Developing Fall-Impact Protection Pad with 3D Mesh Curved Surface Structure Using 3D Printing Technology

polymers

Article Developing Fall-Impact Protection Pad with 3D Mesh Curved Surface Structure Using 3D Printing Technology

Jung Hyun Park and Jeong Ran Lee *

Department of Clothing and Textiles, Pusan National University, Busan 46241, Korea; [email protected] * Correspondence: [email protected]; Tel.: +82-51-510-2841

Received: 9 October 2019; Accepted: 30 October 2019; Published: 1 November 2019

Abstract: In this study, we present the development of fall-impact protection pads for elderly people using three-dimensional (3D) printing technology. To develop fall-impact protection clothing, it is important to maintain the functionality of the protection pad while ensuring that its eﬀectiveness and appearance remain optimal in the process of inserting it. Therefore, this study explores the beneﬁt of exploiting 3D scan data of the human body using 3D printing technology to develop a fall-impact protection pad that is highly suited to the human body shape. The purpose of this study was to present a 3D modeling process for creating curved protective pads comprising a hexagonal mesh with a spacer fabric structure and to verify the impact protection performance by printing curved pads. To this end, we set up a section that includes pads in the 3D human body scan data and extracted body surface information to be applied in the generation of the pad surface. The sheet-shaped hexagonal mesh structure was cut and separated according to the pad outline, and then deformed according to the curved surface of the human body. The pads were printed, and their protection performance was evaluated; a 79.2–81.8% reduction in impact force was observed compared to similar cases in which the pads were not used.

Keywords: impact protection pad; 3D mesh; 3D printing; 3D body scan; 3D modeling; force attenuation

1. Introduction According to a report by the Korea Center for Disease Control and Prevention, an increasing number of elderly people over the age of 65 are being hospitalized due to falls [1]. Furthermore, research indicates that following transport accidents, injuries resulting from falls were the leading cause of hospitalization. Half of the fall patients were hospitalized for more than two weeks, with patients with hip fractures having the longest hospital stays. Falls frequently occur while moving on the road in winter, especially among elderly women. It is becoming increasingly important to prevent the elderly from falling. Wearing hip protectors can prevent falls or reduce the damage caused by falling [2–5]. However, existing hip protectors are not suitable for use with daily clothes; they are not widely utilized because of aesthetic limitations. Furthermore, they are not comfortable [6,7]. Therefore, it is necessary to develop an optimized impact protector with due consideration for body characteristics and motion. Additionally, it is necessary to improve the wearing satisfaction by designing the impact protector in a form that is suited to the shape and motion of the human body while maintaining its protection performance. To develop a fall-impact protection pad that is highly compatible with the human body, utilizing 3D scan data of the human body through 3D printing technology is advantageous [8]. Three-dimensional printing is a manufacturing technology that prints materials layer by layer based

Polymers 2019, 11, 1800; doi:10.3390/polym11111800 www.mdpi.com/journal/polymers Polymers 2019, 11, 1800 2 of 14 on 3D modeling information. This printing technology is widely used in small-scale production and prototype development, because it can produce 3D objects relatively easily and efficiently [9–11]. It is already in use in the fields of machinery, aerospace, automobiles, consumer goods/appliances, and medicine/dentistry [12–19]. The development of 3D computer-aided design (CAD), the emergence of low-cost, personal 3D printers, and the cost reduction of printing materials have fueled the increasing popularity of 3D printers [20–22]. In the fashion field, 3D printed clothing serves mainly artistic purposes; i.e., it is generally not functional. Such technology is actively deployed in producing accessories, such as hats, shoes, bags, and glasses, because they are small in size and require design versatility, as their production involves a wide range of materials and designs [23,24]. Valta and Sun [25] demonstrated the possibility of introducing 3D printed parts of a garment using the fused deposition modelling (FDM) technique. Eom et al. [26] studied the thermal transmission properties of 3D printed materials with a 3D spacer fabric structures for use in cold-weather clothing. Pei et al. [27] investigated the adhesion of polymer materials printed directly onto fabrics using entry-level FDM machines. Sanatgar et al. [28] analyzed 3D printing process parameters such as extruder temperature, construction platform temperature, and printing speed on the adhesion of polymers and nanocomposites to fabric. Although attempts have been made to apply 3D printing technology to various structures and forms in the field of textiles, it is still in the early stages of development [29]. Three-dimensional printing technology can be applied to develop a protective pad with a complex three-dimensional structure to fit the curved shape of the human body. Moreover, the 3D printing method makes it possible to produce prototype products relatively easily with low manufacturing costs compared to the use of conventional foam materials made by injection molding. Therefore, in this study, we propose the design of a curved protection pad based on human body surface information using 3D printing technology that, in addition to being able to produce three-dimensional shapes, has a wide range of shape and structural designs. We further printed out the protection pad and verified the impact protection performance. We extracted the pad surface from the 3D human body scan data of elderly women and applied the hexagonal mesh structure with light and flexible characteristics to the pad shape to develop a pad that provides greater satisfaction in terms of impact protection and wearing comfort. The findings of this study can further reveal the potential of 3D printing technology in the field of functional clothing.

