Environmental Life Cycle Assessment of Nanosilver-Enabled Bandages

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Environmental Life Cycle Assessment of Nanosilver-Enabled Bandages Leila Pourzahedi, and Matthew J. Eckelman Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/es504655y • Publication Date (Web): 09 Dec 2014 Downloaded from http://pubs.acs.org on December 14, 2014

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Environmental Science & Technology

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Environmental Life Cycle Assessment of Nanosilver-Enabled Bandages

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Leila Pourzahedi and Matthew J. Eckelman*

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Department of Civil and Environmental Engineering, Northeastern University, 360 Huntington Avenue, Boston, MA 02115, USA *

Corresponding Author: [email protected], +1 (617) 373 4256

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Abstract Over 400 tons of silver nanoparticles (AgNPs) are produced annually, 30% of which are used in medical applications due to their antibacterial properties. The widespread use of AgNPs has implications over the entire life cycle of medical products, from production to disposal, including but not limited to environmental releases of nanomaterials themselves. Here a cradle-to-grave life cycle assessment (LCA) from nanoparticle synthesis to end-of-life incineration was performed for a commercially available nanosilver-enabled medical bandage. Emissions were linked to multiple categories of environmental impacts, making primary use of the TRACI 2.1 impact assessment method, with specific consideration of nanosilver releases relative to all other (non-nanosilver) emissions. Modeling results suggest that (1) environmental impacts of AgNP synthesis are dominated by upstream electricity production, with the exception of life cycle ecotoxicity where the largest contributor is mining wastes, (2) AgNPs are the largest contributor to impacts of the bandage for all impact categories considered despite low AgNP loading, and (3) impacts of bandage production are several times those bandage incineration, including nanosilver releases to the environment. These results can be used to prioritize research and policy measures in order to improve the overall ecotoxicity burdens of nano-enabled products under a life cycle framework.

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1. Introduction

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For hundreds of years, silver has been widely used in a range of technologies for its physical and chemical properties, such as high electrical conductivity, photosensitivity, and bioactivity. Production and applications of nanosilver have also existed for more than a century, typically in colloidal form, first for ingestion as a medicinal tonic, and later for direct application as a biocidal agent.1 More recently, nanosilver has been incorporated within a broad range of consumer products. Over 400 metric tons of silver nanoparticles (AgNPs) are produced globally each year, with applications in cosmetics, textiles and electronics.2 Almost 30% of this net global production of AgNPs are incorporated into medical supplies and devices due to their bactericidal properties,2 yet there has been limited research on the fate of AgNPs used in health care settings, or on the significance of the life cycle impacts of these nanoparticles in the context of medical devices.3–6

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1.1 Silver Nanoparticle Releases from Textiles

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The growing number of nanosilver applications in consumer products worldwide has led to increasing concerns regarding the potential for direct human exposure and releases to the environment. Both experimental and mathematical models have been developed in past efforts to quantify potential AgNP releases from specific products. For textiles, releases of dissolved silver and silver nanoparticles have been reported at laboratory scales for various AgNP-enabled textiles, particularly during mechanical agitation.14–16 Modeling of AgNP releases from textiles has been used to determine the projected level of nanosilver in effluents, suggesting AgNPincorporated textiles will have higher release probabilities than all other applications, due to relatively high concentrations of particles and continuous use and wash cycles.17 AgNPs released during washing cycles are conveyed with municipal and industrial effluents to wastewater

A commercial application of AgNPs in the medical field is nanosilver-enabled wound dressing that is applied directly to severe burns and open wounds and is the focus of this study. These bandages are produced by embedding silver particles within the fabric of the bandage using a variety of physical techniques including impregnation, deposition, and coating.7 The antimicrobial properties of AgNPs allows the bandages to be effective against common gramnegative and -positive bacteria, thus promoting healing of the wounds and preventing infection.8 AgNPs work against bacteria by releasing silver ions that adhere to the cell membrane, penetrate it, and generate reactive oxygen species that cause oxidative stress and potential DNA damage.8 Numerous experiments have been carried out to test the effectiveness of these particles against common bacteria such as E. coli,9,10 with little evidence of bacterial resistance. These bandages have shown promising results in terms of reducing antibiotics use and decreasing the length of stay in hospitals and increasing healing rate of the wounds.11–13 The potential advantages of using nanosilver-enabled bandages notwithstanding, there is also concern regarding the health and environmental implications associated with the life cycle of AgNPs.


