Additive manufacturing (AM), commonly referred to as 3D printing, is a process in which a physical object is created layer by layer from a digital model, for applications in rapid prototyping, manufacturing, healthcare, etc. (Fig. 1) [1]. Vat Photopolymerization (VPP) is one of the seven categories of AM listed by the international standard ISO/ASTM 52900:2021 (Additive manufacturing — General principles — Fundamentals and vocabulary). VPP encompasses several technologies, including Stereolithography, Digital Light Processing, Continuous Direct Light Processing, and Direct UV Printing. The most employed among these are Stereolithography (SLA) and Digital Light Processing (DLP). Both utilize similar materials, commonly consisting of monomers/oligomers for resin matrix, photoinitiators activated at the printer's light source working wavelength, as well as fillers to improve mechanical properties, and additives to achieve the required properties for various applications.

A prominent area of research is the incorporation of nanoparticles into photopolymerizable formulations to enhance the mechanical properties, dimensional stability, biocompatibility, and chemical resistance, as well as to optimize the electrical or optical properties of materials, particularly in 3D printing applications [2,3]. Since the number of publications in this field continues to grow rapidly, writing a review on this subject would be challenging. Therefore, we focus our review on the impact of nanoparticle incorporation on the photopolymerization process, specifically examining factors such as the rate of photopolymerization, degree of conversion, and initiation/sensitization. We also explore how nanoparticles influence the efficiency of the light-curing process, including effective light utilization, spatial resolution, and printing precision and accuracy in vat photopolymerization (VP) additive manufacturing, where applicable. A comprehensive literature search in the PubMed and Scopus databases conducted using a range of relevant keywords revealed that the publication trend indicates a recent increase in interdisciplinary research and a growing interest in nanoparticle-enhanced photopolymerization (Supplementary Information).

Review articles to date provide comprehensive insights into various advancements in photopolymerization techniques and their applications in 3D printing and nanophotonics. One review [4] provides an in-depth analysis of photon upconversion technologies, which enable spatially confined photoexcitation crucial for achieving high spatial resolution in stereolithography and related 3D printing methods. In employing traditional UV photoinitiators that operate via linear absorption, polymerization is triggered along the entire light path, which reduces spatial precision. In contrast, high spatial resolution is possible with photon upconversion because excitation, and thus polymerization, only occurs where the light intensity is sufficiently high, typically at the focal point. Various upconversion mechanisms, including two-photon absorption, upconversion nanoparticles, and triplet-triplet annihilation, are discussed, highlighting recent advancements in these technologies over the past two years. Another review [5] focuses on the challenges and future potential of near-infrared (NIR) light-induced reversible deactivation radical polymerization (NIR-RDRP) in the preparation of advanced materials such as nanoparticles, precise 3D printing, drug delivery systems, etc. Different NIR-RDRP systems are categorized based on their photoinitiation pathways: upconversion nanoparticle-mediated systems, photocatalyst-mediated systems, photothermal conversion, and two-photon absorption, while giving valuable insights. Lastly, advancements in nanophotonic structures have been explored [6], focusing on tunable and reconfigurable systems that utilize nanoscale photonic effects for advanced photonic applications. The authors emphasize the use of metallic, dielectric, and hybrid photonic structures, exploring both reversible and irreversible changes in nanostructures. Their review discusses how low laser intensities induce reversible changes in nanostructures for tunability, while higher intensities trigger non-reversible changes, such as material structure alterations and melting. [6] offer valuable insights into the underlying physical and chemical processes that drive these effects, essential for advancing nanophotonic technologies.

This article aims to provide a comprehensive overview of the advancements made in photopolymerization enhancement, focusing on the nanoparticles utilized to date. This review begins by examining the types and key characteristics of nanoparticles studied for photopolymerization enhancement. We then explore the methods used to evaluate these enhancements and how they influence the interpretation of results across studies. Finally, the discussion moves to the implications of these findings for optimizing vat photopolymerization processes.