2. Experimental Methods

2.1. 3D Modeling Three-dimensional modeling methods can be divided into polygon, non-uniform rational basis spline (NURBS), and solid modeling. The polygon method is based on triangles and a three-dimensional mesh structure. Although it offers fast and light data calculation, its accuracy is relatively poor. The NURBS method uses lines, surfaces, and solids to create three-dimensional shapes; although it requires more computation power than the polygon method, it enables accurate modeling. The solid method can calculate the weight of the actual material, thus making it possible to perform dynamic simulations and achieve modeling that incorporates the properties of the material. It can be used with airplanes, ships, and architectural designs. The calculation cost of the solid method is the highest. In this study, we use the NURBS method, specifically, Rhinoceros 5.0 (Robert McNeel & Associates, Seattle, WA, USA), i.e., the software of the NURBS method widely used in art, design, and architecture. By enabling accurate numerical modeling and polygon mesh editing, it makes it easy to apply 3D human body scan data to surface pad modeling. Therefore, we used it to model the curved surface pad. The pad designed in this study has the following characteristics: a. Curved surface pad fitted to 3D human shape. b. A pad outline with shape properly outlining the impact protection area. Polymers 2019, 11, 1800 3 of 14 c. A hexagonal three-dimensional unit that is closely connected in honeycomb form, maintaining flexibility due to the connection of each structural unit by thin plate. d. A hexagonal, three-dimensional unit structure composed of a surface layer and a spacer layer with an impact-absorbing property similar to that of the spacer fabric.

To design a protection pad with the above characteristics, a 3D modeling process was performed as follows: a. Setting a baseline over the human 3D body scan data (base surface). b. Creating a pad outline, and using it to create a flat surface. c. Placing a pad outline along the body surface, and using it to create a curved surface (target surface) that reflects the body shape. d. Designing a hexagonal mesh structure and extracting a flat pad shape conforming to the pad outline (flat pad with structural information). e. Completing the curved surface protective pad by transforming the pad of the flat hexagonal mesh structure (flat pad with structural information) according to the human body’s curved surface contours (target curved surface) through the base curved surface.

2.2. 3D Printing of Protective Pads In this study, a 3D model of a protective pad was printed using fused deposition modeling (FDM), the most popular 3D printing method. This method is cost- and time-effective. The Cubic on Single 3D printer (CUBICON Inc., Seongnam-si, Korea), that can use flexible filament material, was used in this study, because it is necessary for the protection to have flexible characteristics. The maximum size that can be printed using the Cubicon Single is 240 190 200 mm (W D H); acrylonitrile butadiene × × × × styrene copolymer (ABS), polylactic acid (PLA), and thermoplastic polyurethane (TPU) filament with a diameter of 1.75 mm can be used. The printer nozzle size is 0.4 mm. NinjaFlex® (NinjaTek, Manheim, PA, USA). A flexible TPU filament was used for printing the pad of the three-dimensional mesh ® structure. The melting point of NinjaFlex is 216 ◦C; it enables 660% elongation and has 85 Shore A hardness. The 3D model, saved as a stereolithography (STL) file, was sliced using the Cubicreator 2.5.2 (CUBICON Inc., Seongnam-si, Korea) software, and the G-code was created through setting adjustments. The nozzle temperature was set to 230 ◦C, and the bed and chamber temperature to 40 ◦C. The thickness of the layer was 0.2 mm, and the printing speed was set to 40 mm/s. Because the protective pads designed in this study were three-dimensional curved surfaces, supporters were created to enable printing. The printing was processed through the temperature setting, filament loading, leveling, extrusion, and layering. After printing, the various part of the pad, from which the supporter was removed, were placed in stretch jersey fabric in accordance with the design, and were combined into a piece of pad by stitching.

2.3. Evaluation of Impact Protection Performance To evaluate the impact protection performance of the pad, a six-pound bowling ball was dropped from varying heights to generate a force in the direction of the ground simulating a situation where the human body is impacted when it falls down. The force generated by dropping the bowling ball from heights of 6 cm, 9 cm, and 12 cm without the pad were measured. Due to the limitation of the equipment, the force at 15 cm and above could not be measured; thus, extrapolation was applied to estimate the value of the force using the results for 12 cm or below. The force generated in the ground direction can be estimated to be ~8000 N at a drop height of 20 cm and ~6500 N at a drop height of 15 cm, as shown in Figure1. Polymers 2019, 11, x FOR PEER REVIEW 4 of 14 Polymers 2019, 11, 1800 4 of 14 Polymers 2019, 11, x FOR PEER REVIEW 4 of 14 9000 9000 8000 8000 7000 7000 6000 6000 5000 5000 4000

Force (N) 4000 3000

Force (N) 3000 2000 2000 1000 1000 0 0 0 3 6 9 12151821 0 3Drop 6 height 9 12151821(cm)

Drop height (cm) Figure 1. Impact forces measured without pad. Figure 1. Impact forces measured without pad. To verify the impact protectionFigure 1. of Impact the 3D forces printing measured pads, without a six-pound pad. bowling ball was dropped To verify the impact protection of the 3D printing pads, a six-pound bowling ball was dropped from heights of 15 cm and 20 cm onto the force plate on which the pad was placed, and the impact from Toheights verify of the 15 impactcm and protection 20 cm onto of the the force 3D prin plateting on pads, which a six-pound the pad was bo wlingplaced, ball and was the dropped impact absorption rate of the pad was obtained by comparing the impact value with the situation with no pad absorptionfrom heights rate of of 15 the cm pad and was 20 cm obtained onto the by force compar plateing on the which impact the value pad waswith placed, the situation and the with impact no (Figure2). The drop test was performed 10 times at each height. To prevent damage to the sensor and padabsorption (Figure rate2). The of thedrop pad test was was obtained performed by 10 compar times ingat each the height.impact Tovalue prevent with damagethe situation to the with sensor no force plate, a 6.0 mm-thick neoprene fabric was laid on the force plate. andpad force (Figure plate, 2). Thea 6.0 drop mm-thick test was neoprene performed fabric 10 timeswas laid at each on the height. force To plate. prevent damage to the sensor and force plate, a 6.0 mm-thick neoprene fabric was laid on the force plate.

FigureFigure 2.2. Picture of the impact performance testtest equipment.equipment.

