{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,9,4]],"date-time":"2026-09-04T15:29:30Z","timestamp":1788535770063,"version":"build-2803163510"},"reference-count":34,"publisher":"MDPI AG","issue":"9","license":[{"start":{"date-parts":[[2026,9,4]],"date-time":"2026-09-04T00:00:00Z","timestamp":1788480000000},"content-version":"vor","delay-in-days":0,"URL":"https:\/\/creativecommons.org\/licenses\/by\/4.0\/"}],"content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":["Dent. J.","Dentistry Journal"],"abstract":"<jats:p>Background\/Objectives: Directly printed orthodontic appliances rely mostly on vat-photopolymerisation resins; pre-polymerised thermoplastics processed by fused deposition modelling (FDM) and selective laser sintering (SLS) are a less-explored, single-component route, but are poorly characterised. This study compared six thermoplastic material\u2013process systems, supplied for biomedical or dental use, using a single intraoral-simulating protocol. Methods: Thin specimens (25 \u00d7 5 \u00d7 0.6 mm) of polyethylene terephthalate glycol (PETG), polycarbonate (PC), polyamide (PA), polyether-ether-ketone (PEEK) and a thermoplastic copolyester elastomer (TPC) printed by FDM, and of polyamide 12 (PA12) by SLS, underwent three-point bending and 10 min stress relaxation at 37 \u00b0C in artificial saliva (n = 5). Differences were assessed using the Kruskal\u2013Wallis test with Dunn post hoc comparisons (Holm correction), and Fourier-transform infrared (FTIR) spectroscopy was used to compare virgin and printed materials for the five FDM polymers. Results: Apparent flexural moduli were 989 (PA12), 750 (PC), 687 (PEEK), 509 (PETG), 85 (PA) and 32 MPa (TPC), a very large material effect (\u03b52 = 0.95), with post hoc comparisons separating only the extremes of that range. Relaxation over 10 min ranged from 2.6% (PC) to 22.1% (PA12) among the five materials in which it could be quantified, with TPC falling below the load-cell resolution; expressed as force, PA12 lost the most (0.86 N) and yet still delivered 3.01 N, which was above the initial force of PEEK, PETG, PA and TPC, though not separable from that of PC, which retained 97.4% of its own. Virgin and printed spectra were concordant (Pearson r \u2265 0.986, against \u2264 0.80 between different materials). Conclusions: The PA12\/SLS system was the stiffest, followed by PC and PEEK, but, as the only laser-sintered material, its polymer and process effects cannot be separated. PETG and PC combine useful stiffness with accessible FDM; PA and TPC are far more compliant. These specimen-level findings provide a screening basis for selecting material\u2013process systems for printed orthodontic appliances, preceding device-level validation.<\/jats:p>","DOI":"10.3390\/dj14090565","type":"journal-article","created":{"date-parts":[[2026,9,4]],"date-time":"2026-09-04T15:11:04Z","timestamp":1788534664000},"page":"565","update-policy":"https:\/\/doi.org\/10.3390\/mdpi_crossmark_policy","source":"Crossref","is-referenced-by-count":0,"title":["Flexural Stiffness and Force Retention of Six 3D-Printed Thermoplastics for Orthodontic Appliances: An In Vitro Screening Study"],"prefix":"10.3390","volume":"14","author":[{"ORCID":"https:\/\/orcid.org\/0000-0003-4680-0989","authenticated-orcid":false,"given":"Marco","family":"Serafin","sequence":"first","affiliation":[{"name":"Department of Biomedical Sciences for Health, University of Milan, 20133 Milan, Italy"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0009-0005-0723-6424","authenticated-orcid":false,"given":"Marina","family":"Borgese","sequence":"additional","affiliation":[{"name":"Department of Medicine and Innovative Technology, University of Insubria, 21100 Varese, Italy"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0001-5078-7284","authenticated-orcid":false,"given":"Elisa","family":"Boccalari","sequence":"additional","affiliation":[{"name":"Department