Soft robotic fabric design,fabrication,and thermoregulation evaluation

Abstract

Usually, traditional insulation materials have a constant thermal resistance value that cannot change within the ambient temperature and will decrease as ambient humidity or external stress increases. Humans heavily rely on heating, ventilation, and air conditioning (HVAC) systems to meet the thermal comfort requirements of their bodies, giving rise to energy waste and global warming. As an infinitely available natural resource, air is one of the most efficient thermal retaining substances known to science. Inspired by soft pneumatic robotics, we propose an architecture for air-driven thermoregulation fabrics called soft robotic fabrics (SRF). By changing the thickness of trapped air layer in fabric system through SRF, wearers could modify garments’ thermal insulation performance. A fabrication method is introduced to rapidly manufacture low-cost pneumatic structures using various types of construction and dimensions. With excellent ductility, elasticity, and compression resistance, the thickness of SRF increases by 12 times or more after inflation, and the fabric even can lift an object 270 times heavier than its weight. The excellent deformability can effectively increase stable air layer between clothing and skin. Based on the Predicted Mean Vote–Predicted Percentage of Dissatisfied model, the thermoregulation capability of SRF helps HVAC expand the temperature setpoint range by 3–8 times when compared with traditional fabrics, and has far-reaching significance in saving energy.

Keywords

Thermal comfort soft robotic pneumatic actuation thermoregulation garment design

Human body temperature is maintained within a very narrow range as humans are homeothermic. In most extreme environments, the skin’s ability to thermoregulate cannot ensure thermal comfort, and clothing is essential for survival, or else the person might be exposed to danger. As a portable environment, a garment is ideal for personal thermal management in both indoor and outdoor conditions. Nevertheless, traditional materials, such as down feather and cotton, have limited thermoregulation capacity due to their constant thermal resistance value (R_ct). In many cases, humans heavily rely on heating, ventilation, and air conditioning (HVAC) systems to help them meet thermal comfort requirements. However, according to the 2018 China Association of Building Energy Efficiency Report, the energy used in the building sector accounts for 20.6% of China’s total energy consumption, about a quarter of which is consumed by HVAC. The corresponding figures for the United States are 40.2% and 33%.^1–3 It is imperative to create a reasonable auxiliary temperature control system or a thermal regulation material to reduce avoidable energy waste due to the excessive increase in indoor thermal temperature.^4–6

Thermal regulation materials or garments are one of the practical and potential solutions. Scholars in textile and clothing research fields have been committed to developing various novel materials or methods of high-performance thermal adjustment.⁵,⁷ Numerous new materials have been proposed, including carbon nano-material,⁸ shape-memory alloys (SMAs),⁹,¹⁰ phase-change materials (PCMs),¹¹or even textiles with biomechanical responses.¹² In recent years, nano-material and nano-composite textiles,²,¹³ bio-inspired materials,^14–17 and 3D printing technologies have also been widely used in the field of intelligent thermoregulatory clothing.⁷,¹⁸,¹⁹ Electrically heated clothing is realized through supplying heat sources to the clothing system, involving energy input. Heating components are usually made by either graphene composite fibers, carbon fiber elements,⁸ or metal heating wires, but at the cost of overheating possibility, energy cost, and safety issues.⁵ Thermal-sensitive PCMs (both liquid–gas and solid–liquid transitions) have been used to fabricate thermal clothing. PCMs store or release latent heat while the state changes within a certain temperature range, which exhibits very promising performances in terms of high thermal-regulation capability.²⁰ Nevertheless, they have relatively low energy efficiencies and poor moisture permeability. The thermal-regulation effect increases with the loading of PCMs; however, the clothing is heavier if it has more PCMs,²¹ and the phase transitions of these materials need to be actuated by a significant temperature difference. Clothing with shape-memory materials (SMMs) has shortcomings with respect to its thickness, stiffness, and high manufacturing costs. Similar to PCMs, these materials have low sensitivity to a small temperature variation, thus are not able to meet human thermoregulatory requirements.^22–24

As stable air conducts heat less readily than any fiber, its higher availability in the textile structure is known to be able to improve the thermal insulation and keep the body warm. With the convergence of fabric technology, lamination technology, and welding technology, air has been used as a filler in recent years instead of the traditional thermal insulators or human–computer interaction medium.²⁵,²⁶ Some garments, for example NuDown Jacket,^27–29 use gas for insulation. Air is pumped into the channels embedded in the cloth with the hand pump, and the thermal insulation is manually adjustable to suit altered environmental conditions. Air can even be replaced by an inert gas like argon with a thermal conductivity of 32% less than that of air. The idea of employing air as clothing insulation was inspired by the diving drysuits used in the deep-water layer at low water temperature (3–4°C). However, most inflatable structures are made of continuous air chambers. Classic inflatable garments with a large area of structures impermeable to water vapor have low breathability, especially when the air chambers cover too much body area. Besides, the inelastic plastic film used in the inflatable structure cannot fit human bodies well after inflation, leading to reduced wear comfort.

Despite significant progress, there remains a long-standing scientific challenge to the development of high-performance inflatable thermoregulation clothing which has a low-cost fabrication method and is eco-friendly, comfortable, and fast manufacturing. Within the last 15 years, studies on soft robotics and mechanisms have gained momentum.^30–42 They mainly involve many fields related to, for example, wearable devices, medical equipment, and item grabbing. Soft robotics have better potential for flexibility and human–robot interaction safety, and pneumatic actuation is one of the essential driving principles.⁴⁰,⁴³ Inspired by the idea of fluid-driven soft robotics, we here propose the design and fabrication methods for architectures of pneumatic fabrics, also called soft robotic fabrics (SRFs). Air, the endless green resources, is used in place of the down feather, releasing the dependence on the down products and rising above the controversies surrounding animal fibers. Present pneumatic garments use air as the “power-driven motor” by utilizing this endlessly available natural resource that just so happens to be one of the most efficient thermal insulating substances known to science, which provides a new choice for heat-retaining apparels. Compared with existing inflatable temperature-adjusting clothing, the textiles in the current study are ergonomic and have better breathability and comfort on the premise of ensuring good thermal insulation performance. Moreover, they were fabricated at multiple scales at extremely low cost. With SRF, clothing has a range of thermal resistance values instead of a fixed value, and wearers can change these values according to their own needs.