2.4. Evaluation of FlexuralFigure Stiffness 2. Picture of the impact performance test equipment. 2.4. Evaluation of Flexural Stiffness To evaluate the flexural stiffness of 3D printing pads, standard test methods of KS K 0350 (ball 2.4. EvaluationTo evaluate of theFlexural flexural Stiffness stiffness of 3D printing pads, standard test methods of KS K 0350 (ball bursting method) were used to determine the displacement distance (mm) when constant forces (5 N, burstingTo evaluatemethod) thewere flexural used tostiffness determine of 3D the printing displa cementpads, standard distance test (mm) methods when constantof KS K 0350forces (ball (5 15 N, and 30 N) were applied by a ball bursting tester (Instron, Norwood, MA, USA) on the circular N,bursting 15 N, and method) 30 N) werewere appliedused to by determine a ball bursting the displa testercement (Instron, distance Norwood, (mm) MA, wh enUSA) constant on the forcescircular (5 type of 3D printing pads. The diameter of test specimens was 10 cm with a thickness of 1 cm (Figure3). typeN, 15 of N, 3D and printing 30 N) were pads. applied The diameter by a ball of bursting test specimens tester (Instron, was 10 cmNorwood, with a thickness MA, USA) of on 1 cmthe (Figurecircular A greater displacement distance indicates more flexible materials. The strap type of the 3D printing 3).type A ofgreater 3D printing displacement pads. The distance diameter indicates of test sp moreecimens flexible was 10materials. cm with Thea thickness strap type of 1 cmof the(Figure 3D pads was also prepared to measure the flexural stiffness of the 3D printing pads using the standard printing3). A greater pads wasdisplacement also prepared distance to measure indicates the moreflexural flexible stiffness materials. of the 3D The printing strap typepads ofusing the the3D test methods of KS K 0642 (flexometer method). The size of test specimens was 2.0 cm 15.0 cm standardprinting padstest methods was also of prepared KS K 0642 to (flexometer measure the method). flexural The stiffness size of of test the specimens 3D printing was pads 2.0× cmusing × 15.0 the× 1.0 cm (Figure3). First, the test specimen was placed on the flat region of a flexometer. Then, the test cmstandard × 1.0 cm test (Figure methods 3). ofFirst, KS Kthe 0642 test (flexometer specimen wamethod).s placed The on size the offlat test region specimens of a flexometer. was 2.0 cm Then, × 15.0 specimen was pushed to a slope region, i.e., 41.5 , as measured using a flexometer. The distance of thecm test× 1.0 specimen cm (Figure was 3). pushedFirst, the to test a slope specimen region, wa◦ i.e.,s placed 41.5°, on as the measured flat region using of a a flexometer. flexometer. Then, The the test specimen was measured when the end-tip of the test specimen touched the slope region of distancethe test ofspecimen the test specimenwas pushed was to measured a slope region,when the i.e., end-tip 41.5°, ofas themeasured test specimen using toucheda flexometer. the slope The distance of the test specimen was measured when the end-tip of the test specimen touched the slope

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Polymers 2019, 11, x FOR PEER REVIEW 5 of 14 Polymers 2019, 11, x FOR PEER REVIEW 5 of 14 the flexometer. A greater length of measured distance indicates higher flexural stiffness materials. Theregion flexural of the sti flexometer.ffness was measuredA greater threelength times of meas and theured average distance value indicates was obtained. higher flexural stiffness materials. The flexural stiffness was measured three times and the average value was obtained.

Figure 3. Specimens for flexural stiffness test. Figure 3.3. Specimens for ﬂexuralflexural stistiffnessﬀness test.test.

2.5. Evaluation of Elongation Rate 2.5. Evaluation of Elongation Rate The elongation rate (%) was measured with standard test methods of KS K 0521 (strip method) by The elongation rate (%) was measured with standard test methods of KS K 0521 (strip method) a universal test machine (Instron 5582, Instron, Norwood, MA, USA). The constant forces (5 N and by a universal test machine (Instron 5582, Instron, Norwood, MA, USA). The constant forces (5 N and 15 N) were applied to measure the elongation of the 3D printed pads. The sample dimensions were set 15 N) were applied to measure the elongation of the 3D printed pads. The sample dimensions were toset 2.0 to cm2.0 cm15.0 × cm15.0 cm1.0 cm× 1.0 (Figure cm (Figure3). The strain3). The rate strain was rate 100 mmwas/ min.100 mm/min. The elongation The elongation was measured was set to 2.0× cm × 15.0× cm × 1.0 cm (Figure 3). The strain rate was 100 mm/min. The elongation was three times and the average value was obtained. measured three times and the average value was obtained. 3. Results and Discussion 3. Results and Discussion 3.1. Processing of Body Scan Data 3.1. Processing of Body Scan Data ToTo designdesign aa protectiveprotective padpad thatthat ﬁtsfits thethe humanhuman bodybody shape,shape, thethe humanhuman bodybody scanscan datadata ofof womenwomen in theirTo 60design s was a protective required. Inpad this that study, fits the 3D human scan data body in shape, the OBJ the format, human an body anthropometric scan data of women data of in their 60 s was required. In this study, 3D scan data in the OBJ format, an anthropometric data of SizeSize Korea,Korea, waswas used. used. The The hip hip circumference, circumference, lateral latera line,l line, and and posterior posterior center center line line were were drawn drawn on theon humanSize Korea, body was scan used. data. The To make hip circumference, the modeling process lateral moreline, and convenient, posterior the center human line body were was drawn divided on the human body scan data. To make the modeling process more convenient, the human body was intodivided horizontal into horizontal planes 6.5 planes cm above 6.5 thecm waistabove circumference the waist circumference and 21.5 cm belowand 21.5 the hipcm circumference;below the hip onlydivided the hipinto part horizontal was used, planes as shown 6.5 cm in Figureabove 4the. waist circumference and 21.5 cm below the hip circumference; only the hip part was used, as shown in Figure 4. (a) (a) (b)

Left Back Top

Figure 4. Procedure of body baseline setting (a) and human body scan data processing (b). Figure 4. Procedure of body baseline setting (a)) andand humanhuman bodybody scanscan datadata processingprocessing ((bb).).