of Biomedical Sciences for Health, University of Milan, 20133 Milan, Italy"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0001-5530-3939","authenticated-orcid":false,"given":"Gilberto","family":"Binda","sequence":"additional","affiliation":[{"name":"Department of Theoretical and Applied Sciences, University of Insubria, 21100 Varese, Italy"},{"name":"Norwegian Institute for Water Research (NIVA), 0579 Oslo, Norway"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0001-6322-1845","authenticated-orcid":false,"given":"Mario","family":"Raspanti","sequence":"additional","affiliation":[{"name":"Department of Medicine and Innovative Technology, University of Insubria, 21100 Varese, Italy"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0001-9646-4958","authenticated-orcid":false,"given":"Piero Antonio","family":"Zecca","sequence":"additional","affiliation":[{"name":"Department of Medicine and Innovative Technology, University of Insubria, 21100 Varese, Italy"}],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"1968","published-online":{"date-parts":[[2026,9,4]]},"reference":[{"key":"ref_1","doi-asserted-by":"crossref","first-page":"105616","DOI":"10.1016\/j.jdent.2025.105616","article-title":"3D-printed clear aligners: An emerging alternative to the conventional thermoformed aligners? A systematic review","volume":"155","author":"Torkomian","year":"2025","journal-title":"J. Dent."},{"key":"ref_2","doi-asserted-by":"crossref","unstructured":"Narongdej, P., Hassanpour, M., Alterman, N., Rawlins-Buchanan, F., and Barjasteh, E. (2024). Advancements in clear aligner fabrication: A comprehensive review of direct-3D printing technologies. Polymers, 16.","DOI":"10.3390\/polym16030371"},{"key":"ref_3","doi-asserted-by":"crossref","first-page":"6246","DOI":"10.1038\/s41598-022-09831-4","article-title":"Thermo-mechanical properties of 3D printed photocurable shape memory resin for clear aligners","volume":"12","author":"Lee","year":"2022","journal-title":"Sci. Rep."},{"key":"ref_4","doi-asserted-by":"crossref","first-page":"23530","DOI":"10.1038\/s41598-025-93026-0","article-title":"Mechanical and viscoelastic properties of a temperature-responsive photocurable resin for 3D printed orthodontic clear aligners","volume":"15","author":"Choi","year":"2025","journal-title":"Sci. Rep."},{"key":"ref_5","doi-asserted-by":"crossref","first-page":"226","DOI":"10.1007\/s00056-023-00511-0","article-title":"Effect of printing orientation on mechanical properties of 3D-printed orthodontic aligners","volume":"86","author":"Camenisch","year":"2025","journal-title":"J. Orofac. Orthop."},{"key":"ref_6","doi-asserted-by":"crossref","unstructured":"Goracci, C., Juloski, J., D\u2019Amico, C., Balestra, D., Volpe, A., Juloski, J., and Vichi, A. (2023). Clinically relevant properties of 3D printable materials for intraoral use in orthodontics: A critical review of the literature. Materials, 16.","DOI":"10.3390\/ma16062166"},{"key":"ref_7","doi-asserted-by":"crossref","first-page":"2163","DOI":"10.1007\/s00784-023-04912-8","article-title":"Fused filament fabrication (FFF): Influence of layer height on forces and moments delivered by aligners\u2014An in vitro study","volume":"27","author":"Spanier","year":"2023","journal-title":"Clin. Oral Investig."},{"key":"ref_8","doi-asserted-by":"crossref","first-page":"2118412","DOI":"10.1155\/2024\/2118412","article-title":"In vitro characterisation of 3D printable filaments subjected to edible liquids: An analysis of fused deposition modelling for intraoral applicability","volume":"2024","author":"Barman","year":"2024","journal-title":"Int. J. Dent."