THESIS

CALIFORNIA ST ATE UNIVERSITY SAN MARCOS

THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE

MASTER OF SCIENCE TN KTNESTOLOGY

TITLE: Impact of Thermoplastic Elastomer versus Neoprene Materials on Skin During Simulated

AUTHOR(S): Morgan Simmons

DATE OF SUCCESSFUL DEFENSE: 05/07/2021

THE THESIS HAS BEEN ACCEPTED BY THE THESIS COMMITTEE IN

PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF

MASTER OF SCIENCE IN KINESIOLOGY

Sean Newcomer May 10, 2021 COMMITTEE CHAIR SIGNATURE DATE

Richard Armenta May 11, 2021 COMMITTEE MEMBER SIGNATURE DATE

JeffNessler May 11, 2021 COMMITTEE MEMBER SIGNATURE DATE

COMMITTEE MEMBER SIGNATURE DATE Impact of Thermoplastic Elastomer versus Neoprene Wetsuit Materials on Skin Temperature During Simulated Surfing

Abstract Introduction: Neoprene alternatives have not been investigated in the context of wetsuit design. Thermoplastic elastomer (TPE) is a potential alternative to neoprene because it has material characteristics superior to neoprene. However, no research exists on the insulating capabilities of TPE in a wetsuit. Therefore, the purpose of the study was to examine skin temperature under TPE and neoprene wetsuit material. We hypothesized that there would be no significant difference in skin between TPE and neoprene. Methods: Eighteen male participants were recruited to complete a 60-minute simulated surfing protocol in an Endless Pool Elite Flume set at approximately 16 ⁰C. Upon arrival, participants provided informed consent and completed both a health/fitness and surf related questionnaire. Subjects were provided a custom 2mm wetsuit and completed a protocol consisting of alternating between sitting, paddling, duck diving, and laying on the board. The were designed with the TPE and neoprene on opposing sides. Subjects were equipped with thermistors to collect skin surface temperatures at 60 second intervals in 16 locations on the body. Subjects provided perceptual data regarding the flexibility and temperature differences perceived immediately following the protocol. Results: There was a significant main effect for wetsuit material for all anatomical locations except for the upper back and upper arm. Specifically, post hoc analysis indicated neoprene was significantly warmer at the upper chest for all time points beyond minute 25. Neoprene was significantly warmer in the lower abdomen, lower arm, and upper leg for all time points beyond minute 5. Neoprene was significantly warmer in the lower back and lower leg for all time points. Of the 16 participants, 18.75% reported they perceived the TPE was warmer while 31.25% perceived the neoprene was warmer. Based on the perceptual data, 50% of the participants perceived no thermal differences bilaterally. Discussion: The findings of the data suggest that TPE is an inferior wetsuit insulator in anatomical locations with prolonged cold-water exposure. Interestingly, perceptual data did not align with the skin temperature data.