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treatment plants (WWTPs),16,18–21 contributing to measurable AgNP concentrations in wastewater.14,15 AgNPs in WWTPs may be removed by treatment processes but could be reintroduced to the environment through the incineration or use of biosolids as soil amendments or, ultimately contributing to terrestrial and aquatic ecotoxicity.16 AgNPs may also be released from textiles during waste management and disposal, either as particulate emissions from incineration or leached from solid waste.22–24 These end-of-life releases are particularly relevant for single-use textiles that are not subject to washing, such as the bandages under consideration here. AgNPs in textiles and medical applications have also been considered in larger-scale release and fate models that have tracked multiple engineered nanomaterials (ENMs) across a range of enduses. Early models based on partition coefficients estimated predicted environmental concentrations in Switzerland;23 these were later extended to include additional environmental compartments and geographic areas, using a probabilistic approach.25 At a global level, production, use, treatment, and disposal pathways have been reported for ten categories of ENMs,2 and later geographically disaggregated to allow for location-specific modeling of predicted concentrations.26 At a more fundamental level, Bayesian modeling has been applied to physicochemical parameters of ENMs to model fate and transport behavior and to estimate some categories of environmental impacts.27 This latter approach is particularly promising as fate and effects can vary significantly even within each class of ENMs depending on parameters such as size, surface chemistry, and environmental conditions variations that are not generally captured in the larger-scale release models.28 1.2 Life Cycle Modeling of Nanomaterials and Nano-enabled Products AgNPs and other ENMs can be released from products not just during cleaning and disposal, but also from product manufacturing and use stages, or at any point during a product’s life cycle, both accidentally and deliberately.22,29 While ENM risk assessment has focused on environmental releases, fate and transport, and subsequent exposure and effects on humans and aquatic invertebrates, impacts from non-nano emissions that occur throughout the ENM life cycle have received less attention. Compared to naturally occurring nanoparticles, ENMs are precisely manufactured to possess certain physical, chemical, or biological properties and many are highly energy intensive to produce, with low material efficiencies compared to other hightech materials and pharmaceuticals.30 Sources of manufacturing emissions and environmental impacts differ depending on the production route, but in general, high energy demands, low material efficiencies, and intensive purification requirements can all contribute to the overall impacts of the nanomaterials and nano-enabled products. Nanomanufacturing impacts derived from conventional emissions may exceed impacts from any potential releases of nanomaterials. For example, an assessment of life cycle aquatic ecotoxicity of CNTs demonstrated that non-nano emissions from CNT synthesis and upstream production of 3 ACS Paragon Plus Environment


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materials and energy resulted in ecotoxicity impacts that were several orders of magnitude greater than those from CNT releases.31 Many of these impacts can occur upstream from the actual synthesis, during production of chemicals or energy,32 and several studies have highlighted the dominant contribution of emissions of upstream electricity generation to overall emissions.33,34 Here, the distribution of life cycle environmental impacts for AgNPs were studied and checked for coherence with prior studies. Nanomaterials are rarely used by consumers in their pure form, thus, modeling of nano-enabled products must consider the benefits and potential impacts of nanomaterials in the context of the entire product and its function in the economy. As Life Cycle Assessment (LCA) models utilize functional comparisons, a product-based rather than a material-based assessment framework for engineered nanomaterials has been recommended.35 Different product categories utilize different concentrations of nanomaterials, and this obviously affects their impacts relative to the larger product. For a nanomaterial-enabled product, the presence of ENMs is not necessarily a driver of total life cycle impacts. For example, in a study of CNTs in electronic memory devices, the environmental impacts of CNTs were insignificant compared to those of the metals (particularly gold) and process inputs used during fabrication, indicating that nano-enabled devices may have negligible nano-related impacts.36 Several studies have examined life cycle impacts of nanosilver applications in textiles. A comparative cradle-to-grave LCA has been performed between a regular T-shirt and one containing AgNPs with two different routes of AgNP synthesis, stating higher global warming potential (GWP) for production stages in comparison to other life cycle phases, due to emissions during silver mining.37 A similar screening-level study for nanosilver containing socks has been performed also considering for different AgNP production methods, concluding that the overall environmental burden of the product is highly dependent on how AgNPs are synthesized and incorporated into the textiles.38 The present study extends this previous work by evaluating life cycle environmental impacts of a single-use, nanosilver-coated medical bandage, Acticoat 7. The study uses LCA to contribute to the investigation of the magnitude and primary sources of environmental impacts from nanoenabled products through three interrelated analyses: (i) process contributions to environmental impacts of AgNP synthesis (based on Acticoat 7 specifications); (ii) relative contribution of AgNPs to the life cycle impacts of the overall medical bandage; and (iii) comparison of production vs disposal stages for various categories of environmental impact.

