Environmental and Health Impacts of Graphene and Other Two-Dimensional Materials: A Graphene Flagship Perspective

Two-dimensional (2D) materials have attracted tremendous interest ever since the isolation of atomically thin sheets of graphene in 2004 due to the specific and versatile properties of these materials. However, the increasing production and use of 2D materials necessitate a thorough evaluation of the potential impact on human health and the environment. Furthermore, harmonized test protocols are needed with which to assess the safety of 2D materials. The Graphene Flagship project (2013-2023), funded by the European Commission, addressed the identification of the possible hazard of graphene-based materials as well as emerging 2D materials including transition metal dichalcogenides, hexagonal boron nitride, and others. Additionally, so-called green chemistry approaches were explored to achieve the goal of a safe and sustainable production and use of this fascinating family of nanomaterials. The present review provides a compact survey of the findings and the lessons learned in the Graphene Flagship.

Boron Nitride Nanosheets Induce Lipid Accumulation and Autophagy in Human Alveolar Lung Epithelial Cells Cultivated at Air-Liquid Interface

Hexagonal boron nitride (hBN) is an emerging 2D material attracting significant attention due to its superior electrical, chemical, and therapeutic properties. However, inhalation toxicity mechanisms of hBN in human lung cells are poorly understood. Here, cellular interaction and effects of hBN nanosheets is investigated in alveolar epithelial cells cultured on porous inserts and exposed under air-liquid interface conditions for 24 h. hBN is taken up by the cells as determined in a label-free manner via RAMAN-confocal microscopy, ICP-MS, TEM, and SEM-EDX. No significant (p > 0.05) effects are observed on cell membrane integrity (LDH release), epithelial barrier integrity (TEER), interleukin-8 cytokine production or reactive oxygen production at tested dose ranges (1, 5, and 10 µg cm-2 ). However, it is observed that an enhanced accumulation of lipid granules in cells indicating the effect of hBN on lipid metabolism. In addition, it is observed that a significant (p < 0.05) and dose-dependent (5 and 10 µg cm-2 ) induction of autophagy in cells after exposure to hBN, potentially associated with the downstream processing and breakdown of excess lipid granules to maintain lipid homeostasis. Indeed, lysosomal co-localization of lipid granules supporting this argument is observed. Overall, the results suggest that the continuous presence of excess intracellular lipids may provoke adverse outcomes in the lungs.

Ultralarge suspended and perforated graphene membranes for cell culture applications

With its high mechanical strength and its remarkable thermal and electrical properties, suspended graphene has long been expected to find revolutionary applications in optoelectronics or as a membrane in nano-devices. However, the lack of efficient transfer and patterning processes still limits its potential. In this work, we report an optimized anthracene-based transfer process to suspend few layers of graphene (1-, 2- and 4-layers) in the millimeter range (up to 3 mm) with high reproducibility. We have explored the advantages and limitations for patterning of these membranes with micrometer-resolution by focused ion beam (FIB) and picosecond pulsed laser ablation techniques. The FIB approach offers higher patterning resolution but suffers from the low throughput. We demonstrate that cold laser ablation is a fast and flexible method for micro-structuring of suspended graphene. One promising field of application of ultimately thin, microporous graphene membranes is their use as next-generation cell culture supports as alternative to track-etched polymer membranes, which often exhibit poor permeability and limited cell-to-cell communication across the membranes. To this end, we confirmed good adhesion and high viability of placental trophoblast cells cultivated on suspended porous graphene membranes without rupturing of the membranes. Overall, there is high potential for the further development of ultrathin suspended graphene membranes for many future applications, including their use for biobarrier cell culture models to enable predictive transport and toxicity assessment of drugs, environmental pollutants, and nanoparticles.

Understanding the Chemical Exposome During Fetal Development and Early Childhood: A Review

Early human life is considered a critical window of susceptibility to external exposures. Infants are exposed to a multitude of environmental factors, collectively referred to as the exposome. The chemical exposome can be summarized as the sum of all xenobiotics that humans are exposed to throughout a lifetime. We review different exposure classes and routes that impact fetal and infant metabolism and the potential toxicological role of mixture effects. We also discuss the progress in human biomonitoring and present possiblemodels for studying maternal-fetal transfer. Data gaps on prenatal and infant exposure to xenobiotic mixtures are identified and include natural biotoxins, in addition to commonly reported synthetic toxicants, to obtain a more holistic assessment of the chemical exposome. We highlight the lack of large-scale studies covering a broad range of xenobiotics. Several recommendations to advance our understanding of the early-life chemical exposome and the subsequent impact on health outcomes are proposed.

Gene expression profiling of human macrophages after graphene oxide and graphene nanoplatelets treatment reveals particle-specific regulation of pathways

Graphene and its derivatives are attractive materials envisaged to enable a wealth of novel applications in many fields including energy, electronics, composite materials or health. A comprehensive understanding of the potential adverse effects of graphene-related materials (GRM) in humans is a prerequisite to the safe use of these promising materials. Here, we exploited gene expression profiling to identify transcriptional responses and toxicity pathways induced by graphene oxide (GO) and graphene nanoplatelets (GNP) in human macrophages. Primary human monocyte-derived macrophages (MDM) and a human macrophage cell line, i.e. differentiated THP-1 cells, were exposed to 5 or 20 μg/mL GO and GNP for 6 and 24 h to capture early and more persistent acute responses at realistic or slightly overdose concentrations. GO and GNP induced time-, dose- and macrophage type-specific differential expression of a substantial number of genes with some overlap between the two GRM types (up to 384 genes (9.6%) or 447 genes (20.4%) in THP-1 or MDM, respectively) but also a high number of genes exclusively deregulated from each material type. Furthermore, GRM responses on gene expression were highly different from those induced by inflammogenic material crystalline quartz (maximum of 64 (2.3%) or 318 (11.3%) common genes for MDM treated with 20 μg/mL GO and GNP, respectively). Further bioinformatics analysis revealed that GNP predominantly activated genes controlling inflammatory and apoptotic pathways whereas GO showed only limited inflammatory responses. Interestingly, both GRM affected the expression of genes related to antigen processing and presentation and in addition, GO activated pathways of neutrophil activation, degranulation and immunity in MDM. Overall, this study provides an extensive resource of potential toxicity mechanisms for future safety assessment of GRM in more advanced model systems to verify if the observed changes in gene expression in human macrophages could lead to long-term consequences on human health.

