7 Scientific Article The Israel Chemist and Chemical Engineer Issue 8 · November 2021 · Kislev 5782 force leading to material deposition, and mechanisms are explained. Table 1 briefly summarizes various methods in terms of the mechanism and key aspects. Printing method Mechanism Materials Maximum speed Feature size Comments Photothermal Local heating by laser – temperature gradient: I) ion separation – local electric field, supports movement of charged particles towards heated area and/or II) particles carried by liquid (bulk) motion towards focal point. Metals, polymers, organic molecules ~1 µm/s 300 nm – 200 μm Reconfigurable printing was demonstrated. Microbubble assisted Local heating increases the vapor pressure until a micro-bubble is formed. Convective flows, capillary forces carry particles towards base of micro-bubble where some are pinned. Metals, polymers, organic molecules 10 mm/s ~510 nm – 50 µm I) Laser modulation forms continuous patterns. II) 3D particle-covered hollow spherical structures were shown. Optical forces Optical forces due to photon momentum conservation (particle >> λ) or electrostatics (size << λ) used to either: I) optically trap and deposit materials at desired locations on the substrate, or II) push toward the substrate (scattering force). Metals, polymers, organic molecules, living cells particle every 5 s Individual objects (mostly) deposited selectively nanometers to micrometers I) Minimal inter-particle distance – 60-150 nm. II) Fixation by gel and electrophoresis. Single photon reactions Polymerization by photons with enough energy to excite electron from ground to higher state close to liquid surface (poor selectivity in z-direction). Layered approach forms 3D polymeric microstructures. Photocurable resins 4000 mm/s 5–70 µm Available commercially. Multi photon reactions Non-linear process – at least two photons required to excite electron, promoting chemical reaction. High energy pulsed lasers polymerize/reduce metal ions. Laser energy tuned to produce sub-diffractionlimit features. 3D microstructures by multi-photon reactions (freely moving focal point). Photocurable resins, metals 0.9 mm/s 65 nm – 5 µm Commercially available. Thermally driven reactions Occur upon heating due to laser light absorption, increasing probability to overcome activation barrier and promote electron transfer. Oxides, metals, polymers, organic molecules, alloys, compounds (molecular) 10 mm/s ~0.7–500 µm Multi layered (2.5D) and simple 3D microstructures were demonstrated. Table 1. Brief comparison of printing methods. Adapted with permission . Copyright 2021, Wiley-VCH.