General scheme of soft robotic fabric development

The general scheme of SRF development includes five primary sections (Figure 1). First, we took air insulation and pneumatic control as main driving principles for actuators and collected various natural and artificial structures⁴⁴,⁴⁵ to design the structure of actuators. Key factors, such as feasibility and human comfort performance, were considered during the development. The possibility of subsequent industrial processing and rapid manufacturing was assessed by taking into consideration the manufacturing difficulty, durability, and controllability of the design. SRF actuators were optimized as filters for best designs and final fabrication. Finally, we did experiments on characterization of materials, such as basic specification parameters, thermal comfort performance, and compress property, and chose comparative fabric specimens to measure the advantages and disadvantages of SRF specimens.

Figure 1.

General scheme of soft robotic fabric development.

Fabrication method and material choices

Based on the existing pneumatic soft-robotic construction and main features, we proposed four types of SRF actuators (Figure 2) combined with the attributes of fabrics. These actuators cover four space dimensionalities, namely points, lines, planes, and space. Actuators of different scales were connected or utilized separately according to specific applications. Unidirectional deformation actuators (UDA) are circular cross-section chambers that deform in one direction and expand into a hemispherical balloon after inflation. UDA monomers are connected in series through tubes to form a dotted network. Meanwhile, bidirectional deformation actuator is a linear tube that deforms in both directions. It transforms from a soft straight hose before inflation to a wave-shaped structure after inflation, and would be fixed in fabric by weaving or knitting to form a deformable fabric structure. One-piece inflatable construction (OPIS) is a kind of inflatable triple-layers composite structure. Computer numerical control (CNC) laser machining techniques are used to obviate the assembly of monomers and the need for manual cutting, thereby ensuring faster manufacture and accurate designs. Material surface deformation (MSD) combines and promotes a negative Poisson’s ratio structure, forming an inflatable space driven by fluid expansion. Part of the MSD surface is coated with infrared-reflective layers that are opaque at infrared wavelengths to prevent the transmission of thermal radiation. The surface infrared reflectance ration is controlled by pneumatic actuation. MSD designed and fabricated following the hybrid soft inflatable manufacturing paradigm has the potential for prospective features, for example adjusting the reflectivity of the building surface to save energy. In this paper, we proposed two types of SRF actuators, OPIS and UDA, with the fabrication process and application prototypes. Furthermore, we characterized and compared the performance of two classes of OPIS and investigated traditional thermal materials. In particular, we focus on thermal comfort performance, durability, and compressive properties.

Figure 2.

Design, shape-change effect, and applications of SRF actuators in different dimensionalities.

One-piece inflatable structure (OPIS)

The fabrication of the proposed inflatable structure consisted of five basic steps (Figure 3): First, a casting mold was manufactured, and fiducial markers were engraved on the mold surface by laser mechanizing. Second, the first layer of silicone was poured, and the mid-layer was placed and be aligned with the fiducial markers. Third, the second layer of silicone was poured, OPIS was cut by CNC laser cutting machine after fully curing. In order to create a thin and breathable structure with good elasticity and flexibility, we chose a flexible silicone (Ecoflex™ 00-30) fluid enough as the surface of OPIS. Soft silicone has low viscosity, which means it could form a film (around 1 mm per layer) during pouring. Once it cured, the rubber could rebound to the original form without distortion after stretching multiple times. OPIS has three layers, in which the mid-layer is a material incompatible with silicone that is preprogrammed as air chambers. The mid-layer plays a role in preventing two silicone layers from adhesion, and the width of the mid-layer is proportional to the final thickness of air chambers. The mid-layer allows for a flexible choice of materials, such as polyvinyl alcohol (PVA), plastic membrane, sulfuric acid paper, or even aerosol clear mold release spray. It is worth noting that release spray only is appropriate for the air chambers with small areas. We used release spray as mid-layers in the design of UDA. As for OPIS, soft plastic films were used to isolate the silicone layers. To prevent mid-layers from slipping in the cavity during wearing, we cut small holes (diameter = 3 mm) at each periphery of mid-layers in advance (Figure 3(e) right). Silicone fused through these small holes and formed rivet-like connections to restrict the movement of mid-layers. Laser machining technology was used to cut materials according to a preprogrammed cutting path that ensured the fast, accurate, and precise fabrication process. At the same time, according to various design requirements, we entered the unit sizes, surface areas, and cutting geometry data into software platforms before the physical construction to enable the programmability of the SRF design. Since OPIS is an inflatable structure, to avoid air leakage in inflatable cavities, it is necessary to prevent the intersection of the laser cutting line and the mid-layer boundary during design. Silicone is not breathable; therefore, to achieve the dual performance of breathability and thermal comfort, an equilibrium point should be found between these two opposite parameters. Following a human thermal and sweat map,⁴⁶ we simulated the distribution of skin sweat glands; more holes were set in high-sweating areas to increase breathability and fewer holes were arranged in low-sweating areas to prevent heat loss (Figure 3(j–h)).

Figure 3.

Overview of the OPIS actuator manufacturing method and design details. (a) Laser-engraving alignment label on the mold surface. (b) The first layer of silicone is cured. (c) Placing isolation film (cut by laser in advance) as mid-layer to prevent silicone adhesion, and the isolated area forms air chambers. (d) Second layer of silicone is cured. (e) A final laser machining step following typical patterns of cutting releases OPIS pieces from surrounding substratum. Design details of mid-layer at the right side. (f) The pre-designed system of laser cutting path. The black line is OPIS outline, and three crosses are used as fiducial markers to align mid-layer positions with silicon cutting path. The green and red traces are cutting paths of the middle layer and silicone layer. The green line is shifted inward by 3 mm ± 0.5 mm from the red line, which means silicone layers have 3 mm ± 0.5 mm adhesion borders. The picture on the right side shows the overlapping path of clothing cutting patterns, mid-layer, and silicone layers. The purple area is the air chamber. (j) Design details of garment prototypes according to the human thermal map. (h) Front and back view of OPIS garment prototypes.

Unidirectional deformation actuator (UDA)

The UDA is a rhombus cross-section chamber that expands into a hemispherical balloon upon pressurization with air (Figure 4(k, l)). Unlike ordinary elastomers (e.g. balloons), this inflatable unibend structure was designed for better wearing comfort. The unidirectional deformation was rooted in the hardness difference between the two layers of silicone. The shore hardness (ASTMD-2240) and tensile strength (ASTMD-412) of Dragon Skin™ 30 on the bottom were far greater than those of Ecoflex™ 00-50 on the top. As a result, the top layer was active and more susceptible to deformation under the same pressure than the strain-limiting layer (bottom layer). Therefore, UDA formed a hemispheroid after inflation. The strength of this design is that the base smoothly fits the skin before and after inflation, freeing wearers from any oppressive or foreign body sensation.

Figure 4.