Novel electrospun chitosan/PEO membranes for more predictive nanoparticle transport studies at biological barriers

The design of safe and effective nanoparticles (NPs) for commercial and medical applications requires a profound understanding of NP translocation and effects at biological barriers. To gain mechanistic insights, physiologically relevant and accurate human in vitro biobarrier models are indispensable. However, current transfer models largely rely on artificial porous polymer membranes for the cultivation of cells, which do not provide a close mimic of the natural basal membrane and intrinsically provide limited permeability for NPs. In this study, electrospinning is exploited to develop thin chitosan/polyethylene oxide (PEO) membranes with a high porosity and nanofibrous morphology for more predictive NP transfer studies. The nanofiber membranes allow the cultivation of a tight and functional placental monolayer (BeWo trophoblasts). Translocation studies with differently sized molecules and NPs (Na-fluorescein; 40 kDa FITC-Dextran; 25 nm PMMA; 70, 180 and 520 nm polystyrene NPs) across empty and cell containing membranes reveal a considerably enhanced permeability compared to commercial microporous membranes. Importantly, the transfer data of NPs is highly similar to data from ex vivo perfusion studies of intact human placental tissue. Therefore, the newly developed membranes may decisively contribute to establish physiologically relevant in vitro biobarrier transfer models with superior permeability for a wide range of molecules and particles.

Hazard assessment of abraded thermoplastic composites reinforced with reduced graphene oxide

Graphene-related materials (GRMs) are subject to intensive investigations and considerable progress has been made in recent years in terms of safety assessment. However, limited information is available concerning the hazard potential of GRM-containing products such as graphene-reinforced composites. In the present study, we conducted a comprehensive investigation of the potential biological effects of particles released through an abrasion process from reduced graphene oxide (rGO)-reinforced composites of polyamide 6 (PA6), a widely used engineered thermoplastic polymer, in comparison to as-produced rGO. First, a panel of well-established in vitro models, representative of the immune system and possible target organs such as the lungs, the gut, and the skin, was applied. Limited responses to PA6-rGO exposure were found in the different in vitro models. Only as-produced rGO induced substantial adverse effects, in particular in macrophages. Since inhalation of airborne materials is a key occupational concern, we then sought to test whether the in vitro responses noted for these materials would translate into adverse effects in vivo. To this end, the response at 1, 7 and 28 days after a single pulmonary exposure was evaluated in mice. In agreement with the in vitro data, PA6-rGO induced a modest and transient pulmonary inflammation, resolved by day 28. In contrast, rGO induced a longer-lasting, albeit moderate inflammation that did not lead to tissue remodeling within 28 days. Taken together, the present study suggests a negligible impact on human health under acute exposure conditions of GRM fillers such as rGO when released from composites at doses expected at the workplace.

Airborne emissions from combustion of graphene nanoplatelet/epoxy composites and their cytotoxicity on lung cells via air-liquid interface cell exposure in vitro

Graphene nanoplatelet (GNP) as a nanofiller improves the mechanical strength, electrical conductivity, and flame retardancy of the polymers significantly. With an increasing number of GNP-reinforced products, a careful safety assessment is needed to avoid social and economic setbacks. However, no study has addressed the effects of combustion-generated emissions from GNP-reinforced products in the lung, the most sensitive exposure route to airborne particles. Therefore, we studied the influence of GNP as a nanofiller on the emitted particles and polycyclic aromatic hydrocarbons (PAHs), and cytotoxicity of the emissions from the combustion of pure epoxy (EP) and GNP-reinforced epoxy (EP-GNP). GNP was not detected in the airborne emissions. PAHs were found in airborne particles of both emissions from EP and EP-GNP, with some differences in their concentrations. A first hazard assessment was performed on human alveolar epithelial cells exposed to the airborne emissions at air-liquid interface conditions. At 24 h and 96 h after the exposure, similar responses were observed between EP and EP-GNP except an acute transient decrease in mitochondrial activity after exposure to the emissions from EP-GNP. Both emissions from EP and EP-GNP had no acute effects on membrane integrity, cell morphology or expression of anti-oxidative stress markers (HMOX1 and SOD2 genes). Meanwhile, both emissions induced the activation of the aryl hydrocarbon receptor (CYP1A1 gene) and a transient (pro-) inflammatory response (MCP-1), but the effects between EP and EP-GNP were not significantly different.

Catching Them Early: Framework Parameters and Progress for Prenatal and Childhood Application of Advanced Therapies

Advanced therapy medicinal products (ATMPs) are medicines for human use based on genes, cells or tissue engineering. After clear successes in adults, the nascent technology now sees increasing pediatric application. For many still untreatable disorders with pre- or perinatal onset, timely intervention is simply indispensable; thus, prenatal and pediatric applications of ATMPs hold great promise for curative treatments. Moreover, for most inherited disorders, early ATMP application may substantially improve efficiency, economy and accessibility compared with application in adults. Vindicating this notion, initial data for cell-based ATMPs show better cell yields, success rates and corrections of disease parameters for younger patients, in addition to reduced overall cell and vector requirements, illustrating that early application may resolve key obstacles to the widespread application of ATMPs for inherited disorders. Here, we provide a selective review of the latest ATMP developments for prenatal, perinatal and pediatric use, with special emphasis on its comparison with ATMPs for adults. Taken together, we provide a perspective on the enormous potential and key framework parameters of clinical prenatal and pediatric ATMP application.

