Guayule rubber medical radiation attenuation Zhenyu Li1 and Katrina Cornish2 1. EnergyEne Inc, Wooster, OH; 2. Department of Food, Agriculture and Biological Engineering, The Ohio State University, Columbus, OH

1 Introduction Radiation attenuation (RA) gloves protect and shield health care professionals (HCP) from occupational exposure to ionizing radiation. Potential end-users include radiologists, cardiologists, surgeons, and technicians who administer radiation examinations and/or perform radiation treatments. The number and types of procedures involved with radiation exposure to HCP in 2006 are shown in Table 1, and this number totaled over 16 million in 2014 in the US (2). Some fluoroscopy-assisted surgeries require the hands of the HCP to be in or near the primary radiation field. Therefore, of all body parts, the hands usually receive the highest cumulative levels of radiation (3-8). The Nuclear Regulatory Commission (NRC) limits the maximum permissible exposure for the hands to 500 mSv/year, while most institutions establish a conservative safety threshold of 50 mSv/year to maintain the exposures As Low As Reasonably Achievable (ALARA). The ALARA level is frequently exceeded for radiation workers in high risk areas (3). Various simulated and real clinical studies reported operators’ hand doses range from 30-80 μSv/min to over 500 μSv/min (4, 7). This translates to only 17 hrs of exposure to 500 μSv/min, 104 h to 80 μSv/min and 278 hrs to 30 μSv/min before the maximum permissible exposure level (500 mSv/year) is reached, all clearly much less than the work load of a practitioner. Hand exposure Table 1. US Medical Procedures with Ionizing is becoming a limiting factor for the number of Radiation Exposure, 2006 (from (1), Table 4.9). procedures some surgeons can perform, with the Procedure in 2006 Number concern being skin tissue damage causing acute or chronic radiation dermatitis (3, 9). Wearing Nonvascular procedures 8,634,600 radiation attenuation (RA) protective gloves can Noncardiac diagnostic arteriography 1,188,000 reduce hand exposure by over 40% (10), thus Noncardiac interventional vascular 1,430,000 lower the accumulative doses and associated Cardiac procedures 4,645,350 long-term health risk. Total 15,897,950 Commercially available disposable RA gloves (Table 2) differ by base elastomer, thickness, tensile properties and degree of attenuation, with attenuation levels depending on the loading of the filler attenuation compounds and film thickness (10-13). Most RA gloves are formulated with hevea (HNR) because of the higher filler capacity and tensile properties compared to materials. However, the amount of radiation-attenuating diluent fillers still causes these gloves to fail the medical glove performance standards (Table 3, ASTM D3577 for surgical gloves, and D3578 for examination gloves). According to FDA regulations, this necessitates double gloving, in which a medical glove must also be worn, to protect against pathogen transmission. The RA gloves are already much thicker than normal medical gloves, and with double-gloving, tactile sensation and hand dexterity is further reduced to the potential detriment of surgical outcome. Guayule (Parthenium argentatum Gray), a shrub from the American Southwest, produces a circumallergenic natural rubber that is softer and more elastic than traditional hevea natural rubber. The linear guayule natural rubber (GNR) polymer also allows a more integrated polymer filler network than the bulkier branched HNR polymer (14, 15). This property, in combination with its very low total protein and high fatty acid content (15), creates more “room” in the matrix enabling higher filler loading. A previous unpublished study indicates that solid GNR matrix can hold up to three times more bio-based filler than HNR, while still maintaining excellent physical properties. We predicted this high loading capacity would extend to radiation attenuation fillers. Hence, we capitalized on these unique properties of GNR to develop radiation attenuation medical glove and address the unmet market need.

2 Methods 2.1 Experimental design The main goal of our study is to prove the feasibility of fabricating radiation attenuation material filled GNR films which can meet the tensile requirements of ASTM D3577 or D3578 standards (Table 3). We used bismuth tri-oxide (Bi2O3) as an example radiation attenuation filler.