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2. Methods and Modeling

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2.1 Scope and System Boundary END-OF-LIFE IMPACTS

CRADLE-TO-GATE LIFE CYCLE IMPACTS textiles, plastics, adhesives, energy

Material Extraction & Production

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mining, industry, transport emissions

Ag, chemicals, energy

AgNP Synthesis

stabilized AgNPs

AgNP, direct and indirect process emissions

Bandage Manufacture

Application and Use

AgNP, direct and indirect process emissions

(potential AgNP releases)

Medical Waste Incineration

particulates and ash

Figure 1. System boundary and life cycle stages. Figure 1 depicts the system boundary for this analysis, showing the life cycle of Acticoat 7 medical bandage. As specified by the manufacturer, AgNPs used in Acticoat 7 are produced via reactive magnetron sputtering.39 The synthesis route and associated material inputs and energy consumption data are described in Section 2.2. Production of basic materials, chemicals, and fuels for subsequent manufacturing steps was traced back to raw material extraction and processing. It should be noted that deposition of particles onto the bandage polymer substrate is also included in the synthesis step as it takes place simultaneously. Textile fabrication and packaging were included in the bandage manufacturing stage described in Section 2.3. Transportation of the final product was also included in this stage. Application of the bandage was assumed to take place at a hospital setting. Resources and supplies required for bandage application during use phase, such as disposable gloves, were excluded from the analysis since they are not specific to the use of nanosilver. While on the wound, bandages are passive and require no additional resource inputs for use. Leaching into the wound and subsequent bodily uptake was estimated from medical literature to be modest, at approximately 0.1% of silver contained in the bandages.40 Another article estimated 10% loss of silver from bandages to the skin surface, which may later be cleaned with sterile pads that are then discarded as medical waste.17 After removal from the wound, used bandages and cleaning pads are treated as biohazardous waste and are either incinerated (assumed here) or sterilized in an autoclave prior to disposal. Acticoat 7’s modeled end-of-life incineration is described in Section 2.4. The scope of the assessment includes all natural resource inputs and emissions that occur at each point along the life cycle, including emissions of AgNPs and other forms of silver during production and incineration, as well all relevant non-nano, non-silver emissions, such as NOx from electric power plants.

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2.2 AgNP Synthesis

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Acticoat 7 antimicrobial barrier dressings use SILCRYSTTM silver nanocrystal coating technology.41 These AgNPs are developed using reactive magnetron sputtering (RMS),39 a form of physical vapor deposition (PVD). PVD is a family of processes in which an elemental, alloy, or compound target is vaporized from a solid phase to atoms or molecules in a low pressure or vacuum chamber.42 Particles can be deposited on a substrate via sputtering, a stage where the solid surface is bombarded with the ionized gas mixture in the chamber.42 The collision results in the ejection of particles and their deposition onto the substrate surface. Sputtering commonly involves plasma and movement of the positively charged ions towards the target surface due to their electric potential difference.42 Magnetrons are often used to provide strong magnetic fields that hold plasma particles close to the target surface.43 Magnetrons can use both direct current (DC) and radio frequency (RF) for sputtering. RF sputtering helps avoid positive charge buildup on the target and can also be used for insulating targets.42

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RMS is a PVD technique in which the deposited film is formed via chemical reaction between the gas mixture and the metal target. In RMS, a mixture of argon and reactive gases is used to deposit a film of oxides or nitrides of the target onto the substrate surface.44 Here, reactive sputtering of silver using RF (13.56 MHz) in argon-nitrogen (RMS-Ar-N) was modeled based on an experimental study and scaled up to the reference flow of 1 kg AgNPs.44In the absence of reported data, 100% conversion of the silver target to AgNPs was assumed. Resource inputs to produce 1 kg of AgNPs using the RMS-AR-N method are 10.4 g nitrogen gas mixed with 124 g argon and 28 kWh of electricity. 90% recycling was considered for all carrier gases, excluding capture and re-pressurization. The RMS-Ar-N method produces particles with a size distribution between 50 to 60 nm.