Addressing microchimerism in pregnancy by ex vivo human placenta perfusion

The physical connection of mother and offspring during pregnancy allows the bi-directional exchange of a small number of cells through the placenta. These cells, which can persist long-term in the recipient individual are genetically foreign to it and therefore fulfill the principle of microchimerism. Over the last years, pioneer research on microchimeric cells revealed their role in immune adaptation during pregnancy and priming of tolerogenic responses in the progeny. However, the mechanisms involved in cell transfer across the placenta barrier remain poorly investigated. In this review, we summarize the evidence of fetomaternal microchimerism, propose a mechanism for cell trafficking through the placenta and discuss the different models and techniques available for its analysis. Likewise, we aim to generate interest in the use of ex vivo placenta perfusion to investigate microchimerism in physiological and pathological settings.

Microfluidic Co-Culture Platform to Recapitulate the Maternal-Placental-Embryonic Axis

Safety assessment of the effects of developmental toxicants on pregnant women is challenging, and systemic effects in embryo-maternal interactions are largely unknown. However, most developmental toxicity studies rely on animal trials, while in vitro platforms that recapitulate the maternal-placental-embryonic axis are missing. Here, the development of a dedicated microfluidic device for co-cultivation of a placental barrier and 3D embryoid bodies to enable systemic toxicity testing at the embryo-maternal interface is reported. The microfluidic platform features simple handling and recuperation of both tissue models, which facilitates post-hoc in-depth analysis at the tissue and single-cell level. Gravity-driven flow enables inter-tissue communication through the liquid phase as well as simple and robust operation and renders the platform parallelizable. As a proof of concept and to demonstrate platform use for systemic embryotoxicity testing in vitro, maternal exposure to plastic microparticles is emulated, and microparticle effects on the embryo-placental co-culture are investigated.

Label-free detection of uptake, accumulation, and translocation of diesel exhaust particles in ex vivo perfused human placenta

Background

Pregnant women and developing fetuses comprise a particularly vulnerable population as multiple studies have shown associations between prenatal air pollution exposure and adverse pregnancy outcomes. However, the mechanisms underlying the observed developmental toxicity are mostly unknown, in particular, if pollution particles can cross the human placenta to reach the fetal circulation.

Results

Here, we investigated the accumulation and translocation of diesel exhaust particles (DEPs), as a model particle for combustion-derived pollution, in human perfused placentae using label-free detection by femtosecond pulsed laser illumination. The results do not reveal a significant particle transfer across term placentae within 6 h of perfusion. However, DEPs accumulate in placental tissue, especially in the syncytiotrophoblast layer that mediates a wealth of essential functions to support and maintain a successful pregnancy. Furthermore, DEPs are found in placental macrophages and fetal endothelial cells, showing that some particles can overcome the syncytiotrophoblasts to reach the fetal capillaries. Few particles are also observed inside fetal microvessels.

Conclusions

Overall, we show that DEPs accumulate in key cell types of the placental tissue and can cross the human placenta, although in limited amounts. These findings are crucial for risk assessment and protection of pregnant women and highlight the urgent need for further research on the direct and indirect placenta-mediated developmental toxicity of ambient particulates.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12951-021-00886-5.

Research on nanoparticles in human perfused placenta: State of the art and perspectives

Increasing human exposure to nanoparticles (NPs) from various sources raises concerns for public health, especially for vulnerable risk groups like pregnant women and their developing fetuses. However, nanomedicine and the prospect of creating safe and effective NP-based formulations of drugs hold great promise to revolutionize treatment during pregnancy. With maternal and fetal health at stake, risks and opportunities of NPs in pregnancy need to be carefully investigated. Importantly, a comprehensive understanding of NP transport and effects at the placenta is urgently needed considering the central position of the placenta at the maternal-fetal interface and its many essential functions to enable successful pregnancy. The perfusion of human placental tissue provides a great opportunity to achieve predictive human relevant insights, circumventing uncertainties due to considerable differences in placental structure and function across species. Here, we have reviewed the current literature on the ex vivo human placenta perfusion of NPs. From 16 available studies, it was evident that placental uptake and transfer of NPs are highly dependent on their characteristics like size and surface modifications, which is in line with previous observations from in vitro and animal transport studies. These studies further revealed that special considerations apply for the perfusion of NPs and we identified relevant controls that should be implemented in future perfusion studies. While current studies mostly focused on placental transfer of NPs to conclude on potential fetal exposure, the ex vivo placental perfusion model has considerable potential to reveal novel insights on NP effects on placental tissue functionality and signaling that could indirectly affect maternal-fetal health.

Investigating the effects of differently produced synthetic amorphous silica (E 551) on the integrity and functionality of the human intestinal barrier using an advanced in vitro co-culture model

E 551, also known as synthetic amorphous silica (SAS), is the second most produced food additive. However, according to the re-evaluation of E 551 by the European Food Safety Authority (EFSA) in 2018, the amount of available data on the oral toxicity of food grade E 551 is still insufficient for reliable risk assessment. To close this gap, this study aimed to investigate six food-grade SAS with distinct physicochemical properties on their interaction with the intestinal barrier using advanced in vitro intestinal co-cultures and to identify potential structure-activity relationships. A mucus-secreting Caco-2/HT-29/Raji co-culture model was treated with up to 50 µg/ml SAS for 48 h, which represents a dose range relevant to dietary exposure. No effects on cell viability, barrier integrity, microvilli function or the release of inflammatory cytokine were detected after acute exposure. Slight biological responses were observed for few SAS materials on iron uptake and gene expression levels of mucin 1 and G-protein coupled receptor 120 (GPR120). There was no clear correlation between SAS properties (single or combined) and the observed biological responses. Overall, this study provides novel insights into the short-term impact of food-relevant SAS with distinct characteristics on the intestinal epithelium including a range of intestine-specific functional endpoints. In addition, it highlights the importance of using advanced intestinal co-cultures embracing relevant cell types as well as a protective mucus barrier to achieve a comprehensive understanding of the biological response of food additives at the intestinal barrier in vitro.