From prior knowledge, we learnt that 120 phr loading of Bi2O3 at 0.28 mm film thickness provides the minimum radiation attenuation required by ASTM D7866 standard (Table 4). We used these values as a baseline to design our RA medical gloves. Bismuth tri-oxide filler loadings of 120 phr and 150 phr, sulfur loading and vulcanization condition was also varied and tested. Target thickness was set at 28 mm and maintained by compound dipping dwell time. In addition, we also observed and tested other factors including former type and the method used to add the Bi2O3 filler into compound. A list of variables tested in this study can be found in Table 6. 2.2 Compounding 2.2.1 Formulation The base compound formulation is shown in Table 5. The sulfur emulsion, antioxidant dispersion, and zinc oxide dispersion were generously donated by Akron Dispersions (Akron, OH, USA); the ammonium hydroxide was supplied by a generic chemical supplier. The xanthate based accelerators ZDNC and DIXP were generously donated by Robinson Brothers (West Bromwich, West Midlands, United Kingdom). Attenuation filler Bi2O3 was sourced from Ferro Corporation (Product 320, Mayfield Heights, OH, USA). 2.2.2 Compound preparation

The desired amount of Bi2O3 was measured and dispersed by adding various amount of deionized water and mixed thoroughly using a handheld mixer. The compound emulsion without attenuation filler and added water was then prepared by mixing the ingredients. The compound emulsion was then added to the Bi2O3 dispersion under slow stirring. Stir speed was gradually increased to make sure Bi2O3 was evenly dispersed in the GNR latex compound. The final compound emulsion with Bi2O3 was filtered through one layer of 110 mesh silkscreen to remove impurity particles and coagulates. Compound is stored in 4-10 °C fridge overnight to allow air bubbles to exit, then used within the next 3-5 days until cumulative coagulates of about 10% total Bi2O3 weight is removed. 2.3 Thin film preparation by dipping Thin film samples were produced by dipping coagulant coated, pre-heated aluminum plate formers (DipTech Systems Inc., Kent, Ohio, USA) into prepared emulsions, followed by heating in a curing oven (Heratherm oven, Thermo Scientific, Waltham, MA, USA) to remove liquids and vulcanize the GNR. Film thicknesses were controlled by compound dwell time. All thin films were generated with a Diplomat computerized latex dipper (DipTech Systems Inc). For each treatment, two film samples were made (identical samples per dip per plate, because both sides of the plate were coated). From each dipping, we chose one side of the film from the plate for tensile testing, the other side was saved for radiation attenuation testing. 2.4 Measurements 2.4.1 Tensile Tensile measurements were performed according to ASTM D412. From the samples chosen, five dumbbell specimens were cut using Die “C”. Specimen thickness was determined as the median of three spots across the testing area measured using a Vernier caliper. The tensile properties of the specimen were determined using a tensiometer (model 3366, Instron, Norwood, MA, USA) with 50 N static load cell (model 2530-50N, Instron), coupled with a high elongation contact extensometer (model 3800, Epsilon Tech. Corp., Jackson, WY, USA). Three key tensile parameters (tensile strength, ultimate elongation, and modulus at 500% strain) were derived from the raw data with the Bluehill program (version 2, Instron). 2.4.2 Former temperature Former temperature was measured using a Milwaukee infrared temperature meter. The aluminum plate formers were painted with Rust-oleum spray paint at the non-dipping area, and temperature was measured on the painted area only. Three readings were taken at different spots for each measurement. 2.5 Tensile data analysis Analysis of variance was carried out to detect significant variation of key film properties caused by compounding and dipping variables. Inter-treatment comparisons were also conducted to find out statistically significant variations. Multivariate linear regression was performed to model film tensile performances in response to changes in compounding and processing variables. Factors were manually selected based on ANOVA result and other observations, and were screened by p-value of less than 0.01 for regression modeling.