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2.3 Bandage Manufacturing and Packaging

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In this study the medical bandage manufacturing process is modeled based on the components of a commercially available wound dressing, Acticoat 7.41 This bandage is composed of five layers infused with AgNPs for faster healing of the wounds. An SEM image of this bandage can be seen in Figure 2 showing the existence of spherical nano-sized silver particles, with composition confirmed by EDAX. The samples were mounted on an aluminum pin using a carbon coated adhesive and sputter coated with conductive platinum prior to imaging. Hence the presence of aluminum, carbon, and platinum in the EDAX plot, respectively. The inner layer and two outer layers are a high density polyethylene (HDPE) mesh, with AgNPs incorporated using the RMS method.45 The two layers in between the HDPE mesh are made of non-woven rayon and polyester fabric. These layers are ultrasonically welded together. Silver concentration for a 10 cm × 12.5 cm piece of Acticoat 7 bandage was empirically derived using acid digestion and shown to be 104 mg.46 The amounts of HDPE and polyester fabric used for a 125 cm2 piece of 6 ACS Paragon Plus Environment

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bandage were also calculated by weight. The ultrasonic welding process was not modeled in the inventory due to unavailability of data. Individual packaging slips were assumed to be from bleached supercalendared paper. A summary of the bandage inventory can be found in Table S2 in the SI.

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Figure 2. Nano scale SEM image of the Acticoat 7 bandage

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2.4 End of Life

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Used bandages are collected in hospitals as hazardous waste and are transported to medical waste incineration plants.47 It was assumed that the packaging and textile burn completely during the process, which is in excess of 800 °C. Behavior of ENMs in incineration plants is highly dependent on the configuration and operating conditions, as well as the physicochemical characteristics of the nanoparticles under study. For example, CeO2 nanoparticles were found in a full-scale test as likely to remain with residues and be deposited in landfills, with current filtration technology capable of removing any nanoparticles in the flue gas.48 Other metallic nanoparticles may volatilize during incineration and condense into nanoparticles after passing through the filters.49 Incinerated AgNPs either remain in the slag or become airborne.2 Even though a majority of these particles can be captured by filtration, it was estimated that between 0.05-1% of the total AgNPs can be released into the atmosphere.2 All other incinerator emissions are modeled using existing unit processes, as described in Section 2.5. Subsequent fate of AgNPs after their release from incinerators was modeled as follows. AgNPs in sealed landfills are assumed to remain deposited there or recirculated with leachate.25 Settling of airborne particles was allocated based on area covered by water and land. For the US, 93.2% of the airborne particles were assumed to settle onto soils and the rest on surface waters.25 Longterm studies on AgNP fate in soils have demonstrated that almost 70 wt% of AgNPs remain in soil or sediment they were added to, with partial transformation to Ag2S in soils and a 7 ACS Paragon Plus Environment


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combination of Ag2S and Ag-sulfhydryl compounds in sediments50,51 This study focuses on the remaining 6.8% of airborne AgNPs that deposit onto surface waters. Nanoparticles in aquatic environments participate in specific processes that should be included in any multi-media fate and transport models, such as aggregation, sedimentation, adsorption, and dissolution.52 Multimedia models have been developed for that aid in determining concentrations of engineered nanoparticles across air, rain, surface water, soil and sediments.53 AgNPs under oxidative environments in water will not undergo complete dissolution, while partial dissolution depends on their size and shape,54 residence time, and ambient temperature.28 Water chemistry is another important factor in dissolution since chlorides, sulfides, dissolved organic carbons (DOCs) and natural organic matter (NOM) directly affect the ion release rate.55 Reaction modeling with inclusion of NOMs can be used to predict Ag ion release,56 but first order kinetics for dissolution have been shown to be adequate.28,52,53,57 Here, we based Ag dissolution rates on recent experimental studies looking at ion release from a range of particle sizes and capping agents into natural waters.58 Particles less than 10 nm in diameter showed almost 80% loss, and larger particles (50 nm) lost at most 50% of their mass over a 4 month period.58 RMS-Ar-N synthesized AgNPs have diameters between 50-60 nm, and so 50% dissociation was assumed here. The characterization factor for ecotoxicity impacts associated with free silver ion fraction of AgNPs in surface waters was used in subsequently LCA modeling (Section 2.5). Table S3 contains the life cycle inventory data for this stage of the life cycle.

