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Considerations for Safe Innovation: The Case of Graphene Margriet V.D.Z. Park,* Eric A.J. Bleeker, Walter Brand, Flemming R. Cassee, Merel van Elk, Ilse Gosens, Wim H. de Jong, Johannes A.J. Meesters, Willie J.G.M. Peijnenburg, Joris T.K. Quik, Rob J. Vandebriel, and Adrien̈ ne J.A.M. Sips Rijksinstituut voor Volksgezondheid en Milieu, 3720 BA Bilthoven, The Netherlands ABSTRACT: The terms “Safe innovation” and “Safe(r)-by-design” are currently popular in the field of nanotechnology. These terms are used to describe approaches that advocate the consideration of safety aspects already at an early stage of the innovation process of (nano)materials and nanoenabled products. Here, we investigate the possibilities of considering safety aspects during various stages of the innovation process of graphene, outlining what information is already available for assessing potential hazard, exposure, and risks. In addition, we recommend further steps to be taken by various stakeholders to promote the safe production and safe use of graphene. KEYWORDS: graphene, safe innovation, production, environmental fate, occupational exposure, consumer exposure, hazard, risk assessment
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exposure (together referred to as safety) information can be collected by different stakeholders to gain insight in potential nanospecific risks of a manufactured nanomaterial and to make a balanced decision in the gates about the fate of an innovation. In the “potential” phase, a safety screening strategy was developed based on nanomaterial properties (www.nanoreg. eu,3 Deliverable and Factsheet D6.04 Inventory of existing regulatory accepted toxicity tests applicable for screening of MNMs, 2016). This strategy could be of use for all actors along the innovation chain but is mainly focused at academics and industry. The “indicator” phase has not been specified yet but aims at more knowledge on the specific uses of the nanomaterial or nanoenabled product, which allows the generation of more specific safety information on the material itself as well as on potential exposure routes. Finally, the safety information to be gathered during the “demonstrator” phase will be for regulatory purposes when the product is close to market. The EU NANoREG project has highlighted various issues that remain to be resolved regarding the regulatory risk assessment of nanomaterials, including the issue of effectively dealing with uncertain risks of such a large variety of nanomaterials. Considering the large commercial interest in graphene products and their fast development, the innovation process of graphene may serve as an example that would benefit from considering safety aspects at various stages of development, according to a similar strategy as suggested for nanomaterials in
nnovation can be seen as a sequence of stages leading from ideas to marketed products. In his Stage-Gate model, Cooper characterized each stage by its readiness to produce materials for market applications (Figure 1).1 Stages can be seen as timeframes in which information on various aspects of the innovation is gathered. This means that information is gathered on technology readiness, market potential, etc. In the present situation, innovations evolve up to stages near market introduction before regulatory risk assessors have expressed the needs to address potential health risks for humans and the environment. The innovation may get caught in a deadlock if by that time it becomes apparent that regulatory requirements or guidance documents do not fully cover the innovative aspects, leading to uncertainty about how to address their potential risks for human and environmental health. Unfortunately, this is the current situation for innovative materials like nanomaterials. In the ideal situation, consideration of safety aspects of innovative material development starts already at early stages of product development, i.e., when the novel material is at the drawing table. First initiatives toward shaping such an approach were developed within the Dutch nanotechnology program NanoNextNL (www.nanonext.nl2), by treating safety aspects as an objective rather than a constraint and by promoting the safe design of products and production processes. Furthermore, within the EU NANoREG project (www.nanoreg.eu,3 D6.3 comparison on toxicity testing in drug development and in present MNMs safety testing, 2014), a strategy has been developed for nanomaterials specifying what information on risks should be considered at each stage of innovation: the “potential” phase, the “indicator” phase, and the “demonstrator” phase (Figure 1). For each stage, various types of hazard and © 2017 American Chemical Society
Received: June 13, 2017 Accepted: September 21, 2017 Published: September 21, 2017 9574
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Figure 1. Stage-Gate innovation process including risk terminology.1 Reprinted with permission. Copyright 1990 Elsevier.