3.2. Creation of Pad Outline and Base Surface In this study, the round pad outline designed using pattern CAD (YUKA) was imported into Rhinoceros 5.0. A sideline was modified by creating a vertical line at the intersection with the hip circumference. As a preprocessing step to modifying the pad outline on a flat surface to fit the 3D

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3.2. Creation of Pad Outline and Base Surface In this study, the round pad outline designed using pattern CAD (YUKA) was imported into Rhinoceros 5.0. A sideline was modified by creating a vertical line at the intersection with the hip Polymerscircumference. 2019, 11, x FOR Asa PEER preprocessing REVIEW step to modifying the pad outline on a flat surface to fit the6 of 3D 14 human body shape, the curved outline located on the plane was divided. The hip line was divided humaninto 10 parts,body shape, following the curved which aoutline vertical located line was on createdthe plane at was the splitdivided. point The and hip extended line was to divided fit the intooutline. 10 parts, To form following the surface, which horizontal a vertical baselines line was were created added at abovethe split and point below and the extended hipline. Toto formfit the a outline.surface usingTo form the the curved surface, outlines, horizontal dividing baselines the outline were intoadded the above top, bottom, and below left, the and hipline. right proved To form to abe surface effective. using Therefore, the curved the outlineoutlines was, dividing split at the the outline pointcorresponding into the top, bottom, to the corner.left, and The right horizontal proved tobaseline be effective. was also Therefore, divided into the sections. outlineThe was elements split at of the the point sections corresponding were grouped, to making the corner. it possible The horizontalto bundle andbaseline move was them also in each divided section. into We sectio createdns. The a network elements surface of the using sections the outlines were grouped, and split makinglines, and it possible set it as theto bundle ‘base surface’ and move (Figure them5). in each section. We created a network surface using the outlines and split lines, and set it as the ‘base surface’ (Figure 5).

Figure 5. StepsSteps ofof creating creating pad pad outline outline and and base base surface: surface: (a) import(a) import pad outlinespad outlines and baselines; and baselines; (b) create (b) createand segment and segment baseline; baseline; (c) divide (c) divide into 10 into sections 10 sections based based on hip on line;hip line; (d) add(d) add horizontal horizontal baselines baselines for forsurface surface creation; creation; (e) segment(e) segment and and group group curve curve according according to section; to section; and (andf) rebuild (f) rebuild curves curves and form and formnetwork network surface. surface. 3.3. Formation of Pad Surface on the Body Surface 3.3. Formation of Pad Surface on the Body Surface The outline of the pad was aligned with the baseline (lateral line, hip line) of the human body The outline of the pad was aligned with the baseline (lateral line, hip line) of the human body and rotated for each divided section to approximate the contours of the human body curved surface. and rotated for each divided section to approximate the contours of the human body curved surface. The lines constituting the pad surface were drawn over the human body surface and deformed to fit The lines constituting the pad surface were drawn over the human body surface and deformed to fit the curved surface shape. Because the 3D body surface was composed of small, triangular meshes, the curved surface shape. Because the 3D body surface was composed of small, triangular meshes, the outlines along the surface were uneven complex lines; therefore, the curves were regenerated to the outlines along the surface were uneven complex lines; therefore, the curves were regenerated to simplify and smooth them. The regenerated curves were used to create the network surfaces that simplify and smooth them. The regenerated curves were used to create the network surfaces that corresponded to the planar network surfaces. The curved network surface was to be used as a ‘target corresponded to the planar network surfaces. The curved network surface was to be used as a ‘target surface’ (Figure6). surface’ (Figure 6). 3.4. Formation of Three-Dimensional Mesh Structure Planar Pad The pad structures were generated using the three-dimensional mesh structure. The diamond-shaped surface layer and the V-shaped spacer layer generated a basic unit, and the units were connected by connectors. The hexagonal structure of the basic unit is densely connected between individual units without empty space. The hexagonal mesh structure can be also flexibly deformed to various directions compared to the rectangular structure. Therefore, due to the connector, the pad moves flexibly with

Figure 6. Steps of creating pad outline and base surface: (a) rotate and place pad outline by section; (b) transform pad outlines to fit surfaces; (c) rebuild curve; and (d) create network surface.

Polymers 2019, 11, x FOR PEER REVIEW 6 of 14

human body shape, the curved outline located on the plane was divided. The hip line was divided into 10 parts, following which a vertical line was created at the split point and extended to fit the outline. To form the surface, horizontal baselines were added above and below the hipline. To form a surface using the curved outlines, dividing the outline into the top, bottom, left, and right proved to be effective. Therefore, the outline was split at the point corresponding to the corner. The horizontal baseline was also divided into sections. The elements of the sections were grouped, making it possible to bundle and move them in each section. We created a network surface using the outlines and split lines, and set it as the ‘base surface’ (Figure 5).