3 Results 3.1 Compounding

Several methods were tested to disperse Bi2O3 into the compound. Adding latex directly into Bi2O3 powder caused the GNR latex to coagulate at the latex-Bi2O3 interface. This was likely due to latex local dehydration caused by the dense and heavy Bi2O3 powder. We then dispersed the Bi2O3 in water, then added GNR latex or GNR compound. This method produced acceptable compound, without large coagulates when filtered through the silk screen. 3.2 Film thickness and appearance The resultant films color (former side, same below) ranged from yellow brown to dark brown (Figure 1. Color variation of fabricated GNR-Bi2O3 film samples) and thickness ranged from 0.24 to 0.31 mm (with a mean of 0.285 ± 0.014, n = 84). 3.2.1 Film color The color of cured films darkened with increased vulcanization temperature and time (Figure 1). By manually pulling the films, we found the lighter color (yellow brown) films were under- vulcanized as they do not possess proper elasticity (deformed after pulling).

Fully vulcanized GNR-Bi2O3 films always had a medium to dark brown color on the former side. It was used as an indicator to determine and optimize vulcanization temperature and time in later experiments. 3.2.2 Film thickness The main factor determining film thickness was dwell time, and 40 s dwell time was used to maintain consistent film thickness of 0.28-0.29 mm. However, we found film thickness was also altered by the former (Error! Reference source not found.Figure 3). 3.2.3 Former used Two types of plate formers were tested first, with thickness of 3.3 mm (thin) and 6.3 mm (thick) respectively. We observed that both film thickness (Figure 3, thick plate resulted thicker film) and degree of vulcanization was affected (thick plate resulted lighter color, data not shown). We measured the former surface temperature change, and found that the thick plate formers heat up and cool down slower than the thin plate formers (Figure 2). During heating, the thick plate formers had about 5 °C lower surface temperature over the first 30 min we measured. This can be responsible for the different degree of vulcanization of films made on these two former types. During cooling, a difference of 3-5 °C was observed in the first 30-60 seconds and the difference further increased with time. This time frame is what it usually takes after the former is moved out of oven and before it enters compound. During the dwell time, and considering the higher heat capacity of the thick formers, this difference can be further enlarged, affecting amount of compound coagulated onto the former. Our film thickness data showed a significant variation of film thicknesses between the two former types (Figure 3). This result also indicated that adjustments in vulcanization process would need to be made, when transitioning to production with actual ceramic glove formers, because of the different heat capacity to the aluminum plate formers used in this initial study. 3.2.4 Added water Amount of added water was altered to determine its effect on dispersing the bismuth tri-oxide filler. After mixing, 18 phr water and 150 phr Bi2O3 was paste-like, and 24 phr water with 150 phr Bi2O3 was smoothie-like. As water was increased to 50 phr, the water and Bi2O3 phases separated very quickly once stirring stopped. We noticed that with 50 phr water, film produced were less vulcanized compared to lower water content compound under the same vulcanization condition (by film color, data not shown), likely a result of increased water evaporation time. Water content also affected Bi2O3 distribution on resulted films. With 50 phr water, we observed many horizontal dotted lines on the non-former- side of films. This was another consequence of increased vulcanization time before the Bi2O3 powder particles could be fixed by vulcanized rubber lattices.

Therefore, we concluded that water should be minimized to only moisten the Bi2O3, and must be kept constant to minimize its impact to vulcanization process. Water loading of 18 phr was used for later studies. 3.3 Film tensile performance A total of 84 films were made. The mean and standard deviation for tensile strength was 22.8 ± 3.5 MPa, ultimate elongation was 765.8% ± 36.5%, and modulus at 500% strain was 3.69 ± 0.82 MPa. Of these 84 samples, 22 passed ASTM D3577 surgical glove standard, and 79 passed the ASTM D3578 examination glove standard. Therefore, we focused our analysis on reproducibly meeting the surgical glove standard for our GNR-Bi2O3 RA medical gloves. 3.3.1 Compounding Tensile strength was found to be positively correlated with sulfur loading and negatively correlated with bismuth tri-oxide loading (Figure 4). The 5 samples that failed the exam glove standard were all compounded with 2.5 phr sulfur and 150 phr Bi2O3, with tensile strength ranging from 14.4- 17.9 MPa. The highest tensile strength obtained was a sample with 3.4 phr sulfur and 120 phr Bi2O3, with tensile strength of 30.4 MPa. Unexpectedly, instead of the bismuth tri-oxide, only sulfur loading was found to significantly affected both ultimate elongation and modulus. Ultimate elongation for all samples passed the examination glove standard, some passed surgical glove standard (Figure 5). Best ultimate elongation was obtained at 3.2 phr sulfur loading, with 150 phr Bi2O3. Elevated sulfur loading also resulted increased modulus (Figure 6). Though in this study, modulus never exceeded the maximum modulus threshold for medical glove standards.