initiative (graphene-flagship.eu8). The Graphene Flagship is tasked with bringing together academic and industrial researchers to take graphene from the realm of academic laboratories into European society in the space of 10 years, thus generating economic growth, new jobs, and new opportunities. In the design of the EU Graphene Flagship project, the European Commission demanded sufficient awareness on the hazard potential and a proactive attitude in order to avoid a myriad of safety issues of this promising material. Nomenclature and Classification of Graphene. Since graphene is used in literature as a general term and is often used for graphene but also for graphene-based materials such as graphene oxide, the EU Graphene Flagship project suggested the use of a nomenclature and classification framework based on three physicochemical descriptors for graphene (Figure 2).7 The number of graphene layers determines the thickness, specific surface area, and the bending elasticity of the material. The number of graphene layers can range up to 10. Materials with a higher number of layers are categorized as ultrafine graphite, with a maximum thickness of 100 nm. The average lateral size of graphene can range from the nanoscale to the microscale (i.e., 10 nm up to various microns). The lateral size defines the maximum size of the graphene and the degree of deformability (the ability of the material to bend). The third property used for the classification of graphene is the carbon to oxygen ratio (C/O ratio). The C/O ratio influences the hydrophobicity of the graphene. The planar surface of the graphene can be functionalized with, e.g., carbonyl, hydroxyl, and epoxy groups. Oxidation of graphene improves its stability when dispersed in aqueous media.7 Based on the classification of these physicochemical properties, Bianco et al. proposed the nomenclature for graphene and graphene-based materials (Table 1).9 Next to these different structural types, graphene may also be functionalized with capping agents or coatings to make it more compatible with its application. For example, for medical purposes, graphene oxide may be coated with polyethylene glycol (PEG) to improve its biocompatibility (e.g., Xu et al.).10 The nomenclature of ISO documents can also facilitate the understanding of the various graphene products. The nomenclature of nanotechnologies as described by ISO TC 229 “Nanotechnologies” provides information on carbon-based nano-objects, whereas graphene characteristics are described in several ISO and IEC documents under development.11−13 For the remainder of this review, a distinction between different
general. If safe innovation aspects of graphene are not taken into account timely, uncertainty about the safety of production, use, and waste handling of graphene-based products remains, which may impact their commercial success, even for products that are already on the market. The aim of this review is therefore to outline what safety aspects can be considered in the “potential”, “indicator”, and “demonstrator” phases of the innovation process of graphene. We will provide an overview of available information for assessing hazard, exposure, and risks and, where relevant, indicate further steps to be taken by various stakeholders to promote the safe production and use of graphene. The Wonders of Graphene. Referred to in the media as the “miracle material of the 21st century”, graphene is a nearly transparent single-atom-thick sheet of carbon atoms ordered in a hexagonal pattern. Graphene is 100 times stronger than steel yet incredibly flexible, extremely electrically conductive, and impermeable to all gases, although the quality of the material plays a major role in achieving these superior properties.4 Due to its monolayer structure, graphene is the only solid material known in which every atom is available for chemical reaction or interaction from two sides. Oxidized forms are often also referred to as graphene, although in these cases, graphene oxide could be considered a better term, as will be discussed in the next section. The discovery of graphene and graphene-based materials is expected to transform the way many consumer and industrial products are manufactured. Its potential applications are numerous, such as advanced food packaging, foldable touch screens for mobile phones and laptops, superprotective coatings for wind turbines and ships, faster Internet speed, and batteries with dramatically higher capacity than anything available today. The number of scientific papers on graphene now exceeds 12000 per year (www.scopus.com5), and commercial interest is equally large, reflected by a total of almost 21000 patents worldwide since 2000 (patentscope.wipo.int6). Graphene is expected to be the focus of even greater interest for industrial applications when mass-produced graphene will have the same outstanding performance as the best samples produced in research laboratories.7 So far, however, researchers have been unable to produce large quantities of high-quality graphene as no scalable production method exists. This has been the subject of ongoing international research. With a budget of €1 billion, the Graphene Flagship represents a new form of joint, EU-coordinated research at an unprecedented scale, forming Europe’s biggest ever research 9575
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Figure 2. Classification of different types of graphene.7 Reprinted with permission. Copyright 2014 Wiley Online Library.
