Ultrafast laser 3D lithography based on non-linear light–matter interactions, widely known as multi-photon lithography (MPL), offers unrivaled precision rapid prototyping and flexible additive manufacturing options. 3D printing equipment based on MPL is already commercially available, yet there is still no comprehensive understanding of factors determining spatial resolution, accuracy, fabrication throughput, repeatability, and standardized metrology methods for the accurate characterization of the produced 3D objects and their functionalities. The photoexcitation mechanisms, spatial-control or photo-modified volumes, and the variety of processable materials are topics actively investigated. The complexity of the research field is underlined by a limited understanding and fragmented knowledge of light-excitation and material response. Research to date has only provided case-specific findings on photoexcitation, chemical modification, and material characterization of the experimental data. In this review, we aim to provide a consistent and comprehensive summary of the existing literature on photopolymerization mechanisms under highly confined spatial and temporal conditions, where, besides the excitation and cross-linking, parameters such as diffusion, temperature accumulation, and the finite amount of monomer molecules start to become of critical importance. Key parameters such as photoexcitation, polymerization kinetics, and the properties of the additively manufactured materials at the nanoscale in 3D are examined, whereas, the perspectives for future research and as well as emerging applications are outlined.

In this study, acrylated epoxidized soybean oil (AESO) and mixtures of AESO and vanillin dimethacrylate (VDM) or vanillin diacrylate (VDA) were investigated as photosensitive resins for optical 3D printing without any photoinitiator and solvent. The study of photocross-linking kinetics by real-time photorheometry revealed the higher rate of photocross-linking of pure AESO than that of AESO with VDM or VDA. Through the higher yield of the insoluble fraction, better thermal and mechanical properties were obtained for the pure AESO polymer. Here, for the first time, we validate that pure AESO and mixtures of AESO and VDM can be used for 3D microstructuring by employing direct laser writing lithography technique. The smallest achieved spatial features are 1 µm with a throughput in 6900 voxels per second is obtained. The plant-derived resins were laser polymerized using ultrashort pulses by multiphoton absorption and avalanche induced cross-linking without the usage of any photoinitiator. This advances the light-based additive manufacturing towards the 3D processing of pure cross-linkable renewable materials.

Materials obtained from renewable sources are emerging to replace the starting materials of petroleumderived plastics. They offer easy processing, fulfill technological, functional and durability requirements at the same time ensuring increased bio-compatibility, recycling, and eventually lower cost. On the other hand, optical 3D printing (O3DP) is a rapid prototyping tool (and an additive manufacturing technique) being developed as a choice for efficient and low waste production method, yet currently associated with mainly petroleum-derived resins. Here we employ a single bio-based resin derived from soy beans, suitable for O3DP in the scales from nano-to macro-dimensions, which can be processed even without the addition of photoinitiator. The approach is validated using both state-of-the art laser nanolithography setup as well as a widespread table-top 3D printer-sub-micrometer accuracy 3D objects are fabricated reproducibly. Additionally, chess-like figures are made in an industrial line commercially delivering small batch production services. Such concept is believed to make a breakthrough in rapid prototyping by switching the focus of O3DP to bio-based resins instead of being restricted to conventional petroleum-derived photopolymers. Bio-based polymers are emerging as replacement for petroleum-derived polymers. The growth of the production and market is 2.05 Mtons of bio-plastics 1 and 700 bilion Euros in Europe only 2. The main advantages of bio-based plastic products compared to the conventional plastics are the preservation of fossil resources by using bio-mass which is a renewable resource and provision of the unique potential of carbon neutrality, as well as bio-degradability of the certain types of bio-plastics which offers additional means of recovery at the end of a product's life 3. The spectrum of bio-based plastics usage varies from nanocomposites 4-8 and films 9-11 to adsorbents 12-14. Vegetable oils are potential starting materials for the preparation of polymers due to their ready availability, inherent bio-degradability, negligible toxicity, and existence of modifiable functional groups 15. Nowadays there are a lot of scientific research dedicated to the light induced polymerization. As there exist diverse technical implementations of this technology, it is known in many names: lithography (stereolithography, digital light processing (DLP)/projection lithography), direct laser writing (DLW) or alternatively laser direct writing (LDW), two-photon polymerization (2PP), nonlinear lithography (NLL), multi-photon lithography (MPL), etc. However, this additive manufacturing process simply can be called by one common name: optical 3D printing (O3DP) as it is based on photons. This rapid prototyping tool is being developed as a choice for efficient and low waste production tool, yet currently associated with mainly petroleum-derived resins 16-19. On the other hand, O3DP in combination with post-processing (thermal-treatment) allows fabrication of free-form structures which can serve as 3D templates f...

Green chemistry and green engineering concepts have been combined to develop novel sustainable polymeric materials. Solvent free photocurable acrylate resins with biorenewable carbon content of 75%–82% suitable for application in DLP 3D printing technology were composed by commercially available bio‐based materials, acrylated epoxidized soybean oil (AESO), isobornyl methacrylate (IBOMA), methacrylic ester (ME), tetrahydrofurfuryl acrylate (THFA), and tetrahydrofurfuryl methacrylate (THFMA). They demonstrated high printing accuracy and good adhesion between layers. The monitoring of photocross‐linking kinetics of high biorenewable content acrylate photoresins by the real‐time photorheometry and analysis of their rheological parameters was carried out. Synthesized polymers exhibited high yield of insoluble fraction and thermal decomposition temperature at the weight loss of 10% above 300°C. Polymers AESO/IBOMA and AESO/THFMA showed the highest values of tensile modulus and tensile strength. Biodegradability of the synthesized polymers AESO/ME, AESO/THFA, and AESO/THFMA was investigated by measuring oxygen consumption in a closed respirometer. Such AESO‐based polymers can be a competitive solution to replace petroleum‐derived polymeric materials in additive manufacturing and reduce the environmental impact.

Photocross linking of the resins composed of plant‐derived monomers, acrylated epoxidized soybean oil (AESO), myrcene (MYR) and vanillin dimethacrylate (VDM) or divinylbenzene (DVB, for comparison), was performed using 2,2‐dimethoxy‐2‐phenylacetophenone as photoinitiator. Photocross‐linking rate and properties of the crosslinked polymers depended on the resin compositions. The higher amount of MYR caused not only the better homogenization and lower viscosity of the resin but also the reduction of polymerization rate and the worse mechanical and thermal properties of the resulting polymers. The higher amount of aromatic component (VDM or DVB) improved mechanical and thermal properties of polymers. Moreover, the use of VDM instead of DVB in the system led to the higher photocross‐linking rate and higher yield of insoluble fraction. The resin composed of only plant‐derived monomers AESO/MYR/VDM, molar ratio 1:1:3, showed characteristics comparable to those of commercial petroleum‐derived photoresins and was selected as a potential renewable photoresin for application in optical 3D printing. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019, 137, 48708.