Recent insights on indirect mechanisms in developmental toxicity of nanomaterials

BACKGROUND

Epidemiological and animal studies provide compelling indications that environmental and engineered nanomaterials (NMs) pose a risk for pregnancy, fetal development and offspring health later in life. Understanding the origin and mechanisms underlying NM-induced developmental toxicity will be a cornerstone in the protection of sensitive populations and the design of safe and sustainable nanotechnology applications.

MAIN BODY

Direct toxicity originating from NMs crossing the placental barrier is frequently assumed to be the key pathway in developmental toxicity. However, placental transfer of particles is often highly limited, and evidence is growing that NMs can also indirectly interfere with fetal development. Here, we outline current knowledge on potential indirect mechanisms in developmental toxicity of NMs.

SHORT CONCLUSION

Until now, research on developmental toxicity has mainly focused on the biodistribution and placental translocation of NMs to the fetus to delineate underlying processes. Systematic research addressing NM impact on maternal and placental tissues as potential contributors to mechanistic pathways in developmental toxicity is only slowly gathering momentum. So far, maternal and placental oxidative stress and inflammation, activation of placental toll-like receptors (TLRs), impairment of placental growth and secretion of placental hormones, and vascular factors have been suggested to mediate indirect developmental toxicity of NMs. Therefore, NM effects on maternal and placental tissue function ought to be comprehensively evaluated in addition to placental transfer in the design of future studies of developmental toxicity and risk assessment of NM exposure during pregnancy.

The impact of synthetic amorphous silica (E 551) on differentiated Caco-2 cells, a model for the human intestinal epithelium

For several decades, food-grade synthetic amorphous silica (SAS) have been used as a technological additive to reduce caking of food powders. Human exposure is thus inevitable and safety concerns are taken seriously. The toxicity of silica in general and SAS in particular has been studied extensively. Overall, there is little evidence that food-grade SAS pose any health risks to humans. However, from the available data it was often not clear which type of silica was used. Accordingly, the latest report of the European food safety authority requested additional toxicity data for well-characterised "real food-grade SAS". To close this gap, we screened a panel of ten well-defined, food-grade SAS for potential adverse effects on differentiated Caco-2 cells. Precipitated and fumed SAS with low, intermediate and high specific surface area were included to determine structure-activity relationships. In a physiological dose-range up to 50 μg/ml and 48 h of incubation, none of the materials induced adverse effects on differentiated Caco-2 cells. This held true for endpoints of acute cytotoxicity as well as epithelial specific measures of barrier integrity. These results showed that despite considerable differences in production routes and material characteristics, food-relevant SAS did not elicit acute toxicity responses in intestinal epithelial cells.

Release of graphene-related materials from epoxy-based composites: characterization, quantification and hazard assessment in vitro

Due to their mechanical strength, thermal stability and electrical conductivity, graphene-related materials (GRMs) have been extensively explored for various applications. Moreover, GRMs have been studied and applied as fillers in polymer composite manufacturing to enhance the polymer performance. With the foreseen growth in GRM production, occupational and consumer exposure is inevitable, thus raising concerns for potential health risks. Therefore, this study aims (1) to characterize aerosol particles released after mechanical abrasion on GRM-reinforced epoxy composites, (2) to quantify the amounts of protruding and free-standing GRMs in the abraded particles and (3) to assess the potential effects of the pristine GRMs as well as the abraded particles on human macrophages differentiated from the THP-1 cell line in vitro. GRMs used in this study included graphene nanoplatelets (GNPs), graphene oxide (GO), and reduced graphene oxide (rGO). All types of pristine GRMs tested induced a dose-dependent increase in reactive oxygen species formation, but a decrease in cell viability was only detected for large GNPs at high concentrations (20 and 40 μg mL^-1). The particle modes measured using a scanning mobility particle sizer (SMPS) were 300-400 nm and using an aerodynamic particle sizer (APS) were between 2-3 μm, indicating the release of respirable particles. A significant fraction (51% to 92%) of the GRMs embedded in the epoxy composites was released in the form of free-standing or protruding GRMs in the abraded particles. The abraded particles did not induce any acute cytotoxic effects.

Nanostructure generation during milk digestion in presence of a cell culture model simulating the small intestine

HYPOTHESIS

The development of advanced oral delivery systems for bioactive compounds requires the fundamental understanding of the digestion process within the gastrointestinal tract. Towards this goal, dynamic invitro digestion models, capable of characterising the molecular as well as colloidal aspects of food, together with their biological interactions with relevant invitro cell culture models, are essential.

EXPERIMENTS

In this study, we demonstrate a novel digestion model that combines flow-through time resolved small angle X-ray scattering (SAXS) with an invitro Caco-2/HT-29 cell co-culture model that also contained a mucus layer. This set-up allows the dynamic insitu characterisation of colloidal structures and their transport across a viable intestinal cell layer during simulated digestion.

FINDINGS

An integrated online SAXS - invitro cell co-culture model was developed and applied to study the digestion of nature's own emulsion, milk. The impact of the invitro cell culture on the digestion-triggered formation and evolution of highly ordered nanostructures in milk is demonstrated. Reported is also the crucial role of the mucus layer on top of the cell layer, protecting the cells from degradation by digestive juice components such as lipase. The novel model can open unique possibilities for the dynamic investigation of colloidal structure formation during lipid digestion and their effect on the uptake of bioactive molecules by the cells.

Transfer and Metabolism of the Xenoestrogen Zearalenone in Human Perfused Placenta

BACKGROUND

Pregnancy is a sensitive condition during which adverse environmental exposures should be monitored thoroughly and minimized whenever possible. In particular, the hormone balance during gestation is delicate, and disturbance may cause acute or chronic long-term health effects. A potential endocrine disruption may be provoked by in utero exposure to xenoestrogens mimicking endogenous estrogens. The mycoestrogen zearalenone (ZEN), a toxic fungal secondary metabolite and mycotoxin found frequently in food and feed, constitutes a prominent example.

OBJECTIVES

We performed a comprehensive assessment of the transfer as well as phase I and phase II metabolism of ZEN at the human placental barrier.