Figure 5. Steps of creating pad outline and base surface: (a) import pad outlines and baselines; (b) create and segment baseline; (c) divide into 10 sections based on hip line; (d) add horizontal baselines for surface creation; (e) segment and group curve according to section; and (f) rebuild curves and form network surface. Polymers 2019, 11, 1800 7 of 14 3.3. Formation of Pad Surface on the Body Surface the movementThe outline of of the the human pad was body aligned to improve with the the baseli wearingne (lateral comfort. line, To hip apply line) a of three-dimensional the human body meshand rotated structure for each to a curveddivided pad, section it is to necessary approximate to create the contours a plane of by the repeating human body each curved unit structure, surface. andThe cuttinglines constituting the repeating the structure pad surface according were drawn to the outline over the shape. human Because body itsurface is easier and and deformed more eﬃ cientto fit tothe manipulate curved surface the curves shape. compared Because the to surfaces3D body or su solids,rface was the composed unit structure of small, forms triangular a wireframe meshes, with curves.the outlines After along repeating the surface the unit were structure, uneven as showncomplex in lines; Figure therefore,7, only the the components curves were corresponding regenerated to to thesimplify pads wereand smooth retained them. by the The pad regenerated outline surface; curves the we unnecessaryre used to create parts werethe network deleted, surfaces as shown that in Figurecorresponded8. to the planar network surfaces. The curved network surface was to be used as a ‘target Polymers 2019, 11, x FOR PEER REVIEW 7 of 14 surface’ (Figure 6). 3.4. Formation of Three-Dimensional Mesh Structure Planar Pad PolymersThe pad 2019 structures, 11, x FOR PEER were REVIEW generated using the three-dimensional mesh structure. The 7diamond- of 14 shaped surface layer and the V-shaped spacer layer generated a basic unit, and the units were 3.4. Formation of Three-Dimensional Mesh Structure Planar Pad connected by connectors. The hexagonal structure of the basic unit is densely connected between individualThe units pad withoutstructures empty were generated space. The using hexago the thnalree-dimensional mesh structure mesh can structure. be also Theflexibly diamond- deformed shaped surface layer and the V-shaped spacer layer generated a basic unit, and the units were to various directions compared to the rectangular structure. Therefore, due to the connector, the pad connected by connectors. The hexagonal structure of the basic unit is densely connected between moves flexibly with the movement of the human body to improve the wearing comfort. To apply a individual units without empty space. The hexagonal mesh structure can be also flexibly deformed three-dimensionalto various directions mesh compared structure to to the a curvedrectangular pad, structure. it is necessary Therefore, to createdue to athe plane connector, by repeating the pad each unit movesstructure, flexibly and with cutting the movementthe repeating of the structure human bo accordingdy to improve to the the outline wearing shape. comfort. Because To apply it is a easier and morethree-dimensional efficient to manipulatemesh structure the to curves a curved compared pad, it is necessary to surfaces to createor solids, a plane the by unit repeating structure each forms a wireframeunit structure, with and curves. cutting After the repeating repeating structure the accordingunit structure, to the outlineas shown shape. in Because Figure it is7, easier only the FigureFigure 6.6. StepsSteps ofof creatingcreating padpad outlineoutline andand basebase surface:surface: (a) rotaterotate andand placeplace padpad outlineoutline byby section;section; componentsand more corresponding efficient to manipulate to the thepads curves were compared retained to by surfaces the pad or solids,outline the surface; unit structure the unnecessary forms ((bb)) transformtransform padpad outlinesoutlines toto ﬁtfit surfaces;surfaces; ( (cc)) rebuild rebuild curve; curve; and and ( d(d)) create create network network surface. surface. partsa werewireframe deleted, with as showncurves. inAfter Figure repeating 8. the unit structure, as shown in Figure 7, only the components corresponding to the pads were retained by the pad outline surface; the unnecessary parts were deleted, as shown in Figure 8.

FigureFigure 7. 7. WireframeWireframe Wireframe of hexagonal hexagonal mesh mesh structures. structures.

FigureFigure 8. 8.Cutting Cutting procedure procedure of the pad pad structure structure to tomatch match pad pad shape. shape. Figure 8. Cutting procedure of the pad structure to match pad shape. TheThe thickness thickness of theof the pads pads was was adjusted adjusted using the the cage cage function. function. To Toachieve achieve a natural a natural appearance appearance whenwhenThe they thickness they were were inserted of inserted the intopads into the was the garment, adjustedgarment, the theusing pads pads the should should cage be function.be thinner thinner To atat thetheachieve edges. a Usingnatural Using the the appearance cage- cage-edit whenfunctionedit they function on were the curves,oninserted the curves, the into thickest thethe thickest garment, part part was the was set pads se tot to 10should 10 mm, mm, be and thinner the the thinnest thinnest at the to edges. 4 to mm, 4 mm, consideringUsing considering the cage- appearance and protection performance, as shown in Figure 9. editappearance function and on the protection curves, performance,the thickest part as shownwas set in to Figure 10 mm,9. and the thinnest to 4 mm, considering appearance and protection performance, as shown in Figure 9.

Polymers 2019, 11, x FOR PEER REVIEW 7 of 14

3.4. Formation of Three-Dimensional Mesh Structure Planar Pad The pad structures were generated using the three-dimensional mesh structure. The diamond- shaped surface layer and the V-shaped spacer layer generated a basic unit, and the units were connected by connectors. The hexagonal structure of the basic unit is densely connected between individual units without empty space. The hexagonal mesh structure can be also flexibly deformed to various directions compared to the rectangular structure. Therefore, due to the connector, the pad moves flexibly with the movement of the human body to improve the wearing comfort. To apply a three-dimensional mesh structure to a curved pad, it is necessary to create a plane by repeating each unit structure, and cutting the repeating structure according to the outline shape. Because it is easier and more efficient to manipulate the curves compared to surfaces or solids, the unit structure forms a wireframe with curves. After repeating the unit structure, as shown in Figure 7, only the components corresponding to the pads were retained by the pad outline surface; the unnecessary parts were deleted, as shown in Figure 8.

Figure 7. Wireframe of hexagonal mesh structures.

Figure 8. Cutting procedure of the pad structure to match pad shape.