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2.5 Life Cycle Assessment Modeling

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This analysis was performed using SimaPro 8.1 as the life cycle modeling platform, which allows for convenient life cycle inventory data management and assessment of environmental impacts stemming from each of the ~10,000 discrete substances emitted over the life cycle of the bandages. Each entry life cycle inventory developed for this study was matched with an appropriate unit process from the US-EI database (Earthshift, Huntington, VT), the ecoinvent database adjusted for U.S. energy inputs, with details provided in the Supplement Tables S1-S3. Environmental impacts stemming from all direct and indirect emissions were modeled using The U.S. EPA’s Tool for the Reduction and Assessment of Chemical and Other Environmental Impacts (TRACI) 2.1 impact assessment model. TRACI is a collection of linked fate-behavioreffect models that considers the following impact categories and equivalent units: ozone depletion (kg CFC-11 eq), global warming potential (kg CO2 eq), and smog formation (kg O3 eq); respiratory effects (kg PM10 eq); water and soil quality impacts including acidification (mol H+ eq) and eutrophication (kg N eq); human health impacts from toxic carcinogenic and noncarcinogenic substances (health comparative toxic units, or CTUh); and ecotoxicity (CTUe). TRACI was chosen because it is a widely used midpoint model that expresses impacts in terms of discrete environmental problems and was developed for the US context. TRACI 2.1 uses the USEtox consensus multi-media fate and exposure model to model effects on human health and ecotoxicity. The USEtox characterization factor for silver ions was combined with estimates of 8 ACS Paragon Plus Environment

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silver dissociation from AgNPs deposited in surface waters to estimate aquatic ecotoxicity due from nanosilver releases. USEtox ecotoxicity characterization factors for metals are interim, and their parameter uncertainty was considered in subsequent analysis, described in the following section.

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Monte Carlo simulation was used for uncertainty analysis associated with all inputs and model parameters. Default, log-normally distributed probability distributions were used from all background US-EI database unit processes. For direct inputs and foreground processes, probability distributions were calculated using the pedigree-matrix approach, with details provided in Table S5. Uniform distributions were assumed for AgNP release from incineration and ecotoxic effects, using the range suggested in the USEtox model. Simulation was performed over 1,000 iterations using SimaPro 8.1 and summary statistics extracted. Simulation results are described for each set of results presented in Section 3.

2.6 Uncertainty Analysis

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3. Results

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The results of this study have been organized to address the three research questions posed in the Introduction section, namely process contribution to environmental impacts of the AgNP synthesis route (Section 3.1), the relative contribution of AgNPs to overall bandage production (Section 3.2) and end-of-life impacts (Section 3.3), and the relative contribution of different life cycle stages (Section 3.4)

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3.1 AgNP Synthesis

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Analysis of process contributions to AgNP synthesis demonstrated in Figure 3, was performed based on the requirements for producing 1 kg of AgNP. Analysis of process contributions reveals that the top contributing processes to the global warming potential (GWP) are all combustion related, specifically of hard coal, natural gas, lignite, and diesel used for power generation. The largest contributor to ecotoxicity is related not to nanosilver releases but to environmental releases from bulk silver processing, namely the disposal of sulfidic tailings from mining and refining processes of copper and lead ores that contain silver, emissions that occur far upstream from AgNP synthesis. The aforementioned processes also contribute significantly to carcinogenic, non-carcinogenic and eutrophication impact categories. Acidification is mainly due to refining of copper sulfide ores through the generation of SOx emissions, blasting, and power generation processes. Figures S1-S10 demonstrate the contributing processes to silver production for each of the TRACI 2.1 impact categories, while Monte Carlo simulation results for all impact categories can be found in Table S4 in the Supplemental Information. Uncertainty for carcinogenicity and ecotoxicity were found to be the highest among all impact categories, with 9 ACS Paragon Plus Environment


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coefficients of variation (CV) of 280-300% and stemming in large part from the uncertainties in incinerator emissions and characterization factors, in particular the interim ecotoxicity characterization factor for free ionic silver. These large uncertainties hinder decision-making on the basis of life-cycle toxicity results, and future efforts should focus on improving modeling capabilities and data quality in order to reduce uncertainty. The environmental impact categories of global warming, fossil fuel depletion, acidification, ozone depletion, and smog formation all had CVs in the 35-50% range.