},{"key":"ref_9","doi-asserted-by":"crossref","first-page":"23453","DOI":"10.1021\/acsami.0c03830","article-title":"3D printing of polyethylene terephthalate glycol\u2013sepiolite composites with nanoscale orientation","volume":"12","author":"Kim","year":"2020","journal-title":"ACS Appl. Mater. Interfaces"},{"key":"ref_10","doi-asserted-by":"crossref","first-page":"107115","DOI":"10.1016\/j.jmbbm.2025.107115","article-title":"Mechanical and adhesive properties of additively manufactured dental tray materials with variable sustainability profiles","volume":"170","author":"Clarkson","year":"2025","journal-title":"J. Mech. Behav. Biomed. Mater."},{"key":"ref_11","doi-asserted-by":"crossref","first-page":"e2361857","DOI":"10.1080\/17452759.2024.2361857","article-title":"Force system of 3D-printed orthodontic aligners made of shape memory polymers: An in vitro study","volume":"19","author":"Sharif","year":"2024","journal-title":"Virtual Phys. Prototyp."},{"key":"ref_12","doi-asserted-by":"crossref","first-page":"336","DOI":"10.1111\/ocr.12537","article-title":"Comparative analysis of mechanical properties of orthodontic aligners produced by different contemporary 3D printers","volume":"25","author":"Zinelis","year":"2022","journal-title":"Orthod. Craniofac. Res."},{"key":"ref_13","doi-asserted-by":"crossref","unstructured":"Zecca, P.A., Bocchieri, S., Borgese, M., Dolci, C., Campobasso, A., Battista, G., Caprioglio, A., and Raspanti, M. (2023). In vitro investigation of the mechanical properties of blended 3D-printing resins for orthodontic aligners: A comparison between commercial resin and nickel-titanium wire. Appl. Sci., 13.","DOI":"10.3390\/app13159020"},{"key":"ref_14","doi-asserted-by":"crossref","first-page":"104045","DOI":"10.1016\/j.jmbbm.2020.104045","article-title":"Mechanical behaviour of 3D printed vs thermoformed clear dental aligner materials under non-linear compressive loading using FEM","volume":"112","author":"Jindal","year":"2020","journal-title":"J. Mech. Behav. Biomed. Mater."},{"key":"ref_15","doi-asserted-by":"crossref","first-page":"466","DOI":"10.1016\/j.ajodo.2025.04.022","article-title":"Absorption and desorption dynamics of thermoformed and 3-dimensional-printed orthodontic aligners","volume":"168","author":"Simunovic","year":"2025","journal-title":"Am. J. Orthod. Dentofac. Orthop."},{"key":"ref_16","doi-asserted-by":"crossref","first-page":"1587","DOI":"10.1016\/j.dental.2022.08.001","article-title":"Initial biocompatibility of novel resins for 3D printed fixed dental prostheses","volume":"38","author":"Wuersching","year":"2022","journal-title":"Dent. Mater."},{"key":"ref_17","doi-asserted-by":"crossref","first-page":"244","DOI":"10.1093\/ejo\/cjac056","article-title":"Leaching from a 3D-printed aligner resin","volume":"45","author":"Willi","year":"2023","journal-title":"Eur. J. Orthod."},{"key":"ref_18","doi-asserted-by":"crossref","first-page":"e116","DOI":"10.1016\/j.ajodo.2022.06.014","article-title":"Cytotoxicity and estrogenicity of a novel 3-dimensional printed orthodontic aligner","volume":"162","author":"Pratsinis","year":"2022","journal-title":"Am. J. Orthod. Dentofac. Orthop."},{"key":"ref_19","doi-asserted-by":"crossref","first-page":"cjaf014","DOI":"10.1093\/ejo\/cjaf014","article-title":"Comparative microscopic analysis of plastic dispersion from 3D-printed and thermoformed orthodontic aligners","volume":"47","author":"Zecca","year":"2025","journal-title":"Eur. J. Orthod."},{"key":"ref_20","doi-asserted-by":"crossref","first-page":"3","DOI":"10.1186\/s40510-026-00608-3","article-title":"Orthodontic derived microplastics impact macrophage differentiation and homeostasis","volume":"27","author":"Warunek","year":"2026","journal-title":"Prog. Orthod."},{"key":"ref_21","unstructured":"(2025). Biological Evaluation of Medical Devices\u2014Part 1: Requirements and General Principles for the Evaluation of Biological Safety Within a Risk Management Process (Standard No. ISO 10993-1:2025)."},{"key":"ref_22","unstructured":"(2017). Biological Evaluation of Medical Devices\u2014Part 4: Selection of Tests for Interactions with Blood (Standard No. ISO 10993-4:2017)."