Table 1. Summary of Nomenclature for Graphene Proposed by Bianco et al.9 graphene multilayer graphene graphene oxide (GO) reduced graphene oxide (rGO) graphene nanosheet graphene nanoribbon
graphite nanoplates/sheets/flakes
a single-atom-thick sheet of hexagonally patterned carbon atoms that is not an integral part of a carbon material, but is freely suspended or adhered onto a foreign substrate. The lateral dimensions of graphene can vary from several nanometres to the microscale. a sheet-like material, either as a free-standing flake or as substrate-bound coating, consisting of a small number (between 2 and about 10) of well-defined, countable, stacked graphene layers of extended lateral dimension. chemically modified graphene prepared by oxidation and exfoliation that is accompanied by extensive oxidative modification of the basal plane. graphene oxide (as above) that has been reductively processed by chemical, thermal, microwave, photochemical, photothermal, or microbial/bacterial methods to reduce its oxygen content. a single-atom-thick sheet of hexagonally patterned carbon atoms that is not an integral part of a carbon material but is freely suspended or adhered on a foreign substrate and has a lateral dimension less than 100 nm. a single-atom-thick strip of hexagonally patterned carbon atoms that is not an integral part of a carbon material but is freely suspended or adhered onto a foreign substrate. The longer lateral dimension should exceed the shorter lateral dimension by at least an order of magnitude to be considered a ribbon, and the shorter lateral dimension (width) should be less than 100 nm to carry the prefix “nano”. graphite materials having a thickness and/or lateral dimension less than 100 nm. The use of nanoscale terminology here can help distinguish these new ultrathin forms from conventional finely milled graphite powders, whose thickness is typically >100 nm. An acceptable alternative term is “ultrathin graphite”.
types of graphene as depicted in Table 1 will be made where necessary. Characterization of Graphene. Commercially available graphene can vary widely, both in the types as outlined above but certainly also in quality. For many manufacturers, the production of high-quality graphene is still a challenge and far from standardized, resulting in a large variation between different producers or even between batches of the same producer. The intended regular flat sheets or flakes of graphene may not be achieved, showing irregularities at the surface or coiling into tube-like structures. In addition, a batch or even a sample of graphene may be heterogeneous, i.e., a mixture of different types. Moreover, similar to nanomaterials in general,14 the properties of graphene may “age”; i.e., they may change over time, depending on storage and exposure time and conditions. To enable “legitimate graphene producers to differentiate themselves from companies that claim to be selling graphene but that are instead producing some other forms of carbon containing materials”, the Graphene Council has suggested the
establishment of a regime to certify the quality and characteristics of commercially available graphene products.15 In the absence of such a regime, any researcher evaluating the health and safety of graphene should thoroughly characterize the exposure relevant material used in their studies. This characterization should include classification according to the relevant ISO/IEC documents or the system described above (Table 1), heterogeneity, identification of surface properties (presence of coating, capping agents, functionalization), irregularities, shape, and analysis of impurities. Production of Graphene. The first publication on the production and characterization of graphene by Geim and coworkers appeared in 2004.16 In 2010, they won the Nobel Prize for their work on the production, isolation, identification, and characterization of graphene.17 Their graphene films were prepared by a simple mechanical exfoliation method in which thin graphene layers were extracted from graphite with a scotch tape and transferred to a silicon substrate.16,17 9576
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ACS Nano Nowadays, graphene can be produced via two different main approaches: a top-down approach in which graphite is used to produce graphene sheets and a bottom-up approach in which graphene is synthesized using a carbon starting material18 (Table 2). Only exfoliation (a top-down approach) and
deposition, where toxic gases, high temperatures, and vacuum are used, as these have risks of their own. Awareness of potential release of graphene at any point during the production process will help to identify adequate safety measures. Additional considerations with regard to the finished graphene product should be reviewed in case graphene is intended to be administered to the human body (biocompatibility) or if at any given time graphene comes into contact with humans or the environment. Both production methods discussed above may lead to a certain level of impurities, in the form of biomolecules or solvents, which may be hazardous. In addition, production does not necessarily take place under sterile conditions, implying that the materials may become contaminated with bacterial endotoxins. Care should be taken that these impurities remain or are reduced to a level below that which may exert adverse effects in biological