Particle Safety Assessment in Additive Manufacturing: From Exposure Risks to Advanced Toxicology Testing

Abstract

Additive manufacturing (AM) or industrial three-dimensional (3D) printing drives a new spectrum of design and production possibilities; pushing the boundaries both in the application by production of sophisticated products as well as the development of next-generation materials. AM technologies apply a diversity of feedstocks, including plastic, metallic, and ceramic particle powders with distinct size, shape, and surface chemistry. In addition, powders are often reused, which may change the particles' physicochemical properties and by that alter their toxic potential. The AM production technology commonly relies on a laser or electron beam to selectively melt or sinter particle powders. Large energy input on feedstock powders generates several byproducts, including varying amounts of virgin microparticles, nanoparticles, spatter, and volatile chemicals that are emitted in the working environment; throughout the production and processing phases. The micro and nanoscale size may enable particles to interact with and to cross biological barriers, which could, in turn, give rise to unexpected adverse outcomes, including inflammation, oxidative stress, activation of signaling pathways, genotoxicity, and carcinogenicity. Another important aspect of AM-associated risks is emission/leakage of mono- and oligomers due to polymer breakdown and high temperature transformation of chemicals from polymeric particles, both during production, use, and in vivo, including in target cells. These chemicals are potential inducers of direct toxicity, genotoxicity, and endocrine disruption. Nevertheless, understanding whether AM particle powders and their byproducts may exert adverse effects in humans is largely lacking and urges comprehensive safety assessment across the entire AM lifecycle-spanning from virgin and reused to airborne particles. Therefore, this review will detail: 1) brief overview of the AM feedstock powders, impact of reuse on particle physicochemical properties, main exposure pathways and protective measures in AM industry, 2) role of particle biological identity and key toxicological endpoints in the particle safety assessment, and 3) next-generation toxicology approaches in nanosafety for safety assessment in AM. Altogether, the proposed testing approach will enable a deeper understanding of existing and emerging particle and chemical safety challenges and provide a strategy for the development of cutting-edge methodologies for hazard identification and risk assessment in the AM industry.

Keywords: adverse outcome; endocrine disruption; genotoxicity; industrial 3D printing; inflammation; mechanism of action; particle emissions.

FIGURE 1

Overview of the papers published over the last 20 years on the topics “microparticle” and “nanoparticle safety,” “additive manufacturing” and “3D printing,” and “additive manufacturing safety” and “3D printing safety.” All publications were found via PubMed literature search.

FIGURE 2

Impact of reuse on the properties of microparticle feedstock powders in AM. Large energy input during production usually elicits formation of satellites (small particles attached to the surface of bigger particles), bonded particles, non-spherical and elongated particles, tightly bound agglomerates, particles with rough/irregular surface topography or “super-ball” particles with increased size in comparison to the virgin particles.

FIGURE 3

Particle and chemical exposure risks in AM. At different stages of the AM process chain particle and chemical emissions may occur, including input of feedstock powders, high-energy production, product post-processing, machine cleaning, and maintenance. Potential particle exposure routes for exposed AM workers involve inhalation, ingestion, and skin absorption.

FIGURE 4

Particles in the AM occupational setting. The particle size distribution was quite similar in all measurement locations. Some larger particles (>1 µm) could be found in the entrance hall and in the process air.

FIGURE 5

Particle’s biological identity. Particles entering protein-rich biological environments are swiftly covered by proteins forming the so-called protein corona or mechanical interface between particles and cell receptors. Particle physicochemical properties are directly dictating the composition of the protein corona and the extent to which proteins adsorb/desorb from the particle’s surface. The protein corona may block or promote receptor recognition and particle internalization by the cells. Particle’s size determines the mechanism of the internalization, and it may involve pinocytosis, endocytosis (caveolin- or clathrin-mediated) and phagocytosis.

FIGURE 6

Particle-induced effects in the human cells. Both in the extracellular and intracellular milieu, particles potentially release additives, monomers, or metallic ions (depending on the particle chemical composition). After recognition and internalization, particles affect cell physiology in numerous ways by involving extensive signaling leading to changes in the expression of target genes, resulting in inflammation, endocrine disruption, genotoxicity, or cytotoxicity. Moreover, metal ions released during dissolution of metal particles may induce oxidative stress that may trigger inflammation and/or cytotoxicity.

FIGURE 7

Next-generation toxicology testing for the particle safety assessment. Integration of the bioassay toolbox, high-content screening, omics technologies, and machine learning models offers excellent input in the development of AOP networks and description of MIEs, KEs, and AOs.