},{"key":"ref_23","unstructured":"(2009). Biological Evaluation of Medical Devices\u2014Part 5: Tests for in Vitro Cytotoxicity (Standard No. ISO 10993-5:2009)."},{"key":"ref_24","unstructured":"(2021). Biological Evaluation of Medical Devices\u2014Part 10: Tests for Skin Sensitization (Standard No. ISO 10993-10:2021)."},{"key":"ref_25","unstructured":"(2009). Pharmatext Search E-Book (Standard No. USP NF 2009 (United States Pharmacopeia 32\/National Formulary 27))."},{"key":"ref_26","unstructured":"(2016). Medical Devices\u2014Quality Management Systems\u2014Requirements for Regulatory Purposes (Standard No. ISO 13485)."},{"key":"ref_27","unstructured":"(2019). Plastics\u2014Determination of Flexural Properties (Standard No. ISO 178:2019)."},{"key":"ref_28","doi-asserted-by":"crossref","first-page":"106337","DOI":"10.1016\/j.jmbbm.2023.106337","article-title":"Physiochemical and mechanical characterisation of orthodontic 3D printed aligner material made of shape memory polymers (4D aligner material)","volume":"150","author":"Atta","year":"2024","journal-title":"J. Mech. Behav. Biomed. Mater."},{"key":"ref_29","doi-asserted-by":"crossref","unstructured":"Serafin, M., Boccalari, E., Borgese, M., Caprioglio, A., Raspanti, M., Binda, G., and Zecca, P.A. (2026). Static and dynamic mechanical properties of 3D-printable aligner resins: An in vitro study with FTIR chemical characterization. J. Funct. Biomater., 17.","DOI":"10.3390\/jfb17070340"},{"key":"ref_30","doi-asserted-by":"crossref","first-page":"1183","DOI":"10.1177\/00220345630420051301","article-title":"Corrosion of gold and amalgam placed in contact with each other","volume":"42","author":"Fusayama","year":"1963","journal-title":"J. Dent. Res."},{"key":"ref_31","doi-asserted-by":"crossref","first-page":"23","DOI":"10.1186\/s40510-023-00475-2","article-title":"Forces and moments generated by 3D direct printed clear aligners of varying labial and lingual thicknesses during lingual movement of maxillary central incisor: An in vitro study","volume":"24","author":"Grant","year":"2023","journal-title":"Prog. Orthod."},{"key":"ref_32","doi-asserted-by":"crossref","first-page":"7398478","DOI":"10.1155\/2024\/7398478","article-title":"The influence of thickness on the mechanical behaviors of 3D printing resins for orthodontic retainers","volume":"2024","author":"Nakornnoi","year":"2024","journal-title":"Int. J. Biomater."},{"key":"ref_33","doi-asserted-by":"crossref","first-page":"11","DOI":"10.2319\/113015-813.1","article-title":"Stress relaxation properties of four orthodontic aligner materials: A 24-hour in vitro study","volume":"87","author":"Lombardo","year":"2017","journal-title":"Angle Orthod."},{"key":"ref_34","doi-asserted-by":"crossref","first-page":"cjad063","DOI":"10.1093\/ejo\/cjad063","article-title":"Effects of intraoral aging on mechanical properties of directly printed aligners vs. thermoformed aligners: An in vivo prospective investigation","volume":"46","author":"Sayahpour","year":"2024","journal-title":"Eur. J. Orthod."}],"container-title":["Dentistry Journal"],"original-title":[],"language":"en","link":[{"URL":"https:\/\/www.mdpi.com\/2304-6767\/14\/9\/565\/pdf","content-type":"unspecified","content-version":"vor","intended-application":"similarity-checking"}],"deposited":{"date-parts":[[2026,9,4]],"date-time":"2026-09-04T15:11:29Z","timestamp":1788534689000},"score":1,"resource":{"primary":{"URL":"https:\/\/www.mdpi.com\/2304-6767\/14\/9\/565"}},"subtitle":[],"short-title":[],"issued":{"date-parts":[[2026,9,4]]},"references-count":34,"journal-issue":{"issue":"9","published-online":{"date-parts":[[2026,9]]}},"alternative-id":["dj14090565"],"URL":"https:\/\/doi.org\/10.3390\/dj14090565","relation":{},"ISSN":["2304-6767"],"issn-type":[{"value":"2304-6767","type":"electronic"}],"subject":[],"published":{"date-parts":[[2026,9,4]]}}}