systems. Where relevant, organic residuals such as volatile solvents may be removed from submicron aerosol particles using thermosdenuder systems.21 Applications of Graphene. As described in the previous paragraphs, graphene has many superior characteristics compared to other materials. Therefore, graphene can be applied in many different branches of industry, for example, in electronics, generation and storage of energy, and bioapplications. Below we will describe a (nonexhaustive) list of the applications of graphene most encountered in literature and the benefits they may have. Energy Generation and Storage. Graphene shows great potential for the generation and storage of energy due to its high electrical and thermal conductivity and high surface area. Therefore, graphene can be applied in, for example, batteries, supercapacitors, and solar panels. The high thermal conductivity is beneficial because great amounts of heat are released during the production of energy. Furthermore, graphene batteries will be lighter than the current generation of commercial batteries while being charged faster.4,19,22 Sensors. Due to its properties, graphene can be used in a broad range of sensors ranging from measuring magnetic fields to DNA sequencing.19 Graphene oxide is used in biosensing as it binds strongly to biomolecules like enzymes, DNA, and antibodies. Besides, graphene oxide quenches nearby fluorescent dyes (via energy transfer from the dye to graphene oxide), it easily protonates analytes, and it can easily conduct electrons.23−25 Composite Materials, Paints, and Coatings. As graphene is highly electrically conductive and has therefore antistatic properties, it can be applied as light striking protection and can be used in conductive ink, whereas the thermal conductivity enables increasing the operating temperature. Because graphene is impermeable to water and all gases, it can reduce moisture uptake, can form a corrosion barrier and can be used in anti-UV coating. Besides, graphene is very strong and can therefore strengthen bulk materials, give resistance to pressure, and can be used as a protective layer against environmental damage while it does not crack. Overall, graphene is transparent, which is advantageous for composite materials, paints, as well as coatings.4,19,22 Electronics. Graphene is flexible, transparent, and strong and has a high electrical conductivity, making graphene suitable for electronics. Graphene might possibly be used in touch screens, rollable e-papers, OLED (organic light-emitting diodes)
Table 2. Overview of Graphene Production Methods18 top-down approaches
bottom-up approaches
exfoliation graphite intercalation micromechanical exfoliation solvent-based exfoliation arc discharge unzipping of carbon nanotubes
chemical vapor deposition epitaxial growth on SiC carbonization
chemical vapor deposition (a bottom-up approach) are commonly used to produce graphene for commercial purposes.4 Even though graphene is now commercially available, a procedure for mass production of graphene is still unavailable. Exfoliation of Graphite. Mechanical exfoliation is a simple method to produce graphene by peeling graphene layers of graphite crystals using a mechanical product like scotch tape. Several more sophisticated exfoliation techniques are available to produce graphene on an industrial scale.4 Liquid-phase exfoliation is another exfoliation technique in which graphene is produced by exposing graphite to an aqueous or nonaqueous solvent with a surfactant. The graphite splits into individual platelets during sonication, ultrasound or shear forces, which eventually results in the formation of monolayer flakes by breaking the layers in the graphite bulk material.19,20 Due to the high hydrophobicity of graphite, liquid-phase exfoliation requires the use of stabilizers or surfactants to enable dispersion in the solution. Biomolecules like proteins, nucleic acids, and polysaccharides can be used for this purpose due to their amphiphilic (i.e., possessing both hydrophilic and lipophilic properties) character. The hydrophobic part of the biomolecule can absorb to the graphene, while the hydrophilic part interacts with the solvent. Some of these biomolecules perform better than synthetic surfactants.20 The disadvantage of exfoliation methods is that sheets are often damaged or agglomerated during the production. In addition, graphite, the precursor of graphene, is scarce.18 Chemical Vapor Deposition. Chemical vapor deposition is a process to produce graphene, which uses a carbon source (e.g., methane gas), a carrier gas (e.g., hydrogen gas), and a metal substrate (e.g., copper). A mixture of the methane and hydrogen gas is sprayed into a preheated reaction chamber, which is on vacuum. The methane reacts with the surface of the metal substrate, resulting in the deposition of carbon atoms on the metal substrate. The hydrogen gas catalyzes this reaction and thereby increases the deposition of graphene on the metal substrate. The disadvantage of this method is the use of toxic gases, high temperatures, and vacuum. The subsequent transfer to another substrate often leads to impurities and defects in the graphene sheets.18,20 Furthermore, it is difficult to obtain a uniform graphene sheet due to the presence of defects, grain boundaries, and the inclusion of thick layers.4 Safer Production Considerations. Momentarily, the production processes of graphene appear to take place mostly on a small scale, but even on a laboratory scale, safety measures need to be in place, especially in the case of chemical vapor 9577