METHODS

Human placentas were perfused with 1μM (318μg/L) ZEN for 6 h. Samples from the maternal and fetal compartment, placental tissue, and fetal plasma were analyzed by a highly sensitive UHPLC-MS/MS assay to detect ZEN as well as nine key metabolites (α-zearalenol, β-zearalenol, zearalanone, α-zearalanol, β-zearalanol, ZEN-14-glucuronide, α-zearalenol-14-glucuronide, β-zearalenol-14-glucuronide, ZEN-14-sulfate).

RESULTS

The model revealed a fast maternofetal transfer of ZEN across the human placental barrier. We also unraveled phase I and phase II metabolism of the parent toxin ZEN into the approximately 70-times more estrogenic α-zearalenol and the less active ZEN-14-sulfate conjugate, which are effectively released into the maternal and fetal circulation in considerable amounts.

CONCLUSIONS

Our findings suggest that exposure to ZEN (such as through consumption of ZEN-contaminated cereal-based products) during pregnancy may result in in utero exposure of the fetus, not only to ZEN but also some of its highly estrogenically active metabolites. In the light of the known affinity of ZEN and potentially co-occurring xenoestrogens to the estrogen receptor, and our results demonstrating placental transfer of ZEN and its metabolites in an ex vivo model, we recommend further research and more comprehensive assessment of gestational exposures in women. https://doi.org/10.1289/EHP4860.

Investigating the accumulation and translocation of titanium dioxide nanoparticles with different surface modifications in static and dynamic human placental transfer models

Titanium dioxide nanoparticles (TiO₂ NPs) are widely incorporated in various consumer products such as cosmetics and food. Despite known human exposure, the potential risks of TiO₂ NPs during pregnancy are not fully understood, but several studies in mice elucidated toxic effects on fetal development. It has also been shown that modifying NPs with positive or negative surface charge alters cellular uptake and abolishes fetotoxicity of silicon dioxide (SiO₂) NPs in mice. Here, we investigated accumulation and translocation of positively charged TiO₂-NH₂ and negatively charged TiO₂-COOH NPs at the placental barrier, to clarify whether surface charge provides a means to control TiO₂ NP distribution at the placental barrier. To ensure outcome relevant for humans, the recently developed in vitro human placental co-culture model and the gold standard amongst placental translocation models - the ex vivo perfusion of human term placental tissue - were employed during this study. Sector field-ICP-MS analysis of maternal and fetal supernatants as well as placental cells/tissues revealed a substantial accumulation of both TiO₂ NP types while no considerable placental translocation was apparent in both models. Characterization of agglomeration behavior demonstrated a strong and fast agglomeration of TiO₂-NH₂ and TiO₂-COOH NPs in the different culture media. Overall, our results indicate that surface charge is not a key factor to steer placental uptake and transfer of TiO₂. Moreover, the negligible placental transfer but high accumulation of TiO₂ NPs in placental tissue suggests that potential effects on fetal health may occur indirectly, which calls for further studies elucidating the impact of TiO₂ NPs on placental tissue functionality and signaling.

Safety Assessment of Graphene-Based Materials: Focus on Human Health and the Environment

Graphene and its derivatives are heralded as "miracle" materials with manifold applications in different sectors of society from electronics to energy storage to medicine. The increasing exploitation of graphene-based materials (GBMs) necessitates a comprehensive evaluation of the potential impact of these materials on human health and the environment. Here, we discuss synthesis and characterization of GBMs as well as human and environmental hazard assessment of GBMs using in vitro and in vivo model systems with the aim to understand the properties that underlie the biological effects of these materials; not all GBMs are alike, and it is essential that we disentangle the structure-activity relationships for this class of materials.

Gold nanoparticle distribution in advanced in vitro and ex vivo human placental barrier models

Background

Gold nanoparticles (AuNPs) are promising candidates to design the next generation NP-based drug formulations specifically treating maternal, fetal or placental complications with reduced side effects. Profound knowledge on AuNP distribution and effects at the human placental barrier in dependence on the particle properties and surface modifications, however, is currently lacking. Moreover, the predictive value of human placental transfer models for NP translocation studies is not yet clearly understood, in particular with regards to differences between static and dynamic exposures. To understand if small (3–4 nm) AuNPs with different surface modifications (PEGylated versus carboxylated) are taken up and cross the human placental barrier, we performed translocation studies in a static human in vitro co-culture placenta model and the dynamic human ex vivo placental perfusion model. The samples were analysed using ICP-MS, laser ablation-ICP-MS and TEM analysis for sensitive, label-free detection of AuNPs.

Results

After 24 h of exposure, both AuNP types crossed the human placental barrier in vitro, although in low amounts. Even though cellular uptake was higher for carboxylated AuNPs, translocation was slightly increased for PEGylated AuNPs. After 6 h of perfusion, only PEGylated AuNPs were observed in the fetal circulation and tissue accumulation was similar for both AuNP types. While PEGylated AuNPs were highly stable in the biological media and provided consistent results among the two placenta models, carboxylated AuNPs agglomerated and adhered to the perfusion device, resulting in different cellular doses under static and dynamic exposure conditions.

Conclusions

Gold nanoparticles cross the human placental barrier in limited amounts and accumulate in placental tissue, depending on their size- and/or surface modification. However, it is challenging to identify the contribution of individual characteristics since they often affect colloidal particle stability, resulting in different biological interaction in particular under static versus dynamic conditions. This study highlights that human ex vivo and in vitro placenta models can provide valuable mechanistic insights on NP uptake and translocation if accounting for NP stability and non-specific interactions with the test system.

Electronic supplementary material

The online version of this article (10.1186/s12951-018-0406-6) contains supplementary material, which is available to authorized users.

Developmental Toxicity of Nanomaterials: Need for a Better Understanding of Indirect Effects

While placental translocation and direct toxicity to fetal tissue of traversed nanomaterials has been a key focus of developmental toxicity studies, the release of maternal and fetal mediators that indirectly interfere with fetal development and health later in life lacks systematic insights and deserves special attention.