Overview of the UDA actuator manufacturing method and design details. (A) Create mold by 3D printing techniques and cover the surface with mold release spray. (B) Soft silicone layer is cured. (C) Option 1 of mid-layer: mold release spray. (D) Option 2 of mid-layer: Soft PET film. (E) Stiff silicone layer is cured. (F) Releasing UDA monomers from mold, and connect them in series by tube. (G—J) Design of UDA modeling and pouring mold. (G) UDA shape-change appearance. (H) pouring mold and front-back view of UDA. (I) Geometric construction details of pouring mold. (J) Geometric construction of UDA soft (gray) and stiff (purple) silicone layer. (K) Deformation processing of UDA. (L) Maximum deformation appearance of UDA.

UDA actuators were quickly fabricated in six simple steps: (1) the release agent was sprayed on the pre-designed casting mold; (2) the first layer of soft silicone was poured; (3) the second layer of release agent was sprayed (or the intermediate isolation layer was placed); (4) the bottom layer of silicone was poured; (5) mold stripping was finished; and (6) each structural primitive was connected. Each structural primitive functions as a pneumatic deformation point and was interconnected by a 1 mm diameter hose to form a chain inflation structure (Figure 4(a–f)). The position of UDA was adjusted according to specific body surfaces; for example, the density of UDA should be reduced for sweaty areas, while appropriately increased for another body surface. The finished interconnected chain inflation structure is sited in the garment interlayer. Shell fabrics protect UDA from skin contact and friction in daily use, increasing the durability and visual aesthetics of the material. Wearers could inflate the UDA system to increase the air layer whenever they feel cold. Rapid prototyping was realized in multiple sizes through 3D printing. By modifying the size and shape of structural primitives, designers could achieve various designs with different inflation thicknesses.

Compared with OPIS, UDA is different in design. The material in the mid-layer in UDA is the same as that in OPIS, although the release spray is a preferred replacement. During inflation, the primary deformation variable comes from UDA’s top layer. To ensure relatively close deformation variables and reduce the risk of air leakage under high pressure, UDA’s top layer is supported with additional surface structures (Figure 4(j)), instead of adopting a planar structure. This structure provides an additional binding force and stretch when the top layer deforms. UDA’s thickness expands nearly 25 times from the original 2.2 mm to 54.2 mm while remaining unbroken, demonstrating excellent deformation effect and stability (Figure 5).

Figure 5.

Thickness change of UDA (large scale) transformation before and after inflation

Performance characterization and testing apparatus

Silicone elastomers and adhesion strength test

A common problem of the inflation structure is air leakage, which hinders its regular performance. A second casting is required in the production of SRF, and the risk of leakage lies when the second casting of silicone fails to be integrated with the first layer. Findings of various experiments and the intrinsic quality of Ecoflex™ 00-30 informed the main reason for this integration failure. When the pouring time of the second casting exceeds the curing time of the first layer, the first silicone is wholly solidified, then the two layers cannot wholly adhere to each other. However, when only partially cured, the silicone is not mechanically strong enough to support the mid-layer. If the mid-layer is placed with vertical deviation or unevenness, the thickness of the silicone will be affected, resulting in dented surfaces (Figure 7 cross-sections). Therefore, the second pouring time should be between silicone pot life (45 min) and cure time (120 min).

With an adhesive strength test and microscopic observations, this experiment attempts to determine the optimal pouring time. The adhesive strength was tested, according to standards ASTM-D413-98 (2017), on six different cure times ranging from 55 minutes to 105 minutes, covering the entire curing stage of Ecoflex™ 00-30. When the pouring time exceeded 65 minutes, the silicone layer was peeled off (Figure 6). Meanwhile, the morphological and structural observation of silicone interfaces was carried out by using the microscope equipped with a digital camera (PAXcam). From the cross-sectional view of 55 min and 65 min specimens, we found that pouring the second layer on uncured silicone led to an uneven interface. Moreover, the uneven thickness of the first silicone layer would cause irregular deformation during inflation (Figure 7). Therefore, the pouring interval of the two castings for this study shall not exceed 65–75 minutes.

Figure 6.

Adhesion strength test results on six cure times from 55 minutes to 105 minutes. The solid black line is the mean resulting from three experiments, and the green shaded area is the standard deviation.

Figure 7.

The morphological and structural observation of silicone interface on six different cure times (55 to 105 minutes). The white area is the first layer and green for the second, all by Ecoflex™ 00-30.

Relationship between deformation uniformity and air chamber distribution

We investigated the influence of hollow shapes and hole pitches on the inflatable structure. Three shapes of holes were measured, including hexagon, circle, and square, to explore the inflate performance of acute angles, obtuse angle, right angle, and curve edges (Figure 8). The positioning of holes was divided into either isometric and asymptotic arrangement. The circles (Figure 8(a)) decreased in diameter, but distance increased gradually from their centers, simulating human skin pores. The area with large holes simulated the high density of pores and vice versa. In future designs, the number and location of holes could be arranged according to the thermal map and sweat map of the human body (Figure 3(j, h)).⁴⁶ According to these specimens, cross-shaped air chambers had good inflation preference. When the spacing between holes was less than 6.8 mm, air could not be filled into composite structures (red circle areas in Figure 9) Moreover, the inflation thickness was inversely proportional to hole spacing.

Figure 8.

Hollow arrangement and fabrication process. (A) Laser cutting path of mid-layer. (B) Laser cutting process. (C) Mid-layer appearance after laser cutting. (D) Mold for silicon cure. (E) Releasing SRF piece from mold.

Figure 9.

Inflatable appearances under difference hole shapes and pitches.

Results

Specification parameters of single-layer and multi-layer fabric specimens

To understand the basic characteristics of the SRF inflatable structure, typical insulation materials were selected as comparative specimens. For this experiment, we selected commonly used thermal insulation fabrics, including blended fabrics at different wool proportions, polar fleece fabric, and 3M Thinsulate™ (Type G) (G type insulation cotton is a medium fluffy, soft thermal material, which is widely used in the field of thermal clothing for a variety of needs, providing a warm, durable and light experience for the daily life of urban people). The specification parameters of single-layer specimens are shown in Table 1, the first seven of which were insulation materials available on the market. F1 was wool blends, F2 was polar fleece fabric, and F3 was the surface fabric for nature down, or cotton-padded jackets. S1 and S2 were SRF specimens studied in the present paper. S1-P and S2-P were the fully inflated states of S1 and S2. To investigate the influence of silicone area on the thermal comfort of SRF structures, S1 and S2 were designed with evenly spaced holes distributed in different hollow ratios.

Table 1.