The thickness of the pads was adjusted using the cage function. To achieve a natural appearance when they were inserted into the garment, the pads should be thinner at the edges. Using the cage- Polymers 2019, 11, 1800 8 of 14 edit function on the curves, the thickest part was set to 10 mm, and the thinnest to 4 mm, considering appearance and protection performance, as shown in Figure 9.

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Figure 9. Adjusting pad thickness. Figure 9. Adjusting pad thickness. After the overall wireframe of the pad was complete, the pipe function was applied to the curve After the overalloverall wireframewireframe ofof the the pad pad was was complete, complete, the the pipe pipe function function was was applied applied to to the the curve curve to to create the surface. It was necessary to group each component, because the pipe sizes of each tocreate create the the surface. surface. It was It necessarywas necessary to group to eachgroup component, each component, because thebecause pipe sizesthe pipe of each sizes component of each component varied. The spacer layer had pipes with a diameter of 1.0 mm, and the surface layer and varied.component The varied. spacer layerThe spacer had pipes layer with had apipes diameter with ofa diameter 1.0 mm, andof 1.0 the mm, surface and layerthe surface and bridge layer hadand bridge had pipes with diameter of 2.0 mm. The pad on which the pipe was formed is as shown in pipesbridge with had diameterpipes with of 2.0diameter mm. The of 2.0 pad mm. on whichThe pad the on pipe which was formedthe pipe is was as shown formed in is Figure as shown 10. in Figure 10.

Figure 10. Grouping each pad component.

3.5. Formation Formation of Curved Pad of Three-Dimensional Mesh Structure The curvedcurved three-dimensional three-dimensional mesh mesh structure structure was generatedwas generated through through the following the following steps. Through steps. Throughthe ‘base the surface’, ‘base surface’, formed formed as described as described in Section in Section 3.2, the 3.2, three-dimensional the three-dimensional mesh mesh structure structure was wasdeformed deformed to ﬂow to alongflow along the ‘target the ‘target surface’, surface’, formed formed as described as described in Section in 3.3 Section and shown 3.3 and in shown Figure 11in. Figure 11.

Figure 11. FormationFormation of curved pad of three-dimensional mesh structure.

3.6. Dividing Pad The pad was divided, because the size of the curved pad exceeded the printable range. The radial split method for dividing pieces into three by 120° from the center point of the pad, and the elliptical split method for dividing pieces into four according to the sideline and curved surface were used. The elliptical split method was used in this study because, as shown in Figure 12, a comparison of the two types of printed pads revealed that the elliptical split method reflected the curved shape well.

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3.6. Dividing Pad The pad was divided, because the size of the curved pad exceeded the printable range. The radial split method for dividing pieces into three by 120◦ from the center point of the pad, and the elliptical split method for dividing pieces into four according to the sideline and curved surface were used. The elliptical split method was used in this study because, as shown in Figure 12, a comparison of the Polymers 2019, 11, x FOR PEER REVIEW 9 of 14 twoPolymers types 2019 of, 11 printed, x FOR PEER pads REVIEW revealed that the elliptical split method reﬂected the curved shape well.9 of 14

Figure 12. Pad-dividing procedure in the radial split method (left) and the elliptical split method Figure 12.12. Pad-dividingPad-dividing procedure procedure in in the the radial radial split split method method (left )( andleft) the and elliptical the elliptical split method split method (right). (right). (right). 3.7. 3D Printing of Final Product 3.7. 3D Printing of Final Product 3.7. 3DFigure Printing 13 shows of Final the Product curved fall-protection pads that ﬁt the 3D human body shape obtained throughFigure 3D printing13 shows and the the curved fusion offall-protection all the pads. Afterpads thethat supporters fit the 3D were human removed body fromshape the obtained printed Figure 13 shows the curved fall-protection pads that fit the 3D human body shape obtained four-piecethrough 3D pads, printing each and component the fusion was of placed all the between pads. After two stretchablethe supporters fabric were layers removed and ﬁxed from as the an through 3D printing and the fusion of all the pads. After the supporters were removed from the inseparableprinted four-piece pad by pads, sewing each the component top and bottom was placed fabrics between with running two stretchable stitch and fabric blind stitch.layers and fixed printed four-piece pads, each component was placed between two stretchable fabric layers and fixed as an inseparable pad by sewing the top and bottom fabrics with running stitch and blind stitch. as an inseparable pad by sewing the top and bottom fabrics with running stitch and blind stitch.

FigureFigure 13.13. Final product of 3D printing of humanhuman bodybody shape.shape. Figure 13. Final product of 3D printing of human body shape. 3.8. Impact Protection Performance 3.8. Impact Protection Performance 3.8. Impact Protection Performance To verify the impact protection performance of the curved three-dimensional mesh protection To verify the impact protection performance of the curved three-dimensional mesh protection pad,To an estimatedverify the impact forceprotection of 6500 performance N was applied of the to curved the pad three-dimensional at a height of 15 cm mesh in the protection vertical pad, an estimated impact force of 6500 N was applied to the pad at a height of 15 cm in the vertical direction.pad, an estimated As shown impact in Figure force 14 of, the6500 average N was peakapplied value to the of the pad vertical at a height force of measured 15 cm in onthe the vertical force direction. As shown in Figure 14, the average peak value of the vertical force measured on the force platedirection. over As 10 repeatedshown in tests Figure was 14, 1182 the average28 N. From peak a value height of of the 20 cm,vertical an impact force measured force of approximately on the force plate over 10 repeated tests was 1182± ± 28 N. From a height of 20 cm, an impact force of approximately 8000plate Nover was 10 applied repeated to tests the pad; was the1182 average ± 28 N. maximumFrom a height impact of 20 value cm, an measured impact force was 1663of approximately78 N. 8000 N was applied to the pad; the average maximum impact value measured was 1663± ± 78 N. 8000 N was applied to the pad; the average maximum impact value measured was 1663 ± 78 N.

