*Editorial* **Editorial for the Special Issue on 3D Printing of MEMS Technology**

**Andrea Ehrmann**

Faculty of Engineering Sciences and Mathematics, Bielefeld University of Applied Sciences and Arts, 33619 Bielefeld, Germany; andrea.ehrmann@hsbi.de

Microelectromechanical systems (MEMS) combine electrical and mechanical functions and are nowadays broadly applied in many technology fields, often as sensors or actors [1]. Opposite to microelectronics and micromechanics, standardization of the production processes is still relatively low. While this poses challenges for the development of new MEMS for specific applications, it also calls for the use of additive manufacturing with its high degree of freedom in design and the large chance of individualization to produce new MEMS [2].

Additive manufacturing, or 3D printing, belongs to the emerging technologies of our time. While previously they were mostly used for rapid prototyping, the technology has been in rapid production for a long time, especially for complicated objects or those with small lot sizes [3]. Most recently, new 3D printing technologies enable the printing of the smallest features on micro- or even nano-scales [4]. At the same time, well-known problems like the waviness of fused deposition modeling (FDM) printed parts, the missing long-term stability of some typical printing materials, or the reduced mechanical properties of 3Dprinted objects still exist and have to be investigated in detail to enable the optimization of these parameters [5].

This Special Issue focusses on all topics dealing with the 3D printing of microelectromechanical systems (MEMS), such as new or advanced features enabled by 3D printing compared to conventional technologies, but also the challenges which still exist of using 3D printing technologies for MEMS and new approaches for how to overcome them.

One of the challenges addressed by the papers in this Special Issue is dimensional accuracy and surface roughness, e.g., in PolyJet printing (Contribution 1) or powder bed fusion (Contribution 2). A high mixing efficiency (Contribution 3) was aimed to be created, as well as the creation of specific optical properties (Contribution 4). Amongst the investigated materials, polyvinyl alcohol (PVA) was investigated as a possible sacrificial material for MEMS devices (Contribution 5), as well as photo-sintered magnetic strontium ferrite samples (Contribution 6), polymethylmethacrylate (PMMA) microfluidic chips (Contribution 7), as well as the in situ annealing of semiconducting ZnO thin films (Contribution 8). Specific structures (Contribution 9) are investigated as well as sensors (Contribution 10), actuators (Contribution 11), and electronic devices (Contribution 12), partly combining an experiment and simulation (Contribution 13). Additionally, a review of the recent developments and applications of 3D-printed MEMSs technology is given (Contribution 14).

To conclude, the papers collected in this Special Issue provide an overview of different additive manufacturing methods, such as fused deposition modeling, photolithography, PolyJet, aerosol jet or inkjet, and materials including diverse polymers and metals. They describe the development of microfluidic and colorimetric devices, micro-resonators and micro-beams, sensors and actuators, and resistors and micro-heaters, as well as the corresponding challenges and proposed solutions. We hope that these papers will inspire more research in the highly topical research area of 3D printing MEMS.

**Conflicts of Interest:** The author declares no conflict of interest.

**Citation:** Ehrmann, A. Editorial for the Special Issue on 3D Printing of MEMS Technology. *Micromachines* **2023**, *14*, 2195. https://doi.org/ 10.3390/mi14122195

Received: 27 November 2023 Accepted: 29 November 2023 Published: 30 November 2023

**Copyright:** © 2023 by the author. 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/).

### **List of Contributions:**


### **References**


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**Sindhu Vijayan 1,2, Pravien Parthiban <sup>1</sup> and Michinao Hashimoto 1,2,\***


**Abstract:** PolyJet™ 3D printers have been widely used for the fabrication of microfluidic molds to replicate castable resins due to the ease to create microstructures with smooth surfaces. However, the microstructures fabricated by PolyJet printers do not accurately match with those defined by the computer-aided design (CAD) drawing. While the reflow and spreading of the resin before photopolymerization are known to increase the lateral dimension (width) of the printed structures, the influence of resin spreading on the vertical dimension (height) has not been fully investigated. In this work, we characterized the deviations in both lateral and vertical dimensions of the microstructures printed by PolyJet printers. The width of the printed structures was always larger than the designed width due to the spreading of resin. Importantly, the microstructures designed with narrow widths failed to reproduce the intended heights of the structures. Our study revealed that there existed a threshold width (*w<sup>d</sup>* 0 ) required to achieve the designed height, and the layer thickness (a parameter set by the printer) influenced the threshold width. The thresholds width to achieve the designed height was found to be 300, 300, and 500 µm for the print layer thicknesses of 16, 28, and 36 µm, respectively. We further developed two general mathematical models for the regions above and below this threshold width. Our models represented the experimental data with an accuracy of more than 96% for the two different regions. We validated our models against the experimental data and the maximum deviation was found to be <4.5%. Our experimental findings and model framework should be useful for the design and fabrication of microstructures using PolyJet printers, which can be replicated to form microfluidic devices.

**Keywords:** microfluidics; PolyJet 3D printing; fidelity of 3D printing