Specification parameters of single-layer fabric specimens

No.	Code	Name	Material	Structure	Thickness	Density	Area Weight	WVT	Standard Moisture Regains
No.	Code	Name	Material	Structure	mm	kg m⁻³	g m⁻²	g ·m⁻² (24 h)⁻¹	%
1	F1	Wool blends	80%Wool, 20%Polyester	Plain weave	4.52	120.09	542.82	623.88	14.86
2	F2	Polyester	11%Wool, 59%Polyester, 30%Spandex	Knitting	3.22	88.30	284.33	674.14	3.34
3	F3	Polyester	100%Polyester	Plain weave	0.06	1031.12	61.87	826.42	0.04
4	G40	3M Thinsulate™ (Type G)	100%Polyester	TIM	7.40	5.98	44.25	747.33	0.52
5	G60	3M Thinsulate™ (Type G)	100%Polyester	TIM	8.00	7.92	63.37	734.02	0.64
6	G80	3M Thinsulate™ (Type G)	100%Polyester	TIM	11.00	7.52	82.71	681.91	0.61
7	G150	3M Thinsulate™ (Type G)	100%Polyester	TIM	17.00	10.17	203.41	676.36	0.59
8	S1	SRF1 deflated	Silicon	HOR = 51%	1.63	333.05	542.87	969.08	0.62
9	S1-P	SRF1 fully inflated	Silicon	HOR = 51%	36.36	14.93	542.87	1029.33	0.62
10	S2	SRF2 deflated	Silicon	HOR = 8%	1.77	1123.63	1438.24	313.79	0.49
11	S2-P	SRF2 fully inflated	Silicon	HOR = 8%	22.96	62.64	1438.24	341.51	0.49

WVT: Water-vapor transmission rate; TIM: Thermal insulation material; HOR: Hollowed-out ratio.

As silicone is not permeable and is usually heavier than ordinary fabrics, the weight of silicone should be reduced as much as possible while ensuring heat insulation. The hollow-out ratio of S1 was 51%, and that of S2 was 8%, as measured according to the national standards in GB/T 3820-1997. S1-P and S2-P had the maximum thickness of the elastic structure after full inflation. The original thickness of SRF was about 1.7 mm and increased after inflation. The thickness of S1 increased by 22 times after inflation (from 1.63 mm to 36.36 mm), compared with 13 times for S2 (from 1.28 mm to 22.96 mm). The specimen used for fabric area weight measurement was 50*50 cm in size. Each test was repeated at least three times before the average value was determined. Moisture regain of the studied fabrics was measured according to GB/T 9995–1997 and in reference to the standard procedures in ASTM E96–2016. As a result, the water-vapor transmission rate (WVT) of the fabrics was determined.

Single-layered fabrics formed double or multi-layered fabric systems in answer to different requirements. The formed systems then helped better understanding of thermal comfort properties of fabrics with different configurations. In this experiment, we fabricated 10 kinds of multi-layered fabric systems for different applications. Their basic properties are shown in Table 2. All 10 fabrics were covered with F3 as outer shells to simulate merchandizing thermal insulation fabrics (Figure 10, left). S1MDK and S1M-PDK were inflated and deflated specimens with two layers of G40 (Figure 10, right). In multi-layered fabric specimens, each layer was finished with stitches to reduce air convection.

Table 2.

Specification parameters of multi-layer fabric specimens

No.	Code	System	Thickness	Density	Area Weight	WVT	Standard Moisture Regains
No.	Code	System	mm	kg m⁻³	g m⁻²	g · m⁻² (24 h)⁻¹	%
1	G40DK	G40+F3	7.54	22.21	167.43	606.14	0.31
2	G60DK	G60+F3	8.13	22.94	186.49	578.79	0.35
3	G80DK	G80+F3	11.13	17.65	196.44	575.83	0.33
4	G150DK	G150+F3	15.11	17.97	271.51	552.18	0.29
5	S1DK	S1+F3 deflated	1.75	380.70	666.60	610.76	0.52
6	S1-PDK	S1+F3 inflated	36.48	18.27	666.60	391.03	0.52
7	S2DK	S2+F3 deflated	1.89	1115.70	1561.97	194.04	0.45
8	S2-PDK	S2+F3 inflated	23.08	67.68	1561.97	198.47	0.45
9	S1MDK	S1+G40+F3 deflated	10.55	71.57	755.11	551.62	0.46
10	S1M-PDK	S1+G40+F3 inflated	45.28	16.68	755.11	352.97	0.46

Figure 10.

Structure of multi-layer(left) and composite(right) fabric specimens. Each layer is finished with stitches to reduce air convection. On the right is the image of the inflated S1DK.

The sensory perception of a fabric is what human hands feel when touching fabrics. It is an important property of fabric texture and affects the wearing comfort. Sensory perception is a complex and comprehensive feeling composed of many factors. PhabrOmeter is conducted to measure sensory perception of fabrics. According to existing studies, PhabrOmeter test results are highly similar to person’s subjective feel with good data stability.⁴⁷ The PhabrOmeter obtains several indicators of the hand-feel value in a single test, such as softness, smoothness, stiffness, drape, or relative hand-feel value. The fabric with lower stiffness is softer and bends more easily. The softness is interpreted as the compression resistance of the fabric, and smoothness as the amount of force required to slide an adult’s fingertips over a piece of fabric. The feel test also provides the results of force-displacement curves for different specimens. The area of the curve reflects the amount of energy required by the sensor to contact the specimen. More precisely, the sensor needs more energy to slide over the specimen, and in this case the feel becomes harder.⁴⁸ Sensory perception of fabric specimens was tested by PhabrOmeter instrument in reference to the standards procedure in AATCC TM 202:2014 (R2014). The test results revealed that inflation has an impact on the hand-feel of the SRF structure. Referring to the three main indicators given by the PhabrOmeter, softness, stiffness and smoothness data (Figure 11), combined with the displacement-load curve (Figure 12), we found that:

Figure 11.

PhabrOmeter fingerprints result of test samples.

Figure 12.

PhabrOmeter Force-displacement curves of SRF sensory perception.

Inflation makes the SRF structure softer, which means that the latter feels better as the former increases;

The softness of S1 (S1DK: softness = 67.360) is higher than that of S2 (S2DK softness = 48.607);

The softness of the double layer SRF structure is only slightly higher than that of the composite structure (S1DK, S1MDK; S1DK-P, S1MDK-P);

S1DK and S1MDK are less stiff than wool blends F1;

S2DK is similar to F1 in terms of resilience and smoothness; however, the former is not as soft as the latter.