These data suggest that surgical RA gloves at 120 phr Bi2O3 can readily be made with moderate tolerance to vulcanization variations. However, at higher filler loadings (e.g. 150 phr Bi2O3), vulcanization condition also plays a critical role. Sulfur must also be adjusted to accommodate both tensile strength and ultimate elongation. Therefore, we further analyzed vulcanization data to find optimal condition for 150 phr Bi2O3 loaded films to meet surgical glove standard. 3.3.2 Vulcanization Vulcanization temperature and time are known to affect GNR film tensile properties in non-linear pattern (16). Thus, these two factors were examined by plotting to find the optimal condition. As shown in

, we analyzed stress and strain data from all samples by vulcanization temperature and compounding formulation. Vulcanization temperatures that resulted tensile strength > 24 MPa and ultimate elongation >750% were marked wither asterisks. The optimal temperature was determined to be 90 °C as it provided more consistent tensile performances that surpassed the surgical glove standard. Similarly, we determined the optimal vulcanization time to be 60-75 min at 90 °C (Figure 8). 3.3.3 Regression modeling According to findings from compounding and vulcanization analyses, sulfur loading, bismuth tri- oxide loading, film thickness, vulcanization temperature and time were tested against the three tensile parameters. Because of the non-linearity relationship between vulcanization temperature and times, these two factors were transformed by Ln and multiplied to use as single factor. Derived regression models are listed below.

Tensile strength = 15.6+3.74*(S phr)-0.113*(Bi2O3 phr)+0.638*Ln(vulc. temp)*Ln(vulc. time) Ultimate elongation = 7.46+6.79*(film thickness)-0.096*Ln(vulc. temp)*Ln(vulc. time) Modulus = -3.3+1.2*(S phr) + 0.18*Ln(vulc. temp)*Ln(vulc. time) Predicted tensile performances were derived based on these models (Table 7), with vulcanization condition set at 90 °C and 70 min. Radiation attenuation from baseline attenuation levels required by ASTM D7866 was also estimated based on Bi2O3 loading proportional to 120 phr and film thickness proportional to 28 mm.

4 Discussion In this study, we demonstrated that it is possible to manufacture prototype guayule natural rubber radiation attenuation gloves that meet medical examination and surgical glove standards. We have targeted the unique physical properties of GNR latex towards a niche market. This is partially because the current limited GNR latex production capacity does not permit large quantities to be supplied to commodity manufacturers. More importantly, this study demonstrates that the distinct tensile profile of GNR latex coupled with its high filler capacity, opens opportunities for innovative product development, allowing new growth points in existing markets. This provides considerable value to the advancement of existing dipped rubber product industries. 4.1 Limitations Vulcanization condition is altered by former heat capacity and likely oven heating speed and capacity. Consequently, in a production setting, further optimization will be needed to reproducibly achieve surgical glove performance of guayule RA gloves with higher Bi2O3 loading. Water content of compound also affects vulcanization condition, something to be noted as Bi2O3 dispersion method may also vary during scale up. Attenuation measurements of film samples are currently being performed but were not available at time of submission of this paper. Therefore, only proportional estimation based on attenuation filler loading and thickness was provided.

We have enrolled 40 sample gloves at 150 phr Bi2O3 loading and 0.30 mm thickness into the ASTM D11.40 committee round-robin this September to analyze both attenuation level and tensile properties of these gloves. The data should provide further guidelines for GNR-Bi2O3 medical glove production. 4.2 Added benefits Guayule natural rubber latex does not contain protein epitopes that cross-react with human Type I IgE antibodies, thus does not cause Type I latex allergy. In addition, it also has an extremely low protein content, making it very unlikely to cause guayule-specific allergies (17-20). In this study, we also used EnergyEne’s novel GNR latex and xanthate based accelerator system, allowing our films to avoid the skin sensitization rashes (Type IV allergies) and contact dermatitis caused by the common chemical cross-linking accelerators usually used with HNR and synthetic polymers. These added benefits made GNR products ideal for medical uses.