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2.4E+00 kg SO2 eq

6.7E+00 kg N eq

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1.6E-04 CTUh

2.4E-01 kg PM2.5

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Figure 3. Process contribution for all TRACI impact categories of RMS-Ar-N synthesis route (silver source is solid silver, gaseous elements are argon and nitrogen). Labels demonstrate the total absolute value of each impact category. Abbreviations: ozone depletion (OD), global warming (GW), photochemical smog (PS), acidification (ACF), eutrophication (EU), human health: carcinogens (HHC), human health: non-carcinogens (HHNC), human health: criteria air pollutants (HHCR), ecotoxicity (EC), fossil fuel depletion (FF).

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3.2 Bandage Production

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Life cycle environmental impacts for the production of Acticoat 7 were modeled to determine the contribution of AgNPs to the overall impacts of bandage manufacture. Figure 4 shows the results for the scenario where RMS-Ar-N would be used to produce and deposit AgNPs. This figure illustrates that the largest contributor to impacts in all categories is nanosilver, even though AgNPs make up just 6% of the bandage mass. CVs from the uncertainty analysis can be seen as the labels in Figure 4. The CV of the ecotoxicity results is among the highest, again due to large uncertainties in the fate and effect modeling for AgNPs emitted from incineration.


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Figure 4 – Bandage manufacturing process contributions. RMS-AR-N AgNPs were assumed to be incorporated. The labels on bars correlate with the CV of that impact category. Abbreviations: ozone depletion (OD), global warming (GW), photochemical smog (PS), acidification (AC), eutrophication (EU), human health: carcinogens (HHC), human health: non-carcinogens (HHNC), human health: criteria air pollutants (HHCR), ecotoxicity (EC), fossil fuel depletion (FF).

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3.3 End of Life

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Used bandages are treated as medical waste and may be incinerated after disposal. Figure 5a below represents a scenario where a 125 cm2 piece of Acticoat 7 and its packaging are burnt and between 0.05 and 1% of the nanosilver in the bandage is released into the atmosphere as particulate emissions. Incineration products of paper and plastic components of the bandage are the largest contributors for most categories of environmental impact, with the exception of ecotoxicity, where the dissociated silver has the highest contribution, as shown in Figure 5a (see Figure S6 in the SI for additional details). For the 6.8% deposition rate onto surface water and 50% dissociation, it was calculated that between 0.002-0.04 mg of silver will dissociate per bandage. Based on the USEtox characterization factor for Ag+ in freshwater (1.94×105 CTUe/kg), the ecotoxicity impact of the dissociated silver ions were calculated to be between 4×10-4 and 8×10-3 CTUe per bandage. For comparison, the normalization factor (average annual impacts per capita) for ecotoxicity from metal pollution in the US has been estimated at 1.1×104 CTUe,59 so these AgNPs make a relatively small contribution to overall ecotoxicity.

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3.4 Life Cycle Stage Comparisons

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Figure 5b illustrates a comparison among the life cycle stages of Acticoat 7 for GWP and ecotoxicity impact categories. Bandage manufacturing is responsible for higher impacts compared to conventional and AgNP emissions during disposal. Impacts from production are 11 ACS Paragon Plus Environment


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predominantly due to non-nano emissions from conventional industrial facilities, especially from silver mining and processing facilities and electric power plants. Thus efforts to reduce environmental impacts, including ecotoxicity specifically, can be advanced by not just reducing the risks of direct nanoparticle exposures but also by improving the energy and material efficiency of AgNP synthesis and reducing industrial process emissions upstream of AgNP production.

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Figure 5. (a) Process contribution to bandage incineration, case of Acticoat 7, for ecotoxicity and GWP, (b) life cycle stage comparison of Acticoat 7 bandage in terms of ecotoxicity and GWP.