Evaluating thermal comfort parameters based on different fabric systems

To understand the performance changes and extreme values of SRF in different structures and designs, we tested the thermal comfort performance of SRF in three types of fabric systems (single-layer, multi-layer, and composite structures) to study the thermal comfort adjustment ability of SRF and compare it with other commercial products. We investigated the SRF fabric thermal comfort properties with three characteristics (thermal resistance, evaporative resistance, and moisture permeability index).²⁵ In accordance with ASTM-F 1868-C (wind speed: 1m s⁻¹, relative humidity: 65 ± 3%, test plate temperature: 35 ± 0.1°C, ambient temperature: 25 ± 0.1°C), the experiment adopted Sweating Guarded Hot Plate (SGHP)-10.5 and tested the single-layered and multi-layered fabrics listed above (Tables 1 and 2).

Thermal resistance

We can determine thermal comfort performance parameters of each specimen through testing, wherein the resistance to dry heat transfer provided by the fabric system and air layer using equation (1),

R_{c t} = \frac{T_{skin} - T_{amb}}{Q / A} (° C \cdot m W^{- 1})

(1)

R_ct = resistance to dry heat transfer provided by the fabric system and air layer (°C · m W⁻¹),

T_skin = temperature at the test plate surface (°C),

T_amb = temperature in the air flowing over the specimen (°C),

Q = power input (W),

A = area of the plate test section (m²),

R_ct0 is the average thermal resistance value measured for the air layer (bare, plate test) from the average thermal resistance value measure for the total fabric system, R_ct. To determine the resistance to dry heat transfer provided by the fabric alone, R_cf, follow equation (2),

R_{c f} = R_{c t} - R_{c t 0}

(2)

In the textile industry, Clo is one of the main parameters for measuring thermal insulation performance. One Clo is the amount of thermal resistance necessary to maintain in comfort for a person who sits quietly or engages in light mental work (whose metabolic heat productions 209.2 KJ m⁻²·h⁻¹) in a room at a temperature of 21°C, the humidity of the air which is less than 50%, and an air movement less than 0.1m/sec. Calculate Clo using equation (3),

1 R_{c f} = 0.155 Clo (° C \cdot m W^{- 1})

(3)

We found that single-layered specimens S2 and especially S1 had low thermal resistance both before and after inflation (Figure 13(a) clo). As a fabric with a higher hollow-out ratio, S1 had thermal resistance values (0.005 clo before inflation, 0.001 clo after inflation) similar to 0.06 mm-thick fabric F3 (0.002 clo). However, after adding surface fabric, S1 and S2 had much better insulation performance following inflation. In the case of multi-layered combinations, SRF specimens had relatively low thermal resistance before inflation, but their insulation performance significantly improved after inflation (S1-PDK 2.27 clo, S2-PDK 2.468 clo), superior to that of G60 (2.17 clo), single-layered wool blended specimen (1.495 clo) and polar fleece specimen (1.412 clo) under the same conditions. According to Figure 13(b), SRF with shell fabric had insulation performance between G60 and G80. The performance of SRF improved when combined with traditional fillings. The inflated combination of G40, S1, and F3 boasted insulation performance equivalent to G150, which meant that SRF can be used as fillers alone, as well as part of the insulation. SRF can be combined with other materials to achieve even better performance and widen the application range of traditional materials. Comparing thermal insulation performance between traditional materials and SRF with exterior shell fabrics (Figure 13(b) clo), the insulation value of 3M Thinsulate™ (Type G) increased by 1.3 times on average, while the performance of SRF series changed drastically. In the case of multi-layered fabric system, the insulation value of S1 and that of S2 increased by 15 times and 14 times after inflation, respectively. Consequently, we drew the following conclusions:

Figure 13.

Evaluating the thermal comfort properties of SRF fabrics by Sweating Guarded Hot Plate test: Thermal (Clo) and evaporative resistance (R_ef) test examine the obstacles of heat and moisture flow from skin to environment. Permeability index (im) is the efficiency of evaporative heat transport in a clothing system. (A) and (B) Comparison of thermal comfort parameters between single-layered and multi-layered samples.

Shell fabric will enhance the thermal insulation of SRF during inflation (S1-PDK, S2-PDK).

In SRF series, the hollow ratio of silicone is proportional to the thermal resistance of the fabric. That is, the less the hole, the better the thermal resistance of the fabric.

The more SRF inflation, the higher the thermal resistance.

Before being inflated, the thermal resistance of the multi-layered SRF series stays at a very low level. After inflation, the thermal resistance rises significantly (14 to 15 times), while the thermal resistance value of ordinary fabrics is fixed and does not have similar characteristics.

Evaporative resistance

Under the same test standard, we calculate the apparent total evaporative resistance of the single-layered and multi-layered specimens using equation (4),

R_{e t} = \frac{P_{skin} - P_{amb}}{Q / A - \frac{T_{skin} - T_{amb}}{R_{c t}}} (P a \cdot m^{2} W^{- 1})

(4)

where:

R_et = resistance to evaporative heat transfer provided by the fabric system and air layer (Pa · m² W⁻¹),

P_amb = water vapor pressure in the air flowing over the specimen (Pa),

P_skin = water vapor pressure at the test plate surface (Pa),

Q = power input (W),

A = area of the test plate (m),

T_skin = temperature at the test plate surface (°C),

T_amb = temperature in the air flowing over the specimen (°C),

R_ct = resistance to dry heat transfer provided by the fabric system and air layer (°C · m W⁻¹),

R_et0 is the average bare plate evaporative resistance. To determine the average apparent intrinsic evaporative resistance of the sample alone (R_ef) using equation (5):

R_{e f} = R_{e t} - R_{e t 0}

(5)

The evaporative resistance test results of single-layered fabrics indicated that 3M Thinsulate™ G Series (G40/60/80) had maximum evaporative resistance among all other specimens, except for S2 (Figure 13(a) R_ef). In G series, G80 had the maximum value, 31.086 Pa · m² W⁻¹; the other values of G series decreased with fabric area weight and thickness. Among SRF series (except for S2), the three specimens had relatively low evaporative resistance value, especially S1(S1 R_et = 1.386 Pa · m² W⁻¹, S1-P R_et = 1.448 Pa · m² W⁻¹), whose value was lower than 0.06 mm polyester specimen F3 (R_et = 2.729 Pa · m² W⁻¹). According to traditional theories, the evaporative resistance of a fabric increases with its thickness within a certain range.⁴⁹,⁵⁰ However, there appeared an opposite pattern for one of the inflated SRF specimens (S2). The evaporative resistance of S2 decreased as thickness increased. We believe that this might be because the impermeability contact area of SRF with the test plate decreased significantly after inflation (Figure 14). The test plate (representing skin) determined more evaporation area after inflation (Figure 13(a) R_ef). Thus, evaporative resistance decreased after inflation. For example, S2’s evaporative resistance decreased from 36.621 Pa · m² W⁻¹ to 12.880 Pa · m² W⁻¹, which was lower than wool blended specimen F2 (3.22 mm thick) and F1 (4.52 mm thick). S2 had maximum evaporative resistance among all single-layered specimens. The main reason was that the hollow-out ratio of S2 was 8%, much less than 51% of S1. As silicone is entirely impermeable, the lower the hollow-out ratio, the higher the evaporative resistance.