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Figure 3. Life cycle of graphene showing factors relevant to consider in a safe innovation approach. (1) Production of graphene: (a) emission to air during production processes; (b) occupational exposure assessment; (c) emission to wastewater during production processes. (2) Use and application of graphene: (a) sensor technology; (b) coatings and paints; (c) drug delivery; (d) information technology; (e) emission to air during consumer use; (f) emission to wastewater during use; (g) consumer exposure assessment; (3) toxicity testing; (a) in vitro toxicity; (b) in vivo toxicity. (4) Waste disposal: (a) agricultural application of wastewater sludge; (b) wastewater treatment. (5) Graphene in surface waters: (a) environmental exposure estimation of organisms in surface water; (b) attachment to naturally suspended particles; (c) settling from the water column to the sediments at the bottom; (d) environmental exposure estimation of organisms in sediment; (e) oxidation of graphene; (f) bioaccumulation via the food chain; (g) emission from wastewater treatment; (h) sorption of other pollutants to graphene in water. (6) Graphene in soil: (a) bioaccumulation via the food chain; (b) terrestrial exposure; (c) retention; (d) sorption of other pollutants to graphene in soil.
products composed of carbon (fullerenes and graphite nanomaterials) but not products containing graphene nanomaterial (accessed 02/08/2017). The Woodrow Wilson consumer product inventory (CPI) database contains over 1800 entries of “nanoproducts” selected from systematic Webbased searches (www.nanotechproject.org), although the nanoclaims of the products have not been verified and could therefore be false. Still, the Woodrow Wilson CPI provides an indication of potential nanomaterials and their application in products.30 The database (accessed on 20/12/2016) only contains two entries for (unsupported claims of) graphene products, both being sporting rackets from one company. The Siren packaging technology of MeadWestvaco (MWV) was the first graphene-based product becoming commercially available. This antitheft packaging, meant for retailers, gives an alarm when a customer tries to steal or open the package. Meanwhile, other, mostly sports related, graphene-based products like tennis rackets, skis, cycling helmets, bicycle race wheels, and cycling shoes are available. It is expected that touch screens and batteries are the next products that will become available via mass production.4,22 Safe Application Considerations. The number of applications of graphene on the market is limited, likely due to the problematic mass production of this material. Before this problem has been solved, it is already worthwhile to consider potential safety issues of the envisioned applications. Especially for biomedical applications, a direct contact and thus interaction of graphene with the human body is expected, and thus its hazardous properties will need to be investigated thoroughly.
devices, transistors, and display backplanes (electronics that control the pixels in a display).4,19 Photonics. Even though graphene is transparent, it interacts with light over a wide spectral range. Compared to other 2D materials, graphene absorbs a significant amount of light (around 2.3%), making graphene suitable for photonic devices like photodetectors and optical modulators.4,19 Biomedical Applications. Biomedical applications of graphene have attracted much interest in the past decade. Graphene research has been conducted in the area of drug delivery, phototherapy, and tissue engineering. Due to its high electron density, graphene can absorb different chemical substances such as drug molecules on its surface, making graphene attractive for drug delivery. Graphene oxide and reduced graphene oxide are efficient in absorbing near-infrared light which results in the generation of heat which can be used for the ablation of tumor tissue (photothermal therapy).26−28 Furthermore, graphene can be used in tissue engineering, which deals with the regeneration of damaged tissues and organs. Graphene can enhance the mechanical strength and the stiffness of the construct, while the high electrical conductivity stimulates the activity, cell growth, and cellular signaling. Besides, graphene and its derivatives can interact with biomolecules like DNA and proteins, and they can stimulate cell adhesion and proliferation.18,29 Graphene Products on the Market. A lot of effort is spent on the development of graphene-based products. However, until now, only a few of these applications seem to have reached the market. The Nanodatabase at DTU Denmark (nanodb.dk/en) contains currently 3005 products including 9578