Polymers 2019, 11, 1800 10 of 14 Polymers 2019, 11, x FOR PEER REVIEW 10 of 14

9000

8000 No Pad With Pad 7000

6000

5000

4000 Force Force (N) 3000

2000

1000

0 15 cm 20 cm Drop Height (cm)

Figure 14. Impact protection performance test with the curved three-dimensional mesh protection pad Figure 14. Impact protection performance test with the curved three-dimensional mesh protection and with no pad from a height of 15 and 20 cm, respectively. pad and with no pad from a height of 15 and 20 cm, respectively. The percentage of force attenuation (%) was calculated to obtain the impact reduction performance of theThe curved percentage three-dimensional of force attenuation mesh protection (%) padwas using calculated the following to obtain Equation the impact (1). reduction performance of the curved three-dimensional mesh protection pad using the following Equation (1). ! F𝐹T PercentagePercentage ForceForce AttenuationAttenuation (%) == 11− ×100 (1) − F0 × (1) 𝐹 wherewhere FFTT is thethe impactimpact forceforce (N)(N) usingusing thethe protectionprotection padpad andand F00 is the impact force (N) without it, respectively.respectively. We We achieved percentage percentage force force attenuations attenuations (%) (%) of of 81.8 81.8 and and 79.2 79.2 with with drop drop heights heights of of15 15cm cm and and 20 20cm, cm, respectively. respectively. As As shown shown in in Figure Figure 15, 15 ,Tsushima Tsushima et et al. al. reported reported a a range range of 20.1 to 42.642.6 usingusing conventionalconventional hip hip protectors protectors fabricated fabricated using, using, e.g., e.g., polyurethane polyurethane and and polystyrene polystyrene elastomer elastomer [30]. The[30]. percentagesThe percentages of the of forcethe force attenuations attenuations (%) (%) for for Tsushima Tsushima et al.et al. were were reproduced reproduced based based onon thethe impactimpact forceforce fromfrom variousvarious ﬂoorfloor materialsmaterials withoutwithout aa hiphip protector.protector. Thus,Thus, thethe resultsresults ofof thethe percentagepercentage forceforce attenuationattenuation indicated indicated that that the curvedthe curved three-dimensional three-dimensional mesh protectionmesh protection pad provided pad provided excellent impactexcellent protection impact protection performance. performance.

3.9. Flexural100 Stiffness The bending90 stiffness tests of the circular pads with hexagonal mesh structures and flat mesh 81.8 structures were80 performed to investigate the structural effect on the pads. As shown79.2 in Figure 16, the displacement distance of the hexagonal mesh structure was 4.02 mm with an applied force of 5 N, 70 8.94 mm with an applied force of 15 N, and 14.59 mm with an applied force of 30 N, respectively, which was greater60 than those of the flat mesh structure. By flexometer analysis, the test specimens were pushed to the slope region of the flexometer. The measured distances of the hexagonal mesh 50 and flat mesh structures were 67 mm and 140 mm,41.1 respectively.42.6 Thus, the hexagonal mesh structure showed higher40 flexibility than the34.7 flat mesh structure in both the circular and linear shape specimens. Therefore, the30 pad with the hexagonal mesh structure is expected be better suited to the movement of the human body, due20.1 to its excellent flexibility. 20 10 Percentage Force Attenuation (%) Attenuation Force Percentage 0 Test 1 Test 2 Test 3 Test 4 This Study This Study (15 cm) (20 cm)

Polymers 2019, 11, x FOR PEER REVIEW 10 of 14

9000

8000 No Pad With Pad 7000

6000

5000

4000 Force Force (N) 3000

2000

1000

0 15 cm 20 cm Drop Height (cm)

Figure 14. Impact protection performance test with the curved three-dimensional mesh protection pad and with no pad from a height of 15 and 20 cm, respectively.

The percentage of force attenuation (%) was calculated to obtain the impact reduction performance of the curved three-dimensional mesh protection pad using the following Equation (1).

𝐹 Percentage Force Attenuation % = 1− × 100 (1) 𝐹 where FT is the impact force (N) using the protection pad and F0 is the impact force (N) without it, respectively. We achieved percentage force attenuations (%) of 81.8 and 79.2 with drop heights of 15 cm and 20 cm, respectively. As shown in Figure 15, Tsushima et al. reported a range of 20.1 to 42.6 using conventional hip protectors fabricated using, e.g., polyurethane and polystyrene elastomer [30]. The percentages of the force attenuations (%) for Tsushima et al. were reproduced based on the impact force from various floor materials without a hip protector. Thus, the results of the percentage Polymersforce attenuation2019, 11, 1800 indicated that the curved three-dimensional mesh protection pad provided11 of 14 excellent impact protection performance. 100 90 81.8 80 79.2 Polymers 2019, 11, x FOR PEER REVIEW 11 of 14 70 Figure 15.60 Percentage force attenuations. Tests 1 and 2 were conducted with a polyurethane hip protector on concrete and wooden floors, respectively; Tests 3 and 4 were conducted with a polystyrene50 hip protector on concreate and wooden floors, respectively [30]. 41.1 42.6 40 3.9. Flexural Stiffness 34.7 30 The bending stiffness20.1 tests of the circular pads with hexagonal mesh structures and flat mesh structures were20 performed to investigate the structural effect on the pads. As shown in Figure 16, the displacement10 distance of the hexagonal mesh structure was 4.02 mm with an applied force of 5 N, Percentage Force Attenuation (%) Attenuation Force Percentage 8.94 mm with an applied force of 15 N, and 14.59 mm with an applied force of 30 N, respectively, 0 which was greater than those of the flat mesh structure. By flexometer analysis, the test specimens Test 1 Test 2 Test 3 Test 4 This Study This Study were pushed to the slope region of the flexometer. The measured distances(15 cm) of the hexagonal(20 cm) mesh and flat mesh structures were 67 mm and 140 mm, respectively. Thus, the hexagonal mesh structure showedFigure higher 15. Percentageflexibility than force the attenuations. flat mesh structure Tests 1 and in 2both were the conducted circular and with linear a polyurethane shape specimens. hip