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3.5. Discussion and Implications

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AgNP-enabled medical devices are an important category of nano-enabled products.60 These results characterize and quantify the life cycle impacts associated with the Acticoat 7 bandage at three levels: AgNP synthesis, AgNP impacts relative to the entire bandage, and across bandage life cycle stages. Impacts associated with AgNP synthesis dominate the cradle-to-grave impacts of the bandage, and emissions from AgNP and bandage production are several times more 12 ACS Paragon Plus Environment

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impactful than emissions from bandage incineration, including direct releases of AgNPs. For comparison, Walser et al. reported that AgNP synthesis dominated the cradle-to-gate impacts of a nano-enabled t-shirt when a plasma polymerization with Ag co-sputtering process was assumed, but were relatively insignificant when flame-spray pyrolysis AgNP synthesis was considered.37 These diverse results underscore the need to use product-specific synthesis data as opposed to generic production inventories when assessing nano-enabled products. AgNPs are finding increasing direct contact applications, yet there has been limited research on the fate of AgNPs used in health care settings, or on the significance of the life cycle impacts of these nanoparticles in the context of medical devices. Wound dressings were chosen here as the product under study as a representative of this category with high nanoparticle loading. But nanosilver is present in several types of medical supplies and a wide range of consumer products. Manufacturers choose synthesis route and subsequent purification and surface functionalization of AgNPs depending on performance specifications for the intended application, given rise to a wide range of physicochemical properties. In the context of life cycle assessment, these differences should be considered when determining an appropriate functional unit. It is important to note that the AgNP physicochemical properties and morphology affects not just product function but also environmental fate and transport of silver particles in the environment and their stability. Hence, particles developed with different techniques will both perform very differently and have different environmental impacts.7 It is therefore critical that life cycle design and assessment including consideration of ENM size, shape, and surface chemistry.61–63 The fate of AgNPs in products and in the environment is under active investigation and further work will serve to reduce the uncertainty of the results presented here. For example, it has been previously shown that only 10% of AgNPs are released from bandages during use,64 but additional empirical data of AgNPs leaching from bandages and subsequent fate will support the modeling of additional routes of AgNP disposal and potential release. Second, the fate of AgNPs in incineration plants or other solid waste management facilities has not yet been fully investigated.1765 It is hypothesized that AgNP will partially volatilize in the high furnace temperatures and then condense and have surface reactions with other flue gas constituents,17 but physicochemical characterization and toxicity testing of AgNPs following incineration has yet to be performed. Sulfidation in particular has been shown to reduce the toxicity of AgNPs in the environment, due to reduction of Ag+ availability in the environment.20,56,66 A general difficulty of applying LCA to nanomaterials and nano-enabled products is that the use confers superior properties or performance that is not attainable by the current technologies to which they are to be compared. In the case of bandages, AgNPs might replace the use of a topical antibiotic such as triclosan, and one could use standard system expansion techniques in LCA to show the benefits of reducing triclosan manufacturing. But other benefits of using AgNPs would be more difficult to capture using current LCA methods, such as reduced bacterial 13 ACS Paragon Plus Environment


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resistance to antibiotics.9,10 One area of potential future research is to develop quantitative techniques for assessing the potential health and societal benefits of nanomaterial applications that can been expressed in the same units as current LCIA methods, such as disability-adjusted life years (DALYs), as was reported recently for a LCA of a CNT-enabled gas sensor.67 Finally, as with many emerging technologies, the data used in this and nearly every other LCA study for engineered nanomaterials were gathered from bench-scale investigations. Commercial production facilities are more efficient in terms of material and energy use, hence our results should be seen as an upper bound for impacts from the production phase. Here 100% conversion of the silver target to AgNPs was assumed, and in general yield values are not reported in the literature and must also be bounded in nano LCA studies. Life cycle impacts will likely decrease as nanomaterial production volumes grow and nanotechnologies become more mature, but life cycle modeling can continue to identify opportunities for process improvements and provide context for understanding relative risks and benefits of nanotechnologies.

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Supporting Information

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Detailed descriptions of life cycle inventory data, life cycle impact assessment results, and uncertainty analysis results are provided in the Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.

445

Acknowledgments

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We acknowledge NSF award SNM-1120329 as well as the George J. Kostas Nanoscale Technology and Manufacturing Research Center at Northeastern University. We would also like to thank J. Isaacs, D. Meyer, and P. Pati for helpful discussions.


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Ag+ Ag+

Ag+

ACS Paragon Plus Environment

Environmental Life Cycle Assessment of Nanosilver-Enabled Bandages

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