Figure 14.

Surface morphologic changing of SRF (before and after inflation).

The evaporative resistance test results of multi-layered fabric specimens indicated that evaporative resistance of all fabrics increased after adding an outer shell (Figure 13(b) R_ef). The outer shell made fabric systems catch sufficient air. Too much stable air would increase the thermal resistance of the fabric but at the expense of moisture permeability. Among all specimens, S1DK had minimum evaporative resistance before and after inflation. After adding a double polyester wadding interlayers, S1’s value increased significantly, with S1M-PDK having the maximum value. Among all Thinsulate™ specimens, G80DK had the maximum evaporative resistance, which, however, was still lower when compared with SRF specimens, S2DK/S2DK-P, and S1MDK/S1M-PDK. This indicated that the thermal and evaporative resistance of SRF materials changed significantly after inflation.

Meanwhile, the evaporative resistance performance of S1DK and S2DK revealed that the larger hollow-out area allowed the fabric to have better thermal insulation and permeability. By comparing the results of SRF series and G series, the evaporative resistance of S2 was too high and should be avoided in future designs. S1DK performed wonderfully in the evaporative resistance test, where the evaporative resistance of deflated S1DK was the smallest among all specimens (16.536 Pa · m² W⁻¹). After inflation, the evaporative resistance of S1DK-P (41.305 Pa · m² W⁻¹) increased, but was still lower than G40DK (46.097 Pa · m² W⁻¹). Notably, the thermal resistance value of S1DK-P was higher than G60, which means that, compared with G40 and G60, the S1 structure had better moisture permeability and higher thermal insulation performance. S1MDK had a high evaporative resistance due to the stacking of multiple layers of fabric, as the thickness of the fabric would significantly affect its water-vapor transmission.⁵¹

In this section, we discussed the extreme value of the thermal comfort performance of SRF when it was fully inflated or deflated. However, in most cases, the wearer can adjust the SRF system according to the environment. Moreover, according to the test standards of ASTM-F 1868-C, the ambient temperature is 25°C, and it is more inclined to simulate the warm environment in practice; hence, the overall specimen humidity resistance results are more massive. Future research should be done to supplement the humidity and heat test data in cold environments. We infer that the above situation led to the higher humidity resistance of the SRF specimen. From the perspective of clothing design and specific applications, although some SRF fabrics had higher moisture resistance than other specimens, considering the actual situation, such as extreme cold conditions, the clothing’s warmth and air impermeability are vitally important. Besides, as the rate of sweating of human skin significantly reduces in cold conditions, the moisture resistance of the fabric is not an essential factor in cold environments. S2DK and S1MDK with higher moisture resistance had better windproof effects and were more suitable for cold environments. Also, the hot plate test was used to simulate the situation in which the human skin directly contacts the fabric. In practical applications, the wearer would wear underwear or fleece in a warm jacket (such as a down jacket). Therefore, similar to fleece, SRF structures (e.g. S1DK) can be used as insulating clothing between underwear and windproof jackets. The S1DK or S1MDK has a better windproof effect and can be used directly as a top layer windproof thermal jacket. Therefore, we gained the following insights from this test:

Following the same patterns with thermal resistance performance, the evaporative resistance of multi-layered fabrics is higher than that of single-layers—the thicker the fabric, the higher the evaporation resistance. However, the relationship between thermal resistance and evaporation resistance should also be considered in combination with the moisture permeability index.

The hollowing-out ratio of silicone is inversely proportional to the evaporation resistance. Too much silicone coverage area will increase the SRF moisture resistance. In future designs, the surface area of silicone structure should be reduced to promote evaporation performance.

When the parameters of the SRF silicone structure are accurately calculated, low evaporation resistance, and high heat retention performance (S1DK) can be achieved simultaneously.

Different SRF structures can be used in different thermal clothing designs, such as thermal underwear or windproof jackets, which means that SRF has flexible application possibilities in clothing design.

Comparison of moisture permeability index

Permeability index (im) is the efficiency of evaporative heat transport in a clothing system. To calculate the permeability index for fabrics use equation (6),

i m = 0.0094 (I_{t} / R_{e t})

(6)

where: im = permeability index (dimensionless),

I_t = insulation value determined in accordance with equation (2) (clo),

R_et = evaporative resistance (Pa · m² W⁻¹).

According to the moisture permeability index results of single-layered specimens, except for S2, the 10 specimens had roughly equal permeability, ranging from 0.52 to 0.74. im is a moisture permeability index of the fabric (Figure 13(a) im). The thermal balance of the human body in a high-temperature and high-humidity environment is more easily maintained by the fabric with a higher im value, whereas lower im indicates poor permeability. Overall, the permeability of S1 was better than that of S2, and the im value of S1-P (0.64) was similar to that of F1 (0.66). According to the im results of the multi-layered specimens, S1-PDK and G80DK had better permeability, followed by that of S1MDK (Figure 13(b) im). S2 had minimum values in both forms, meaning that a silicone area which was too large could badly affect the permeability of the specimen. In terms of the fabric’s breathability, multi-layers have worse performance than single-layers. However, in SRF specimens, the decrease in the breathability of S1 and S2-P was significantly higher than in other specimens. Even though higher im value suggests better vapor permeability at high temperatures and in high-humidity environments, permeability is less important than insulation performance, as all specimens are meant to be used in cold environments. Moreover, the dimensionless index in the permeability test method has its own limitations. Previous studies believe that this index should be treated with caution.⁵²,⁵³ The explanatory effectiveness of im is not a crucial factor; instead, it can only be used as a reference value. The results regarding the permeability and evaporative resistance of the specimens should also be considered to further understand the moisture permeability of materials comprehensively.⁴⁹,⁵⁰

Personal thermal management systems and temperature setpoints expansion capability