1 Introduction Surfing is an action sport that is rapidly increasing in popularity. It is estimated that Americans account for 2.1 million of the 37 million surfers worldwide [1-3]. Though surfing is typically associated with warm and tropical climates, surfing occurs in a broad spectrum of environments, despite moderate to cold water and ambient air temperatures. Ensuring surfers are well equipped with a wetsuit to properly aid in thermoregulation while allowing for optimal performance is imperative. Since cold water stress is a great risk for surfers, understanding the physiological toll that cold air and water exposures have on competitive and recreational surfers is essential [5-11]. However, limited research exists on the effectiveness of neoprene wetsuits in terms of insulation. Wetsuits are traditionally made of neoprene, a synthetic rubber material. Neoprene is produced through the polymerization of chloroprene. Wetsuits are designed to let small amounts of water in to create a thin layer of water between then skin and the suit [4]. The body then heats this layer of trapped water close to body temperature [4]. Closed cell neoprene, or foamed neoprene, used in wetsuit construction contains bubbles of air, nitrogen, or hydrogen gas, creating the insulating capacity to keep wetsuit users warm during activity. [4, 12]. Though the layer of water between the body and the suit provides added insulation in a wetsuit, the main determinant of the insulating capabilities is the foamed neoprene [4, 12]. Current research suggests that neoprene wetsuits may pose both environmental and health related [13-15]. Evidence of chloroprene exposure in rats and mice has been linked to carcinogenicity at various organ sites [14]. Evidence also suggests that high doses of chloroprene can be absorbed through the skin [13]. Consequences for toxin exposure from chloroprene range from dermatitis to increase cancer risk [13, 14]. Cases of chloroprene rubber exposure in Armenia, China, and Russia have also been linked to an increased risk of developing liver cancer [14]. Chloroprene is currently classified as a 2B IARC, indicating it is reasonably projected to be a human carcinogen based on experimental trials in animals [13, 14]. Additionally, synthetic rubbers such as polychloroprene produce high levels of waste [16]. In 2004, synthetic rubbers created 47,000 tons of waste with only 44% recycled [16]. Rubber pyrolysis is a common practice in recycling synthetic rubbers, but recycling polymers such as polychloroprene can be a challenge since polychloroprene contains chlorine, meaning attempts at recycling the material could further damage the materials when trying to separate out the chlorine [16, 17]. Furthermore, safe, and efficient methods of recycling polychloroprene are still being pursued and modified. Neoprene use in wetsuits comes with the risk of toxin exposure and minimal recycling opportunities, therefore, exploring alternative materials in wetsuit design could lead to more sustainable production. Thermoplastic elastomers are polymer materials that contain the thermal properties of thermoplastics coupled with the flexibility of rubber [18, 19]. The elasticity and thermal capabilities of TPE implies room for further innovation and future applications [18]. The dynamic components of thermoplastic elastomers make it a competitive substitute for neoprene in wetsuit design since TPE waste and defects can be recycled [19]. Specifically, the thermal properties of TPE can be reversed so it can be processed through melt extrusion and injection molding [18]. The cost effective and recyclable properties of TPE indicates it would be a viable replacement for neoprene. Waste reduction is not the only benefit TPE presents. TPE is typically used in situations where thermal stability and wear resistance are required [20]. Since 2012, there has been a significant increase in TPE use for medical application [21]. Thermoplastics have been used for further developing artificial skin and prosthetics [22, 23]. Thermoplastic polyurethane elastomer (TPU) has been shown to reduce surface friction at the skin of the lower leg and minimize skin irritation [23]. Thermoplastic polyurethane has also been evaluated as an insulating material for long life cardiac pacing leads [24]. Test results indicate that thermoplastic polyurethanes are non‐carcinogenic, non-toxic, exceptionally tough, hydrolytically stable, and extremely static in a natural environment [20-24]. Because polyurethanes have high levels of tensile strength, very thin layers are required to decrease the diameter of the leads with the same insulating capacity [24]. If TPE provides similar insulation to neoprene with thinner layers, it could enable surfers to maintain preferred body temperatures while increasing the tensile strength, making the wetsuit more durable. TPE encompasses characteristics that theoretically rival those of neoprene in wetsuit design. However, there is no research directly comparing the insulating capacity of TPE and neoprene in wetsuits. Therefore, the purpose of the study was to examine skin temperature under TPE and neoprene wetsuit material . We hypothesized that there would be no significant difference in skin temperatures between TPE and neoprene during a simulated surfing protocol.

2 Methods 2.1 Participants: Eighteen male recreational surfers ages 18-50 were recruited for this study. Subjects were required to have a minimum of 5 years of shortboard surfing experience and fit into either a medium or large custom 2mm wetsuit. Subjects were a convenience sample recruited via word of mouth at California State University San Marcos and throughout county beaches. Exclusion criteria included restrictions on age (18-50) and sex (male). Subject characteristics are reported in Table 1. Participants filled out an Informed Consent form and an AHA/ACSM Health/Fitness Facility Preparticipation Screening Questionnaire prior to beginning the protocol. A Surfing and Physical Activity Questionnaire was also completed to gauge surfing preference, competency, and frequency.