Table 2. Commercially available disposable RA gloves Basic Info Tensile Performance Attenuation % Tensile Ultimate Modulus Glove Brand Base Attenuat. Thickness 60 80 100 120 Strength Elongat. (MPa @ Polymer agent (mm) kVp kVp kVp kVp (MPa) (%) 500%) ASTM Standard* Exam I (natural) 18 650 5.5 29 22 18 15 ProGuard HNR PbO 0.32 13 n/a n/a 52 41 35 29

RadiaXon HNR Bi2O3 0.38 14-16 n/a n/a 58 49 41 35 XP HNR PbO 0.40 14 n/a n/a 57 51 44 35

Ansell HNR Bi2O3 0.30 13 500 n/a 58 49 41 35 Kiran HNR Bi+other 0.35 14-16 700 n/a 63 53 46 41 ASTM Standard* Exam II (synthetic) 14 650 2.8 29 22 18 15 IBI CR W 0.30 13 n/a n/a 56 47 41 37 Specs collected from manufacturer or vendor websites. * For Tensile, ASTM D3578, Exam I (natural), or Exam II (synthetic); * For Attenuation, ASTM D7866

Table 3. Specification for Rubber Surgical/Examination Gloves (Natural) Minimum Minimum Minimum Maximum Standard # Polymer Type Thickness Tensile Strength Ultimate Modulus at 500% (mm) (MPa) Elongation (%) strain (MPa) ASTM D3577 Surgical I (natural) 0.10 24 750 5.5

ASTM D3578 Exam I (natural) 0.08 18 650 5.5

Table 4. Specification for Radiation Attenuating Protective Gloves

Energy Levels 60 kVp 80 kVp 100 kVp 120 kVp ASTM D7866 Minimum attenuation 29% 22% 18% 15%

Table 5. Base compounding recipe

Chemical phr* GNR latex 100

NH4OH 0.72 Antioxidant 2.3 ZnO 0.5 ZDNC 0.9 DIXP 1.7 Sulfur 3.2 *phr, parts per hundred rubber dry weight

Table 6. Variables involved in this study

Variable Range Unit Note

Sulfur 0.25 0.34 phr Compound ingredient. Rubber crosslinker.

Bi2O3 120 150 phr Compound ingredient. Radiation attenuation filler.

Water 20 50 phr Compound ingredient. Disperse RA filler.

Different types of former. For samples we use Former 0.33 0.63 mm plates, for actual gloves use hand former.

Thickness 0.27 0.31 mm Variation caused by fabrication process.

Cure temp 70 105 °C Vulcanization process. Affects tensile performance. Cure time 35 105 min Nonlinear relationship to tensile performances.

Table 7. Predicted tensile and attenuation performances A B C D E F Sulfur (phr) 3.2 3.3 3.4 3.2 3.4 3.6

Bi2O3 (phr) 120 140 140 150 180 200 Cure temp (°C) 90 90 90 90 90 90 Cure time (min) 70 70 70 70 70 70 Thickness (mm) 0.28 0.28 0.3 0.28 0.3 0.3

Tensile strength (MPa) 26.20 24.32 24.69 22.81 20.17 18.66 Ultimate elongation 752% 752% 766% 756% 766% 766% Modulus at 500% (MPa) 3.98 4.10 4.22 3.98 4.22 4.46 Estimated attenuation 100% 117% 125% 127% 161% 179% (% to ASTM baseline)

Figure 1. Color variation of fabricated GNR-Bi2O3 film samples. Photos taken from former side of the films dipped with thick formers and vulcanized at 90 °C for (A) 40 min, (B) 50 min, (C) 60 min, and (D) 70 min, respectively.