Translocation of silver nanoparticles in the ex vivo human placenta perfusion model characterized by single particle ICP-MS

With the extensive use of silver nanoparticles (AgNPs) in various consumer products their potential toxicity is of great concern especially for highly sensitive population groups such as pregnant women and even the developing fetus. To understand if AgNPs are taken up and cross the human placenta, we studied their translocation and accumulation in the human ex vivo placenta perfusion model by single particle ICP-MS (spICP-MS). The impact of different surface modifications on placental transfer was assessed by AgNPs with two different modifications: polyethylene glycol (AgPEG NPs) and sodium carboxylate (AgCOONa NPs). AgNPs and ionic Ag were detected in the fetal circulation in low but not negligible amounts. Slightly higher Ag translocation across the placental barrier for perfusion with AgPEG NPs and higher AgNP accumulation in placental tissue for perfusion with AgCOONa NPs were observed. Since these AgNPs are soluble in water, we tried to distinguish between the translocation of dissolved and particulate Ag. Perfusion with AgNO3 revealed the formation of Ag containing NPs in both circulations over time, of which the amount and their size in the fetal circulation were comparable to those from perfusion experiments with both AgNP types. Although we were not able to clarify whether intact AgNPs and/or Ag precipitates from dissolved Ag cross the placental barrier, our study highlights that uptake of Ag ions and/or dissolution of AgNPs in the tissue followed by re-precipitation in the fetal circulation needs to be considered as an important pathway in studies of AgNP translocation across biological barriers.

An advanced human in vitro co-culture model for translocation studies across the placental barrier

Although various drugs, environmental pollutants and nanoparticles (NP) can cross the human placental barrier and may harm the developing fetus, knowledge on predictive placental transfer rates and the underlying transport pathways is mostly lacking. Current available in vitro placental transfer models are often inappropriate for translocation studies of macromolecules or NPs and do not consider barrier function of placental endothelial cells (EC). Therefore, we developed a human placental in vitro co-culture transfer model with tight layers of trophoblasts (BeWo b30) and placental microvascular ECs (HPEC-A2) on a low-absorbing, 3 µm porous membrane. Translocation studies with four model substances and two polystyrene (PS) NPs across the individual and co-culture layers revealed that for most of these compounds, the trophoblast and the EC layer both demonstrate similar, but not additive, retention capacity. Only the paracellular marker Na-F was substantially more retained by the BeWo layer. Furthermore, simple shaking, which is often applied to mimic placental perfusion, did not alter translocation kinetics compared to static exposure. In conclusion, we developed a novel placental co-culture model, which provides predictive values for translocation of a broad variety of molecules and NPs and enables valuable mechanistic investigations on cell type-specific placental barrier function.

Impact of particle size and surface modification on gold nanoparticle penetration into human placental microtissues

Aim: Nanoparticle-based drug carriers hold great promise for the development of targeted therapies in pregnancy with reduced off-target effects. Here, we performed a mechanistic in vitro study on placental localization and penetration of gold nanoparticles (AuNPs) in dependence of particle size and surface modification. Materials & methods: AuNP uptake and penetration in human placental coculture microtissues was assessed by inductively coupled plasma-mass spectrometry, transmission electron microscopy and laser ablation-inductively coupled plasma-mass spectrometry. Results: Higher uptake and deeper penetration was observed for smaller (3–4 nm) or sodium carboxylate-modified AuNPs than for larger (13–14 nm) or PEGylate AuNPs, which barely passed the trophoblast barrier layer. Conclusion: It is possible to steer placental uptake and penetration of AuNPs by tailoring their properties, which is a prerequisite for the development of targeted therapies in pregnancy.

A 3D co-culture microtissue model of the human placenta for nanotoxicity assessment

There is increasing evidence that certain nanoparticles (NPs) can overcome the placental barrier, raising concerns on potential adverse effects on the growing fetus. But even in the absence of placental transfer, NPs may pose a risk to proper fetal development if they interfere with the viability and functionality of the placental tissue. The effects of NPs on the human placenta are not well studied or understood, and predictive in vitro placenta models to achieve mechanistic insights on NP-placenta interactions are essentially lacking. Using the scaffold-free hanging drop technology, we developed a well-organized and highly reproducible 3D co-culture microtissue (MT) model consisting of a core of placental fibroblasts surrounded by a trophoblast cell layer, which resembles the structure of the in vivo placental tissue. We could show that secretion levels of human chorionic gonadotropin (hCG) were significantly higher in 3D than in 2D cell cultures, which indicates an enhanced differentiation of trophoblasts grown on 3D MTs. NP toxicity assessment revealed that cadmium telluride (CdTe) and copper oxide (CuO) NPs but not titanium dioxide (TiO₂) NPs decreased MT viability and reduced the release of hCG. NP acute toxicity was significantly reduced in 3D co-culture MTs compared to 2D monocultures. Taken together, 3D placental MTs provide a new and promising model for the fast generation of tissue-relevant acute NP toxicity data, which are indispensable for the safe development of NPs for industrial, commercial and medical applications.

Nanoparticle transport across the placental barrier: pushing the field forward!

The human placenta is a multifunctional organ constituting the barrier between maternal and fetal tissues. Nanoparticles can cross the placental barrier, and there is increasing evidence that the extent of transfer is dependent on particle characteristics and functionalization. While translocated particles may pose risks to the growing fetus particles may also be engineered to enable new particle-based therapies in pregnancy. In both cases, a comprehensive understanding of nanoparticle uptake, accumulation and translocation is indispensable and requires predictive placental transfer models. We examine and evaluate the current literature to draw first conclusions on the possibility to steer translocation of nanoparticles. In addition, we discuss if current placental models are suitable for nanoparticle transfer studies and suggest strategies to improve their predictability.

A micropatterning approach to study the influence of actin cytoskeletal organization on polystyrene nanoparticle uptake by BeWo cells

The microcontact printing technique was successfully applied to study nanoparticle uptake in dependence on different actin cytoskeletal organizations.