Table 1. Subject characteristics

Subjects Height (m) Mass (kg) Age Years Surfing Competency Surfing (1-10) Hours/Week 18 1.7±0.4 72.0±5.6 24.5±6.9 12.4±8.8 7.2±1.2 8.1±4.0 Subject characteristics reported as mean±SD

2.2 Thermistors: Following the completion of the forms, subjects were equipped with 1-Wire iButton Thermocron model DS1921L thermistors. The manufacture reported accuracy was ±0.5⁰C. However, thermistor accuracy was also assessed prior to each protocol by placing them on a Four E’s Scientific Hotplate set at 32⁰ C for a 5-minute protocol collecting data at 60s intervals. Thermistors surpassing the set limitations of 1⁰ C were disposed of and replaced. A total of 16 thermistors were placed in 8 locations on both the left and right side of each subject for bilateral comparison of the neoprene and TPE sides. The thermistors were placed on the skin above the superior pectoralis major, rhomboid, lateral triceps brachii, the inferior rectus abdominis, inferior portion of latissimus dorsi, flexor carpi radialis, lateral vastus laterali, and medial gastrocnemius (Figure 1) [10, 11]. Thermistors used at each location were randomized between subjects. The thermistors were secured to the skin with 6 cm x 7 cm, 3M Tegaderm Film. Surface skin temperatures were recorded on the thermistors at 60 second intervals. At the completion of the study, data was extracted from the thermistors and uploaded into an Excel Spreadsheet for further analysis.

Fig 1. Anatomical locations of the 16 thermistors from the anterior and posterior views.

2.3 Wetsuit: One of four 2 mm custom fullsuit wetsuits were provided for the subject to wear during the protocol. Each wetsuit was designed with half neoprene and half TPE, vertically split on either side of the middle seam. The wetsuits were constructed so that the neoprene and TPE were on different sides for each medium and large wetsuit. The study was a double-blind study. Visually, there was no difference between the left and right sides of the wetsuits (Figure 2).

Fig 2. The custom wetsuit was designed so half of the wetsuit was neoprene and the other half TPE, split along the sagittal plane. There were no visual differences in the materials on either side. 2.4 Experimental Protocol: After the subject was equipped with the thermistors and wetsuit, participants completed a simulated surfing protocol. The protocol was performed in the Endless Pool Elite Flume (Commercial Elite Endless Pools®; Aston, PA) for a total of 60 minutes at a set temperature of approximately 16⁰C. Water temperature, ambient air temperature, and relative humidity were recorded for each protocol. A standardized 5’10*19 ¾*2 ¼ surfboard was used for each subject. The Global Water Flow Meter (Global Water Instrumentation; College Station, TX) was installed prior to each protocol to measure the speed of the water flow in the flume. The subjects were asked to paddle against three speeds throughout the protocol: 1.2 m/s, 1.4 m/s, and 1.6 m/s. The speed was increased every 20 minutes. Subjects began the protocol by entering the flume and laying on the board for the first 60 seconds. Simultaneously the current was turned on and the subject was instructed to perform a shallow duck dive before beginning to paddle for the next 60 seconds. Following the paddling interval, the subject sat on the board for another 60 seconds. The subject continued to alternate between sitting, duck diving/paddling, laying, duck diving/paddling for the remaining 60 minutes. Perceptual data was recorded following the termination of the protocol. Subjects were asked if they were able to detect a difference in flexibility or warmth between the two sides of the wetsuit. 2.5 Data Analysis: The data obtained from the thermistors was downloaded in OneWire Viewer and then uploaded into Excel. Once the data was compiled in Excel it as uploaded into Matlab for further analysis. The thermistor data was compiled into 12 intervals (epochs) by averaging temperature every 5 minutes across the 60-minute protocol [26]. A two-way repeated measures ANOVA compared wetsuit material (TPE and neoprene) across time (12 epochs) [26]. Benjamini-Hochberg post hoc analysis was conducted in Excel. 3 Results 3.1 Environmental Conditions: The standardized protocol duration was 60 minutes for each subject. The average ambient air and water temperatures were 23±5.1 ⁰C and 16.0±0.1 ⁰C respectively. The Endless Pool Elite Flume was set to a standardized temperature of 16 ⁰C for each protocol to imitate water temperatures of San Diego beaches. The average relative humidity was 51.7±18.3 ⁰C. 3.2 Surface Skin Temperatures: There was a significant main effect for time for the upper chest, the upper back, the upper arm, the lower abdomen, the lower back, the lower arm, the upper leg, and the lower leg. There was a significant main effect for wetsuit material for the upper chest, the lower abdomen, the lower back, the lower arm, the upper leg, and the lower leg. Lastly, there was a significant interaction effect for the upper chest, the upper arm, lower arm, the upper leg, and lower leg. Values are indicated in Table 2. Post hoc analysis of paired t-tests with a Benjamini-Hochberg adjustment indicated skin temperatures were significantly warmer under the neoprene material at the upper chest for all time points beyond minute 25. Neoprene was significantly warmer at the lower abdomen, lower arm, and upper leg for all time points beyond minute 5. Skin temperatures were significantly warmer under the neoprene at the lower back and lower leg for all time points (Fig. 3). Table 2. Statistical analysis results Upper Upper Upper Lower Lower Lower Upper Lower Chest Back Arm Abdomen Back Arm Leg Leg