Plate former heating test

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) 65 Thick plate

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°

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e

r 55

u

t

a r

e 45

p

m e

t 35

25 0 5 10 15 20 25 30 time (minutes) Figure 2. Surface heating and cooling speeds vary between the thick and thin plate formers.

Film thickness vs plate + water

thick plate a 50 phr water

thick plate a 24 phr water

thin plate b 24 phr water

0.0 0.1 0.2 0.3 0.4 Film thickness (mm) Figure 3. Film thickness is significantly affected by former used, but less likely by added water. Treatments with different letter notations are significantly different (p<0.01). Error bar represents standard deviation of samples. Thick plate, 50 phr water: mean of 0.2688 ± 0.0126, n = 16; Thick plate, 24 phr water: mean of 0.2905 ± 0.0126, n = 40; Thin plate, 24 phr water: mean of 0.2817 ± 0.0017, n = 12.

Tensile stress vs Sulfur + Bi2O3

30 c

) b b a

P surgical glove(min)

M a (

20

h t

g exam glove(min)

n

e

r

t

s

e

l 10

i

s

n

e T 0 2.5 S 3.2 S 3.4 S 3.4 S 150 Bi2O3 150 Bi2O3 150 Bi2O3 120 Bi2O3

Figure 4. Tensile stress variation from sulfur and bismuth tri-oxide loadings. Treatments with different letter notations are significantly different (p<0.01). Error bar represents standard deviation of samples.

2.5 S, 150 Bi2O3: mean of 19.09 ± 1.85, n = 24;

3.2 S, 150 Bi2O3: mean of 23.47 ± 2.38, n = 12;

3.4 S, 150 Bi2O3: mean of 22.43 ± 2.29, n = 24;

3.4 S, 120 Bi2O3: mean of 26.54 ± 1.968, n = 24.

Tensile strain vs Sulfur + Bi2O3

1000 a’ a b b ) 800

surgical glove (min)

%

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n exam glove (min)

i 600

a

r

t

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0 2.5 S 3.2 S 3.4 S 3.4 S 150 Bi2O3 150 Bi2O3 150 Bi2O3 120 Bi2O3 Figure 5. Tensile strain variation from sulfur and bismuth tri-oxide loadings. Treatments with different letter notations are significantly different (p<0.01; a and a’, p<0.05). Error bar represents standard deviation of samples.

2.5 S, 150 Bi2O3: mean of 774.5 ± 31.1, n = 24;

3.2 S, 150 Bi2O3: mean of 805.2 ± 24.0, n = 12;

3.4 S, 150 Bi2O3: mean of 744.2 ± 36.4, n = 24;

3.4 S, 120 Bi2O3: mean of 759.1 ± 28.5, n = 24.

Modulus vs Sulfur + Bi2O3 )

a 6

P medical glove (max)

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M 0 2.5 S 3.2 S 3.4 S 3.4 S 150 Bi2O3 150 Bi2O3 150 Bi2O3 120 Bi2O3 Figure 6. Modulus at 500% strain from sulfur and bismuth tri-oxide loadings. Treatments with different letter notations are significantly different (p<0.01). Error bar represents standard deviation of samples.

2.5 S, 150 Bi2O3: mean of 2.98 ± 0.38, n = 24;

3.2 S, 150 Bi2O3: mean of 2.83 ± 0.21, n = 12;

3.4 S, 150 Bi2O3: mean of 4.16 ± 0.64, n = 24;

3.4 S, 120 Bi2O3: mean of 4.34 ± 0.51, n = 24.

Figure 7. Tensile performances by curing temperature and compounding formulation. Based on data from all 84 samples. Optimal curing temperature for each compounding formula is determined (** for both parameters passing surgical glove standard; * for only one). Dotted line represents minimum tensile strength and ultimate elongation requirement of surgical glove standard.

Figure 8. Tensile performances by vulcanization condition, based on data from 20 samples made with 150 phr Bi2O3 and 3.4 phr sulfur loadings. Optimal vulcanization time for each vulcanization temperature is determined (** if both parameters passing surgical glove standard; * for only one). Dotted line represents minimum tensile strength and ultimate elongation requirement of surgical glove standard.

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