Bidirectional Transfer Study of Polystyrene Nanoparticles across the Placental Barrier in an ex Vivo Human Placental Perfusion Model

BACKGROUND

Nanoparticle exposure in utero might not be a major concern yet, but it could become more important with the increasing application of nanomaterials in consumer and medical products. Several epidemiologic and in vitro studies have shown that nanoparticles can have potential toxic effects. However, nanoparticles also offer the opportunity to develop new therapeutic strategies to treat specifically either the pregnant mother or the fetus. Previous studies mainly addressed whether nanoparticles are able to cross the placental barrier. However, the transport mechanisms underlying nanoparticle translocation across the placenta are still unknown.

OBJECTIVES

In this study we examined which transport mechanisms underlie the placental transfer of nanoparticles.

METHODS

We used the ex vivo human placental perfusion model to analyze the bidirectional transfer of plain and carboxylate modified polystyrene particles in a size range between 50 and 300 nm.

RESULTS

We observed that the transport of polystyrene particles in the fetal to maternal direction was significantly higher than for the maternal to fetal direction. Regardless of their ability to cross the placental barrier and the direction of perfusion, all polystyrene particles accumulated in the syncytiotrophoblast of the placental tissue.

CONCLUSIONS

Our results indicate that the syncytiotrophoblast is the key player in regulating nanoparticle transport across the human placenta. The main mechanism underlying this translocation is not based on passive diffusion, but is likely to involve an active, energy-dependent transport pathway. These findings will be important for reproductive toxicology as well as for pharmaceutical engineering of new drug carriers.

Weathering of a carbon nanotube/epoxy nanocomposite under UV light and in water bath: impact on abraded particles

Weathering processes can influence the surface properties of composites with incorporated nanoparticles. These changes may affect the release behavior of nanoparticles when an abrasion process is applied. Therefore, the influence of two different weathering processes, immersion in water and exposure to UV light, on the properties of abraded particles from a carbon nanotube (CNT)/epoxy nanocomposite was investigated. The investigation included the measurement of the weathering impact on the surface chemistry of the exposed samples, the particle size of abraded particles, the quantity of exposed CNTs in the respirable part of the abraded particles, and the toxicity of abraded particles, measured by in vitro toxicity tests using the THP-1 monocyte-derived macrophages. The results showed that weathering by immersion in water had no influence on the properties of abraded particles. The exposure to UV light caused a degradation of the epoxy on the surface, followed by delamination of an approx. 2.5 μm thick layer. An increased quantity of exposed CNTs in abraded particles was not found; on the contrary, longer UV exposure times decreased the released fraction of CNTs from 0.6% to 0.4%. The toxicity tests revealed that abraded particles from the nanocomposites did not induce additional acute cytotoxic effects compared to particles from the neat epoxy.

Carbon Nanotubes Released from an Epoxy-Based Nanocomposite: Quantification and Particle Toxicity

Studies combining both the quantification of free nanoparticle release and the toxicological investigations of the released particles from actual nanoproducts in a real-life exposure scenario are urgently needed, yet very rare. Here, a new measurement method was established to quantify the amount of free-standing and protruding multiwalled carbon nanotubes (MWCNTs) in the respirable fraction of particles abraded from a MWCNT-epoxy nanocomposite. The quantification approach involves the prelabeling of MWCNTs with lead ions, nanocomposite production, abrasion and collection of the inhalable particle fraction, and quantification of free-standing and protruding MWCNTs by measuring the concentration of released lead ions. In vitro toxicity studies for genotoxicity, reactive oxygen species formation, and cell viability were performed using A549 human alveolar epithelial cells and THP-1 monocyte-derived macrophages. The quantification experiment revealed that in the respirable fraction of the abraded particles, approximately 4000 ppm of the MWCNTs were released as exposed MWCNTs (which could contact lung cells upon inhalation) and approximately 40 ppm as free-standing MWCNTs in the worst-case scenario. The release of exposed MWCNTs was lower for nanocomposites containing agglomerated MWCNTs. The toxicity tests revealed that the abraded particles did not induce any acute cytotoxic effects.

Transfer studies of polystyrene nanoparticles in the ex vivo human placenta perfusion model: key sources of artifacts

Nanotechnology is a rapidly expanding and highly promising new technology with many different fields of application. Consequently, the investigation of engineered nanoparticles in biological systems is steadily increasing. Questions about the safety of such engineered nanoparticles are very important and the most critical subject with regard to the penetration of biological barriers allowing particle distribution throughout the human body. Such translocation studies are technically challenging and many issues have to be considered to obtain meaningful and comparable results. Here we report on the transfer of polystyrene nanoparticles across the human placenta using an ex vivo human placenta perfusion model. We provide an overview of several challenges that can potentially occur in any translocation study in relation to particle size distribution, functionalization and stability of labels. In conclusion, a careful assessment of nanoparticle properties in a physiologically relevant milieu is as challenging and important as the actual study of nanoparticle-cell interactions itself.

Profilin 1 is required for peripheral nervous system myelination

Myelination allows rapid saltatory propagation of action potentials along the axon and is an essential prerequisite for the normal functioning of the nervous system. During peripheral nervous system (PNS) development, myelin-forming Schwann cells (SCs) generate radial lamellipodia to sort and ensheath axons. This process requires controlled cytoskeletal remodeling, and we show that SC lamellipodia formation depends on the function of profilin 1 (Pfn1), an actin-binding protein involved in microfilament polymerization. Pfn1 is inhibited upon phosphorylation by ROCK, a downstream effector of the integrin linked kinase pathway. Thus, a dramatic reduction of radial lamellipodia formation is observed in SCs lacking integrin-linked kinase or treated with the Rho/ROCK activator lysophosphatidic acid. Knocking down Pfn1 expression by lentiviral-mediated shRNA delivery impairs SC lamellipodia formation in vitro, suggesting a direct role for this protein in PNS myelination. Indeed, SC-specific gene ablation of Pfn1 in mice led to profound radial sorting and myelination defects, confirming a central role for this protein in PNS development. Our data identify Pfn1 as a key effector of the integrin linked kinase/Rho/ROCK pathway. This pathway, acting in parallel with integrin β1/LCK/Rac1 and their effectors critically regulates SC lamellipodia formation, radial sorting and myelination during peripheral nervous system maturation.