The main function of a building’s heating and cooling is to maintain the thermal comfort of occupants. Nevertheless, HVAC systems in buildings consume huge amounts of energy. Taking the United States as an example, HVAC energy consumption accounts for 13% of all building energy consumption. Compared with adjusting the temperature and humidity balance of the entire building, a Localized Thermal Management System aims to change the thermal environment around the human body and has higher energy efficiency.⁵⁴ For instance, consider that there are four adults (surface area = 1.8 m²) in a residential room with a floor area of 186 m² room (surface area of 335m²); the HVAC system needs to adjust the surface area 83.6 m² per resident to meet the thermal comfort requirements (over 46 times the surface area of a person). Available data has determined a very conservative estimate of 15% savings for an expansion of setpoints by 2.2°C (4°F) in each direction,⁵⁴ which has a transformative potential regarding the nation’s electricity usage, consumption of fuels, and greenhouse gas emissions. Thermal comfort is a psychological state that expresses satisfaction with the thermal environment and is evaluated through subjective evaluation (ANSI/ASHRAE Standard 55), following the Fanger’s Predicted Mean Vote (PMV) model,⁵⁵ which has been widely adopted as the standard for thermal comfort for conditioned environments. The ANSI/ASHRAE Standard 55 also combines with the Predicted Percentage of Dissatisfied (PPD) model to evaluate the occupants’ comfort perceptions.

Through the hot plate test in the previous section, we determined the range of SRF thermal resistance, evaporation resistance and moisture permeability index, and found that it had great spring-weight warmth and superior moisture management. This section further discusses the potential of thermostatic fabrics in building energy efficiency. Based on the PMV–PPD model, we used the Center for the Built Environment (CBE) thermal comfort online calculator⁵⁶ to calculate the comfort temperature zones with four parameters for all specimens: clothing insulation (clo), airspeed, metabolic rate, and relative humidity. The environmental parameters were identical with the hot plate test (air speed = 1m s⁻¹, relative humidity = 65%), metabolic rate was 1 met (an adult in a sedentary state 1 met = 58 W m⁻²). The results revealed that SRF series had a great potential to achieve thermal comfort in an expanded ambient temperature range (S1DK = 15.7 to 32.2°C, S2DK = 14.6 to 32°C, S1MDK = 4.6 to 18.3°C) (Figure 15). The program objective of ARPA-E was to achieve thermal comfort in an expanded ambient temperature range of 18.9°C to 26.1°C. S1DK and S2DK completely covered or even exceeded this range, indicating that SRFs had great potential for energy saving. Among all specimens, S1DK and S2DK had the widest comfortable temperature range, nearly three times wider than that of the other specimens. The SRF series had a temperature range three to four times (F1, F2) wider than that of traditional thermal fabrics and eight times (F1) wider than that of polyester pieces.

Figure 15.

Insulation performance and temperature setpoint expansion capability (psychometric). The abscissa is operative temperatures of HVAC devices; Ordinate is humidity ratio. By given these four parameters (Clothing thermal resistance range, metabolic rate = 1 met, Air speed = 1m s⁻¹, and relative humidity = 65%), occupants will find thermal comfort in a wider range of temperatures if clothing level can change within the given range. The wider the shaded area, the stronger the ability of the clothing to expand the temperature setpoint, and the more energy efficient it is. Among them, the thermal comfort zone of S1MDK is beyond the range that the chart can be debugged. In the figure, the blue color block is used for trend addition as a reference. The D-value of comfort temperature and network interface is given in this figure as well.

Inflatable capacity, thermal insulation, and metabolism

The insulation value of clothing is measured in clo, which is used to calculate the insulation value of clothing required by the human body to reach thermal balance under given physiological metabolic strain through a mathematical equation (Table 3).⁵⁷ In a low-temperature environment, the insulation performance of clothing is particularly important. Figure 16 shows the Insulation value required for clothing under different work intensities in cold and hot environments. Previous studies have established that the thicker stable air layer, the better its insulation performance.⁵ Due to the large volume of cavity inside the inflated SRF structure, convection inside the cavity has to be considered. In our current experiment, the (SGHP)-10.5 sweating hot plate was used to test the inflatable airbag (S1 + F3, area = 50 cm²) according to ASTM-F1868-C. Under a constant environment, the airbag was inflated using a hand-held rubber air pump (single pump’s inflatable capacity is about 75 ml). The inflatable fabric in this test could hold 36–40 pumps at most. If 1P = 1 pump’s inflatable capacity, then 0 ≤ P ≤ 40.

Table 3.

Examples of basic insulation values (Clo) of clothing⁵⁷

clo	Garment ensemble
3–4.5 clo	Arctic clothing systems
2.6 clo	Undershirt, underpants, insulated trousers, insulated jacket, overtrousers, socks, shoes, hat, gloves
2.0 clo	Underpants, undershirt, shirt, trousers, jacket, over jacket, overtrousers, socks, shoes, hat, gloves
1.5 clo	Briefs, T-shirt, shirt, fitted trousers, insulated coveralls, calf-length socks, shoes
1.0 clo	Briefs, undershirt, underpants, shirt, overalls, calf-length socks, shoes
0.5 clo	Briefs, short-sleeve shirt, fitted trousers, calf-length socks, shoes

Figure 16.

Insulation values needed to maintain low-level physiological strain (neutral thermal sensation) at varying temperatures ( Note: table is modified from ISO 7243 2017 and Encyclopaedia of Occupational Health and Safety 4th Edition Part VI general hazards, Chapter 42 - Heat and Cold).

Based on the association between the Insulation value of textile and the ambient temperature and that between the thermal insulation performance of the airbag and the inflatable capacity, we established the relationship between the ambient temperature and the inflatable capacity of the airbag (Figure 17). This example assumes that airbag’s Insulation value was only related to the inflatable capacity but not subject to changes in the external ambient temperature. According to ASTM-F1868-C, the simulated human skin temperature is constant at 35°C; hence, formula 6 was deduced. The formula suggests that at a positive temperature, the inflatable capacity of the airbag is inversely proportional to the ambient temperature while at a temperature below 0°C, a directly proportional tendency is observed. When the ambient temperature is constant, the inflatable capacity is inversely proportional to the human metabolic rate (M = Q/A).

P = 111.1097 （\frac{T_{skin} - T_{amb}}{Q / A}） - 5.6078 \approx 111 （\frac{T_{skin} - T_{amb}}{Q / A}） - 5.6

(6)

Figure 17.

Relationship between insulation value and inflating volume.