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concentrations below 4 × 104/cm3), and elemental carbon concentrations were mostly below the detection limit, indicating very low exposure to graphene or any other particles. In another study,36 the exposure at a workplace manufacturing nanoscale graphene platelets was monitored in order to study the effectiveness of a variety of control measures during two processes. For example, during product harvesting, the airborne concentration at the source during the collection of products from the discharge vessel was measured at 2.27 and 0.017 mg/ m3 dependent on the presence of isolation valves.36 Consumer Exposure. At present, only few consumer products are known to contain graphene, and little to no information is available to be able to determine the exposure levels to graphene resulting from the use of graphene-based products. In view of the large variation of graphene-based products envisioned, oral and dermal exposure may play a more relevant role for consumer exposure compared to occupational exposure. Understanding the potential release during use and waste treatment of graphene-based consumer products is crucial to estimate exposure levels. Graphene exposure of the consumer will be dependent on the concentration and on how graphene is applied in the product, its potential for migrating out of the product, and the intensity of product use. Controlling Human Exposure to Graphene. Exposure Reduction Methods. In the previously mentioned NIOSH report, release of particles in the manufacturing location during several phases of graphene production was monitored, and the effectiveness of several control measures such as canopy hoods and exhaust ventilation were evaluated.34 Flexible enclosures were recommended to prevent release during material preparation and product harvesting in the refining process. Higher air velocities were preferred to provide good containment during product transfer. Because particle concentrations in both the production areas and the nonproduction areas were in the same order of magnitude, using separate ventilation systems and maintaining a positive pressure for the nonproduction areas was recommended. Similar exposure control measures may be worth implementing in other graphene manufacturing locations, as well. The type and effectiveness of potential exposure reducing measures will depend on the scale at which graphene is produced. Where the activities of graphite exfoliation and CVD took place at lab scale and could therefore be performed under one fume hood, this proved to be adequate as a method to reduce the number of graphene particles in the air.35 On the other hand, larger graphene production volumes via CVD may require a clean room (class 10000) with exhaust ventilation,35 which are costly and may only be feasible to invest in at later stages of the innovation process. Also, certain adaptions in process design, including isolation valves, wait time, a ventilated enclosure, a fume extractor, and an exhaust fan, may be effective in reducing the exposure to graphene.36 Reducing inhalation exposure from sprays containing graphene may be achieved by using nozzles that produce a relative large mist droplets.37 With the aim of controlling nanomaterial exposure at the workplace, various existing exposure control measures have recently been evaluated.38 This review includes a number of technical reports and guidelines published by (inter)national organisations and standardization bodies including NIOSH, EPA, OECD, and ISO that are aimed at controlling nanomaterial exposure at the workplace. Personal Protective Equipment. Material safety data sheets (MSDSs) of graphene often mention safety measures, including
While graphene incorporated in batteries, sensors, electronics, and photonics may not appear to pose a direct threat to human health or the environment during use of these products, the safety of using graphene in these products needs to be assessed throughout the life cycle, i.e., including the phase where products are discarded. In the light of Safe-by-Design, avoiding specific hazardous properties may be considered to achieve a level of inherent safety of graphene, as will be discussed later. Human Exposure to Graphene. Exposure to graphene could occur during the entire life cycle from production to waste (Figure 3). Potential exposure of workers, i.e., occupational exposure, may be relevant during all stages of product development, while direct exposure of consumers (including sensitive subpopulations) will only occur when handling finished products. The material is known to have a relative low density; i.e., bulk density of graphene nanoplatelets of up to several nm is on the order or 0.03 to 0.1 g/cm3, whereas the bulk density of graphite is 2.09−2.23 g/cm3. When aerosolized, graphene will have aerodynamic characteristics that are very similar to spherical particles, 3 and a length of at least 10 μm are not efficiently cleared by lung macrophages because they are unable to completely engulf them, causing a burst in free radicals and release of inflammatory mediators which attract macrophages and neutrophils.74 This “frustrated phagocytosis” is the biological mechanism at the basis of asbestos-related mesothelioma. The relatively large surface dimensions of graphene in combination with potential sharp edges may cause similar problems as with fibers. Some types of graphene may develop into fiber-shaped material, since the flakes or sheets may coil after production. It has also been reported that graphene materials with sharp edges may spontaneously penetrate cell membranes.75 Penetration of graphene may induce serious malformations and scarring of the tissues if the material is not completely engulfed by phagocytic cells. On the other hand, little pathology is expected at concentrations up to a few milligram per cubic meter in case a graphene flake (
