Citation: Dominici, S.; Kamranikia, K.; Mougin, K.; Spangenberg, A. Smart Nematic Liquid Crystal Polymers for Micromachining Advances. Micromachines 2023, 14, 124. https://doi.org/10.3390/ mi14010124 Academic Editor: Jianguo Guan Received: 5 December 2022 Revised: 22 December 2022 Accepted: 26 December 2022 Published: 1 January 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). micromachines Review Smart Nematic Liquid Crystal Polymers for Micromachining Advances Sébastien Dominici 1,2 , Keynaz Kamranikia 1,2 , Karine Mougin 1,2 and Arnaud Spangenberg 1,2, * 1 Institut de Science des Matériaux de Mulhouse (IS2M), CNRS–UMR 7361, Université de Haute-Alsace, 15 rue Jean Starcky, 68057 Mulhouse, France 2 Université de Strasbourg, 67000 Strasbourg, France * Correspondence: arnaud.spangenberg@uha.fr Abstract: The miniaturization of tools is an important step in human evolution to create faster devices as well as precise micromachines. Studies around this topic have allowed the creation of small-scale objects capable of a wide range of deformation to achieve complex tasks. Molecular arrangements have been investigated through liquid crystal polymer (LCP) to program such a movement. Smart polymers and hereby liquid crystal matrices are materials of interest for their easy structuration properties and their response to external stimuli. However, up until very recently, their employment at the microscale was mainly limited to 2D structuration. Among the numerous issues, one con- cerns the ability to 3D structure the material while controlling the molecular orientation during the polymerization process. This review aims to report recent efforts focused on the microstructuration of LCP, in particular those dealing with 3D microfabrication via two-photon polymerization (TPP). Indeed, the latter has revolutionized the production of 3D complex micro-objects and is nowadays recognized as the gold standard for 3D micro-printing. After a short introduction highlighting the interest in micromachines, some basic principles of liquid crystals are recalled from the molecular aspect to their implementation. Finally, the possibilities offered by TPP as well as the way to monitor the motion into the fabricated microrobots are highlighted. Keywords: nematic liquid crystal; two photons polymerization; micro-machines 1. Introduction Autonomous micro-actuators [1] have attracted the fascination of many researchers over the last few years for possible applications in the future. Micro-devices that can swim in various media [2–4], jump [5], crawl [6,7], or roll [8,9] on hard surfaces upon an adapted external stimulus spring from the imaginations of scientists for a world where injuries and diseases could be healed [10,11], oceans depolluted, or electric circuits could be even smaller. The inspiration for the larger part of the publications comes from nature itself reproducing the crawling of worms, opening of flowers [12,13], swimming of fishes, etc. Promising works have emerged in various fields such as drug delivery [14], in vitro fecundation [15], and robotics [16] where the materials were chosen to respond to a magnetic field [17], electric field [18], or even a light source [19]. Hard materials were commonly used but are not suitable for precise motion at the micron scale. Some materials such as polymers, along with hydrogels [20] or elastomers are soft and robust materials that can be tuned to be responsive and change their properties over time. Polymers’ biocompatibility offers a broad range of sensitive applications that can be coupled to biodegradability for their use in nature. They are generally low-cost materials but also easy to process and some are responsive to external stimuli. Finding the best material for micromachining design is a complex task where many directions are taken. Some favor pH activation [21] or magnetic activation, while others still favor light activation. Light activation is cheap and easy to use in a wide range of wavelengths making it a primary choice of the stimulus Micromachines 2023, 14, 124. https://doi.org/10.3390/mi14010124 https://www.mdpi.com/journal/micromachines