Therefore,protector the on pad concrete with and the wooden hexagonal ﬂoors, mesh respectively; structure Tests is expected 3 and 4 were be conductedbetter suited with to a polystyrene the movement of thehip human protector body, on concreatedue to its and excellent wooden flexibility. ﬂoors, respectively [30]. 18

16 Hexagonal 14.59 Flat 14

9.36 10 8.93 8 6.06 6 4.02 4 2.45 Displace me ne t distance 2(mm)

0 51530 Force (N)

Figure 16. The flexural stiffness tests of 3D printing pads with hexagonal mesh and flat mesh structures, Figure 16. The flexural stiffness tests of 3D printing pads with hexagonal mesh and flat mesh with applied forces of 5 N, 15 N, and 30 N, respectively. structures, with applied forces of 5 N, 15 N, and 30 N, respectively. 3.10. Elongation Rate 3.10. Elongation Rate The elongation rate (%) was also measured with the hexagonal mesh and flat mesh structures. As shownThe elongation in Figure 17rate, the (%) elongation was also ratesmeasured of the with hexagonal the hexagonal mesh structure mesh and were flat 8.20% mesh and structures. 29.90% withAs shown applied in forcesFigure of 17, 5 Nthe and elonga 15 N,tion respectively. rates of the In hexagonal the case of mesh the flat structure mesh structure, were 8.20% the and elongation 29.90% rateswith wereapplied 1.80% forces and of 5.50% 5 N and with 15 applied N, respectively. forces of 5 In N the and case 15 N, of respectively.the flat mesh Since structure, a greater the elongation raterates indicates were 1.80% a more and stretchable 5.50% with pad, applied the pad forces with of the 5 hexagonalN and 15 meshN, respectively. structure showed Since a 4.6- greater and 5.4-timeselongation higher rate indicates elongation a more rates stretchable than the pad pad, with the the pad flat with mesh the structure. hexagonal Therefore, mesh structure the hexagonal showed mesh4.6- and structure 5.4-times has higher better stretchabilityelongation rates than than the flatthe meshpad with structure. the flat mesh structure. Therefore, the hexagonal mesh structure has better stretchability than the flat mesh structure.

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Hexagonal 29.90 30 Flat 25

10 8.20 Elongation rate(%) Elongation 5.50 5 1.80 0 515 Force (N)

Figure 17. The elongation rates of the hexagonal mesh and flat mesh structures with applied forces of 5Figure N and 17. 15 The N, respectively. elongation rates of the hexagonal mesh and flat mesh structures with applied forces of 5 N and 15 N, respectively. 4. Conclusions 4. Conclusions In this study, using 3D printing technology, we designed a new three-dimensional mesh pad, and developedIn this study, a fall using protection 3D printing pad that technology, fits the human we designed body structure a new three- and whichdimensional yields satisfactory mesh pad, impactand developed protection a fall performance protection bypad utilizing that fits thethe 3Dhuman human body body structure scan data and of wh elderlyich yields women. satisfactory When animpact impact protection force in performance the range that bycan utilizing occur the in daily3D human life was body applied scan data to the of protectiveelderly women. pad, itWhen was reducedan impact by force 79.2 %in orthe more. range The that uniformly can occur repeated in daily hexagonallife was applied mesh unitsto the presented protective in pad, this it study was arereduced structurally by 79.2 flexible,% or more. because The uniformly they are connected repeated hexagonal by thin bridges, mesh units and the presented curved protectivein this study pads are reflectstructurally the three-dimensional flexible, because body they shape, are connected making them by thin suitable bridges, for daily and use.the Moreover,curved protective the modeling pads wasreflect dynamic the three-dimensional to develop impact body protection shape, protectorsmaking them reflecting suitable the bodyfor daily shape. use. Through Moreover, several the iterativemodeling experiments, was dynamic we to developed develop impact a reasonable protection and delicateprotectors modeling reflecting method the body that shape. yielded Through results whichseveral are iterative applicable experiments, to research we ondeveloped clothing a and reasonable other fields. and delicate Existing modeling 3D printing method technology that yielded has beenresults deployed which are to applicable produce very to research hard products; on clothing however, and other in this fields. study, Existing the printing 3D printing conditions technology were finelyhas been set, deployed in consideration to produce of the very complex hard products; characteristics however, of flexible in this filament study, the materials. printing Finally, conditions we believewere finely that ourset, findingsin consideration foreground of the the complex possibility characteristics of using 3D of printing flexible technology filament materials. to print complex Finally, andwe believe elaborate that shapes our infindings the field foreground of functional the clothing.possibility of using 3D printing technology to print complex and elaborate shapes in the field of functional clothing. Author Contributions: J.H.P. performed the experiments. J.R.L. conceived the research idea. J.H.P. and J.R.L. wroteAuthor the Contributions: main manuscript J.H.P. text. performed the experiments. J.R.L. conceived the research idea. J.H.P. and J.R.L. wrote the main manuscript text. Funding: This research was supported by Basic Science Research Program through the National Research Foundation ofFunding: Korea (NRF) This fundedresearch by was the Ministrysupported of by Education Basic Science (2015R1D1A1A09057667 Research Program and through 2019R1A6A3A01096512). the National Research ConflictsFoundation of Interest:of KoreaThe authors(NRF) declarefunded noby conflict the ofMinistry interest. of Education (2015R1D1A1A09057667 and 2019R1A6A3A01096512).

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