Gene expression profiling of immune-competent human cells exposed to engineered zinc oxide or titanium dioxide nanoparticles

A comprehensive in vitro assessment of two commercial metal oxide nanoparticles, TiO₂ and ZnO, was performed using human monocyte-derived macrophages (HMDM), monocyte-derived dendritic cells (MDDC), and Jurkat T cell leukemia-derived cell line. TiO₂ nanoparticles were found to be non-toxic whereas ZnO nanoparticles caused dose-dependent cell death. Subsequently, global gene expression profiling was performed to identify transcriptional response underlying the cytotoxicity caused by ZnO nanoparticles. Analysis was done with doses 1 µg/ml and 10 µg/ml after 6 and 24 h of exposure. Interestingly, 2703 genes were significantly differentially expressed in HMDM upon exposure to 10 µg/ml ZnO nanoparticles, while in MDDCs only 12 genes were affected. In Jurkat cells, 980 genes were differentially expressed. It is noteworthy that only the gene expression of metallothioneins was upregulated in all the three cell types and a notable proportion of the genes were regulated in a cell type-specific manner. Gene ontology analysis revealed that the top biological processes disturbed in HMDM and Jurkat cells were regulating cell death and growth. In addition, genes controlling immune system development were affected. Using a panel of modified ZnO nanoparticles, we obtained an additional support that the cellular response to ZnO nanoparticles is largely dependent on particle dissolution and show that the ligand used to modify ZnO nanoparticles modulates Zn²⁺ leaching. Overall, the study provides an extensive resource of transcriptional markers for mediating ZnO nanoparticle-induced toxicity for further mechanistic studies, and demonstrates the value of assessing nanoparticle responses through a combined transcriptomics and bioinformatics approach.

Effects of subtoxic concentrations of TiO2 and ZnO nanoparticles on human lymphocytes, dendritic cells and exosome production

Metal oxide nanoparticles are widely used in the paint and coating industry as well as in cosmetics, but the knowledge of their possible interactions with the immune system is very limited. Our aims were to investigate if commercially available TiO(2) and ZnO nanoparticles may affect different human immune cells and their production of exosomes, nano-sized vesicles that have a role in cell to cell communication. We found that the TiO(2) or ZnO nanoparticles at concentrations from 1 to 100μg/mL did not affect the viability of primary human peripheral blood mononuclear cells (PBMC). In contrast, monocyte-derived dendritic cells (MDDC) reacted with a dose dependent increase in cell death and caspase activity to ZnO but not to TiO(2) nanoparticles. Non-toxic exposure, 10μg/mL, to TiO(2) and ZnO nanoparticles did not significantly alter the phenotype of MDDC. Interestingly, ZnO but not TiO(2) nanoparticles induced a down regulation of FcγRIII (CD16) expression on NK-cells in the PBMC population, suggesting that subtoxic concentrations of ZnO nanoparticles might have an effect on FcγR-mediated immune responses. The phenotype and size of exosomes produced by PBMC or MDDC exposed to the nanoparticles were similar to that of exosomes harvested from control cultures. TiO(2) or ZnO nanoparticles could not be detected within or associated to exosomes as analyzed with TEM. We conclude that TiO(2) and ZnO nanoparticles differently affect immune cells and that evaluations of nanoparticles should be performed even at subtoxic concentrations on different primary human immune cells when investigating potential effects on immune functions.

Knocking at the door of the unborn child: engineered nanoparticles at the human placental barrier

Exposure of pregnant women and their unborn children to engineered nanoparticles (NPs) is not yet of major public concern. However, this may soon change in light of the ever-increasing production of NPs and the continuous appearance of novel NP-containing consumer products. However, NPs may not only pose risks to exposed individuals; they offer major potential for the development of novel therapeutic strategies to treat specifically either the mother or the developing foetus. Hence there is every reason to explore the transplacental transfer of engineered NPs in more detail, and to find answers to the vast number of open questions in this fascinating field of research.

Efficient internalization of silica-coated iron oxide nanoparticles of different sizes by primary human macrophages and dendritic cells

Engineered nanoparticles are being considered for a wide range of biomedical applications, from magnetic resonance imaging to "smart" drug delivery systems. The development of novel nanomaterials for biomedical applications must be accompanied by careful scrutiny of their biocompatibility. In this regard, particular attention should be paid to the possible interactions between nanoparticles and cells of the immune system, our primary defense system against foreign invasion. On the other hand, labeling of immune cells serves as an ideal tool for visualization, diagnosis or treatment of inflammatory processes, which requires the efficient internalization of the nanoparticles into the cells of interest. Here, we compare novel monodispersed silica-coated iron oxide nanoparticles with commercially available dextran-coated iron oxide nanoparticles. The silica-coated iron oxide nanoparticles displayed excellent magnetic properties. Furthermore, they were non-toxic to primary human monocyte-derived macrophages at all doses tested whereas dose-dependent toxicity of the smaller silica-coated nanoparticles (30nm and 50nm) was observed for primary monocyte-derived dendritic cells, but not for the similarly small dextran-coated iron oxide nanoparticles. No macrophage or dendritic cell secretion of pro-inflammatory cytokines was observed upon administration of nanoparticles. The silica-coated iron oxide nanoparticles were taken up to a significantly higher degree when compared to the dextran-coated nanoparticles, irrespective of size. Cellular internalization of the silica-coated nanoparticles was through an active, actin cytoskeleton-dependent process. We conclude that these novel silica-coated iron oxide nanoparticles are promising materials for medical imaging, cell tracking and other biomedical applications.