Time (p<0.001, back (p<0.001, (p<0.001, (p<0.001, (p<0.001, p<0.001, (p<0.001, 2 2 2 2 2 2 2 η partial = (p<0.005, η partial = η partial = η partial η partial η partial η partial 2 0.595) η partial 0.434) 0.732) =0.671) 0.640 =0.889 =0.942). =0.268)

Material (p<0.01, (p<0.001, (p<0.005, (p<0.001, (p<0.001, (p<0.001, 2 2 2 2 2 2 η partial = η partial η partial = η partial η partial = η partial = 0.291), =0.539), 0.393), =0.573) 0.717) 0.802)

Interaction (p<0.01, (p<0.005, (p=0.001, (p=0.005, (p=0.001, 2 2 2 2 2 η partial=0.192), η partial = η partial η partial = η partial = 0.295) =0.362) 0.355) 0.408)

Figure 3.a Upper Chest Figure 3.b Upper Back

Figure 3.c Upper Arm Figure 3.d Lower Arm

Figure 3.e Lower Abdomen Figure 3.f Lower Back

Figure 3.g Upper Leg Figure 3.h Lower Leg

Fig. 3 Comparisons of the neoprene and the TPE wetsuit materials at locations a-h over time (12 epochs). All graphs were scaled between 22⁰C-38⁰C. Bars represent the standard error. Asterisks show areas of statistical significance.

3.3 Perceptual data: Perceptual differences in temperature were recorded for 16 of the subjects immediately following the protocol. Of the 16 participants, 18.75% reported they perceived the TPE was warmer while 31.25% perceived the neoprene was warmer. Based on the perceptual data, 50% of the participants perceived no thermal differences bilaterally. Perceptual data on flexibility was also obtained. Out of the 16 subjects, 25% reported they perceived the TPE side of the wetsuit was more flexible than the neoprene side while 31.25% reported they perceived the neoprene side of the wetsuit was more flexible than the TPE side. Interestingly, 43.75% of participants perceived no differences in flexibility between the TPE and the neoprene. All perceptual data is reported in Table 2. Table 2. Perceptual data

