: a review

Photo-curable resins based on multifunctional acrylate monomers are commonly applied as thin films (e.g. protective coatings, printing inks, etc.) and in recent years are also used for the fabrication of bulk objects such as dental fillings and 3D-printed parts. While rapid curing and good spatial resolution are advantages to these systems, brittleness and poor impact resistance due to inhomogeneous polymer architecture and high crosslink density are serious drawbacks. By comparison, epoxy thermoset resins suffered many years ago from similar problems, but since then are found in ever demanding applications thanks to a variety of approaches to increase polymer toughness. Based on these successes, researchers have tried to translate strategies for toughening epoxy resins to photopolymer networks. Therefore, this review surveys relevant scientific papers and patents on the development of crosslinked epoxy-based polymers and also photo-curable polymers based on multifunctional acrylates with improved toughness. Strategies developed to reduce brittleness include working with monomers, which intrinsically give tougher polymers, particulate additives, and alternate forms of polymerization and polymer architecture (e.g., dual-cure networks, interpenetrating networks, thiol–ene chemistry). All of these strategies have advantages and yet application specific rigours must also be considered before and during formulation development.

Lithography-based ceramic manufacture (LCM) of auxetic structures: present capabilities and challenges

Auxetic metamaterials are known for having a negative Poissons ratio (NPR) and for displaying the unexpected properties of lateral expansion when stretched and densification when compressed. Even though a wide set of micro-manufacturing resources have been used for the development of auxetic metamaterials and related devices, additional precision and an extension to other families of materials is needed for their industrial expansion. In addition, their manufacture using ceramic materials is still challenging. In this study we present a very promising approach for the development of auxetic metamaterials and devices based on the use of lithography-based ceramic manufacturing. The process stands out for its precision and complex three-dimensional geometries attainable, without the need of supporting structures, and for enabling the manufacture of ceramic auxetics with their geometry controlled from the design stage with micrometric precision. To our knowledge it represents the first example of application of this technology to the manufacture of auxetic geometries using ceramic materials. We have used a special three-dimensional auxetic design whose remarkable NPR has been previously highlighted.

Lithography-Based Ceramic Manufacturing: A Novel Technique for Additive Manufacturing of High-Performance Ceramics

Albeit widely established in plastic and metal industry, additive manufacturing technologies are still a rare sight in the field of ceramic manufacturing. This is mainly due to the requirements for high performance ceramic parts, which no additive manufacturing process was able to meet to date.The Lithography-based Ceramic Manufacturing (LCM)-technology which enables the production of dense and precise ceramic parts by using a photocurable ceramic suspension that is hardened via a photolithographic process. This new technology not only provides very high accuracy, it also reaches high densities for the sintered parts. In the case of alumina a relative density of over 99.4 % and a 4-point-bending strength of almost 430 MPa were realized. Thus, the achievable properties are similar to conventional manufacturing methods, making the LCM-technology an interesting complement for the ceramic industry.

Tissue Engineering Scaffolds for Bone Repair: General Aspects

Hard tissue repair is a very relevant and challenging area for the emerging fields of tissue engineering and biofabrication due to the very complex three-dimensional structure of bones, which typically include important variations of porosities and related mechanical properties. The need of porous and rigid extra cellular matrices, of structural integrity, of functional gradients of mechanical properties and density, among other requirements, has led to the development of several families of biomaterials and scaffolds for the repair and regeneration of hard tissues, although a perfect solution has not yet been found. Further research is needed to address the advantages of different technologies and materials for manufacturing enhanced, even personalized, scaffolds for tissue engineering studies and extra cellular matrices with outer geometries defined as implants for tissue repair, as the niche composition and 3D structure play an important role in stem cells state and fate. The combined employment of computer-aided design, engineering and manufacturing (also CAD-CAE-CAM) resources, together with rapid prototyping procedures, working on the basis of additive manufacturing approaches, allows for the efficient development of knowledge-based functionally graded scaffolds for hard tissue repair in a wide range of materials and following biomimetic approaches. In this chapter we present some design and manufacturing strategies for the development of knowledge-based functionally graded tissue engineering scaffolds aimed at hard tissue repair. A complete case of study, linked to the development of a scaffold for tibial repair is also detailed to illustrate the proposed strategies.

Poly(vinyl alcohol) based monomers for lithography-based 3D fabrication

Lithography based Solid Freeform Fabrication (SFF) methods (e.g. stereolithography allow the fabrication of cellular structures with defined pore sizes and geometries. Achievable wall thicknesses range down to 50 μm.

Additive Manufacturing Technologies for the 3D Fabrication of Biocompatible and Biodegradable Photopolymers

High molecular weight vinyl esters and carbonates based on oligo(ethylene glycol), oligomeric fatty acids and poly(hexamethylene carbonate), as alternatives for potentially cytotoxic acrylate based monomers have been structured by Additive Manufacturing Technologies (AMTs) like Microstereolithography (μ-SLA), Digital Light Processing (DLP) and Two-Photon Induced Photopolymerization (TPIP). With these techniques feature resolutions down to 10 μm (μ-SLA and DLP) or even 200 nm (TPIP) can be obtained. This new class of monomers exhibits LC 50 values for cytotoxicity up to two orders of magnitude lower than acrylate references. Beside a high reactivity of the resin, the shrinkage and the mechanical properties of the final part material are another essential parameter. Low molecular weight monomers are very reactive and lead to densely cross-linked materials which suffer from high shrinkage and strains within the cured material. Therefore, mixtures of high molecular weight vinyl esters/carbonates with low molecular weight crosslinkers have been evaluated regarding their photoreactivity and mechanical properties.

New 3D-Biophotopolymers with Selective Surface-cell Interactions for Regenerative Medicine

Additive Manufacturing Technologies (AMTs) have become an appealing method for the fabrication of 3D cellular scaffolds for tissue engineering and regenerative medicine. To circumvent the use of (meth)acrylate based photopolymers, that suffer from skin irritation and sometimes cytotoxicity, new monomers based on vinyl esters, carbonates and carbamates were prepared. The new materials, giving poly(vinyl alcohol) upon hydrolysis, showed similar results compared to (meth)acrylate references concerning the photoreactivity and mechanical properties, yet being significantly less cytotoxic. To study the kinetics of hydrolytic degradation, the influence of the different polymerizable groups was investigated by hydrolysis of model compounds under alkaline conditions. We were able to show that the ester moiety of a vinyl ester based polymer could be used to immobilize alkaline phosphatase, therefore they exhibit the ability to immobilize enzymes for selective cell adhesion. Finally, 3D test structures by AMT techniques could be fabricated and in-vivo testing thereof proofed the biocompatibility of vinyl ester-based scaffolds.