ICE | The Israel Chemist and Chemical Engineer | Issue 8

10 Scientific Article The Israel Chemist and Chemical Engineer Issue 8 · November 2021 · Kislev 5782 to the focal point, and 2D/3D structures are produced inside the liquid resin bulk (Figure 2e). In this non-linear process with threshold effect, the laser energy can be tuned to produce sub-diffraction-limit features. Multi-photon and single photon polymerizations require similar components (see Section 3.1): building blocks and photo-sensitizers/initiators. Multi-photon reduction requires electron acceptors (metal ions) and photo-reducing agents. The reduction process was suggested to form NPs and structures of metal by nucleation, growth, and aggregation. Commercial units are available for high-resolution polymeric 3D printing (e.g., by Nanoscribe and Microlight3D) and are used for various applications. 3.3. Thermally driven reactions When heat is generated by absorption of the laser, there is a greater probability that the reaction activation barrier will be overcome and electron transfer promoted (Figure 2f). It was suggested by Lachish-Zalait et al. [16] that initiation of the deposition process requires adsorption to the substrate of very small amounts of precipitate, which absorb laser radiation and undergo thermal decomposition. Metals as well as oxidized metals are produced from precursor solutions that consist mostly of metal ions, which have an activation energy that is higher than that of the photons. The reaction is sometimes faster than expected due to the local rise in temperature, explained by the gradient in temperature between the spot of the laser and the medium; the resultant convection flows provide a steady supply of precursors. Minimal diameters of microstructures (circular deposition) and linewidths are ~0.7–500 µm. Recent studies suggested a major role for microbubbles [17,18] attributed to gradients that lead (see Section 2.1.2) to convection currents; in combination with capillary flow and liquid evaporation around the microbubble. These currents were thought to increase ion concentration and even lead to supersaturation. Due to the higher temperatures and concentrations, material is deposited around the threephase (liquid/gas/solid) contact interface. Greenberg et al. [17] showed deposition at the interface of NPs that were formed in liquid (along with materials that were synthesized locally). Laser modulation prevents pinning of the bubble to the deposited material and improves control over its size. 4. Discussion Due to the similarity between the various methods, the differences in underlying mechanism, operation, and characteristics of the printed material can easily be missed. There is no “perfect” method. Figure 3 reveals that certain applications require a specific method. For example optical forces, while inferior in terms of speed, are the only method that allows particles to be placed in close proximity (60 nm apart) and living cells to be manipulated. Reactions of single photons offer the greatest speed but are limited to printing of polymers. Methods that enable microfabrication of various materials are advantageous, as flexibility in material choice allows better compatibility for diverse applications. Some methods demonstrated deposition of various materials. Directed assembly of preformedmetals, polymers and organic materials can be expanded to deposit NP dispersions of other materials. Another advantage of such deposition is exceptional control over size and shape of particles forming the microstructure, as specific properties can be chosen. Local synthesis is obviously not applicable to living cells. Various applications benefit from minimal feature size. Photo-thermal, micro-bubble assisted, multi photon and thermally driven printing all achieve sub-micron feature size, while single-photon reactions are limited to microns. While diffraction is considered a limiting barrier, features with a smaller size of tens of nm were achieved by reactions with multiple photons due to non-linear processes. The size range provided by one laser passage is significant for applications that require a feature size much larger than the minimal value. Repeated printing with a small feature size not only requires significant time but may also provide subpar quality due to boundaries between adjacent features. Diverse feature size is also required for connection of macro-sized objects to micro and sub-microstructures. Figure 3. (a) Various aspects of laser-based printing from liquids – green (excellent), yellow (mediocre), red (poor). (b) Printing methods applicable for various material families and (c) linewidths. (d) Abbreviations. Reproduced with permission [4]. Copyright 2021, Wiley-VCH.