Flexibility Perception (n=16) Thermal Perception (n=16) TPE n= 4 n= 3 Neoprene n= 5 n= 5 Equal n= 7 n= 8 4 Discussion Neoprene is a toxic synthetic rubber that produces excessive waste in wetsuit manufacturing. In contrast, TPE does not share the health limitations of neoprene. Any waste or deformities that occur during production with TPE are easily recycled due to the unique thermal properties of the material. The purpose of the study was to examine skin temperature under TPE and neoprene wetsuit material . We hypothesized that there would be no significant difference in skin temperatures when comparing data from the TPE half of the wetsuit to the neoprene half of the wetsuit during a simulated surfing protocol. The results from the current study do not support our hypothesis since skin temperatures under the neoprene wetsuit material were significantly warmer than under wetsuit TPE in six of the eight measured location. Interestingly, data from the current study suggests that subjects were unable to distinguish these differences in skin temperatures between materials. During a surfing session, only 4-8% of the time is spent with the whole body out of the water in wave riding [2, 3, 5, 6]. However, most of the surf session is spent paddling or stationary. Paddling accounts for 44-58% of a surf session, during which the upper chest, lower abdomen, lower arm, upper leg, and lower leg are all submerged in water [2, 3, 5, 6]. Similarly, stationary periods consisting of sitting or lying on the board consist of 28-42% of a surf session [2, 3, 5, 6]. In the sitting position, the lower abdomen, lower back, lower arm, upper leg, and lower leg are submerged in the water. Furthermore, the data indicates the regions with greater skin temperature differences are also the regions with the greatest amounts of cold-water exposure during a surf session. Regions including the upper chest, lower abdomen, lower back, lower arm, upper leg, and lower leg are exposed to cold water for extended durations compared to the upper back and the upper arm during a surf session [10, 11]. There were no significant differences in skin temperatures in the upper back or the upper arm, indicating TPE and neoprene have a similar thermal impact in cases of reduced cold-water exposure. However, the data reveals TPE is less effective as an insulator compared to neoprene as cold-water exposure increases over time. Prolonged thermal resistance is key to optimize performance during a surf session. Since TPE use in wetsuit design is an experimental concept, the specific variable contributing to thermal disparities when compared to neoprene is unknown. Variations in the foaming process during TPE production is a potential explanation for the differences in insulation [4]. Future studies should examine how the production of TPE differs from neoprene in wetsuit development. While it is important to investigate the insulating differences between TPE and neoprene, the perceptual differences should also be further evaluated. Eight (50%) subjects reported perceiving no differences while 3 (18.75%) subjects claimed they were more insulated in the TPE and 5 (31.25%) claimed they were more insulated in the neoprene. Interestingly, previous research suggests humans can perceive temperature differences as small as 0.003 ⁰C in ambient air [27]. Interestingly, previous research indicates surfers could detect a 1.5 °C change in skin temperature during a surf session. However, the 1.5 °C was detected in the upper chest and back. Comparatively, the average temperature change in the upper chest and upper back in our study were 0.4°C and 0.2°C, respectively, indicating the temperature differences in our study may not have been great enough to detect. Although the perceptual data should be considered from a manufacturing standpoint, considering temperature changes in skeletal muscle should also be considered to determine the threshold where surfing performance begins to decline. The nontoxic and environmental benefits of TPE rival the limitations of neoprene. However, statistical analysis revealed that TPE does not insulate as effectively as neoprene during a simulated surf session. Our results failed to support our hypothesis that there would be no significant differences in skin temperatures under TPE compared to neoprene materials. Regions with the longest periods of cold- water exposure saw the greatest differences in skin temperature. Modifications in production must be assessed to improve the thermal impact of TPE before considering replacing neoprene in wetsuit design. Producing larger air bubbles in foamed neoprene has proven to increase its insulating capabilities [4]. Future studies should investigate if this principle can be transferred to foamed TPE. Future studies should also investigate the probability of surfers wearing a thicker wetsuit made of TPE as a tradeoff to avoid toxin exposure in neoprene wetsuits.

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