Case 1: When the wearer works at a light intensity (M = 100^a) at 20°C, substitute the required inflatable capacity for P:

P = 111 （\frac{35 - 20}{100}） - 5.6 = 11

Case 2: When the wearer works at a heavy intensity (M = 230) at –10°C, substitute the required inflatable capacity for P:

P = 111 （\frac{35 + 10}{230}） - 5.6 = 16

In the future, to create a more personalized inflation system, sensors would be integrated to detect the metabolism of the human body, simultaneously overlooking the ambient temperature and the wearer’s metabolic rate.

A single-layered SRF fabric meets the insulation performance required for moderate–slightly heavy–heavy work intensities at a temperature from –40°C to 30°C, moderate–slight work intensities from –13°C to 30°C, and light work intensity from 0°C to 30°C (Figure 18). With excellent insulation performance, S1 composite has an insulation value of 3–4.13 clo, second only to Arctic clothing systems (3–4.5 clo). Overall, SRF provides more choices and greater flexibility to garment developers. Designers can choose single-layered or multi-layered SRF fabrics based on different application scenarios. Judging from test results, single-layered SRF is mostly suitable for daily wear. For example, the annual temperature in the Yangtze River Delta region in China ranges from 1 to 35°C. SRF fabrics meet the thermal comfort requirements under most working conditions (about 80%). In a cold environment (0–30°C), SRF composite is more suited for rest and light workload and SRF single-layer fabric for medium to a heavy workload.

Figure 18.

SRF insulation values vs. atmosphere temperature and metabolic rate.

Figure 19.

Compressive strength test

Compressive strength and durability

Traditional insulation materials such as down feathers, cotton, and polyester wadding are fluffy and soft to retain stable air in the fabrics to the maximum extent. However, one of their disadvantages is that their thickness is significantly reduced when they are subjected to external force. What is more, down’s insulation performance weakens as fluffiness decreases, which happens after a long time of use. Therefore, another factor in evaluating the performance of an insulation material is the shape-holding ability. Whether the inflatable structure can maintain original thickness under external stress needs further verification. In this experiment, we chose an OPIS whose original thickness was 1.77 mm. After inflation, it increased nearly 13 times to 23 mm. As there is no standard for the compression performance of inflatable fabrics, this article refers to the shape retention test standards of typical insulation fillers. An effective and rapid inspection method provided in the measurement standards of FZ/T 64003-2011 was used to test the compression resistance of SRF specimens. The specific test method involves three steps: (1) cutting the specimens into 20 cm²; (2) loading 2 kg weights three times for 30 s each time to measure the heights of specimen at its four corners, with the average value taken as h₀; and (3) loading a 4 kg weight for 30 s and measuring the average height as h₁ and the average height after 3 min as h₂. To calculate the resilience rate for fabrics use equation (7) ⁵⁸:

h = \frac{h_{2} - h_{1}}{h_{0} - h_{1}} \times 100 %

(7)

According to FZ/T 64003-2011, the resilience ratio should exceed 70%, and the resilience ratio of SRF is 89%, which figure roughly reached the standard rates.

Additionally, SRF specimens can lift objects several orders of magnitude heavy than themselves. For instance, a 97 g OPIS specimen (area: 20 cm²) can lift a 2.6 kg object using positive internal air pressure to 23.73 mm (Figutr 19). With excellent compressive strength, SRF could be an excellent insulation filling material in high-pressure environments, for example deep-sea operations. The washing durability of SRF structures was also examined by washing experiments as specified in the GB/T 8629-2001. The delicate washing cycle was selected for washing the SRF specimen. Experiments show that the SRF structure can be washed at least 100 times without being damaged. SRF garments can last for at least 10 years if continuously worn in certain areas (washed twice a month), for example, China's Yangtze River Delta (from October to February). This indicates that SRF structures are highly stable and durable. As silicone rubber is hydrophobic, it does not shrink, nor does its insulation performance decrease after washing. Hence, SRF materials help resolve the washing-related shortcomings of down, wool or cotton.

Reflectivity test

Textiles with a great opaque ability at infrared wavelengths prevent the transmission of thermal radiation from the human body when the ambient temperature reduces. To promote the thermal insulation performance of SRF materials, we coated a reflecting material layer on the SRF surface. Three black silicone specimens were made in the same patch for the experiment. In particular, specimen 1 was black silicone with no coating, specimen 2 was silicone with coating on one side, and specimen 3 was silicone with coating on both sides. The Fourier Transform Infrared Spectrometer (FTIR) spectrum test result concerning the transmittance discrepancy of three specimens is presented in Figure 20. The area enclosed by the curve and the X-axis was calculated through the MATLAB irregular curve surface integral: A_all = 282157.8; A_half = 282247.6; and A_black = 310926.2. The transmittance ratio of specimen 1 (black) increased by 9.3%, compared with that in the other two specimens with coating materials. Therefore, this design further enhanced the thermal performance of SRF.

Figure 20.

Fourier Transform Infrared Spectrometer (FTIR) image of all-coated, half-coated, and silicone-only samples.

Conclusion and discussion

We have developed a flexible pneumatic structure used in the thermal insulation material for human thermal regulation. The structure utilizes air to deform and adjust stable air layers to improve the insulation performance of the fabric. Although air is virtually weightless and low cost, has good insulation performance, provides a source of clean energy, and does not cause pollution, it is rarely directly applied for insulation fabrics. This study aims to bridge this gap. Compared with SMMs, PCMs, and electric braking materials, fluid-driven actuation is a more efficient and active method that makes structural deformation independent of temperature, humidity, power, or magnetism. This paper introduces the production process and features of two typical pneumatic actuators. To ensure the reliability, repeatability, and rapid prototyping of inflatable structures, we carried out experiments to test specific parameters, such as peel strength, casting time, and laser cutting parameters.

Common insulation fabrics were selected (wool blends, polar fleece, and 3M Thinsulate™ with high thermal performance) to compare with SRF in terms of thermal and evaporative performance. Our analysis reveals that the standard moisture regains of single-layered SRF (about 0.56%) are far lower than those of wool blends (14.86%) and polar fleece (3.34%), while similar to that of 3M specimens (the average of standard moisture regains of all SRF specimens = 0.59%). The WVT of the single-layered SRF is also far higher than that of other specimens, indicating that the structure of SRF has good permeability but low hydrophilicity. The permeability of all fabrics decreases when an exterior shell fabric is added. The WVT of multi-layered SRF series (except S2) ranges between 353 g m⁻² per day and 611 g m⁻² per day. Compared with 3M Thinsulate™ series, wool blends, and polar fleece, SRF series are acceptable for portable applications.

In the thermal comfort performance test, we have found that inflation does not change SRF’s thermal comfort performance. However, the inflated SRF’s thermoregulation ability increases drastically with thin shell fabrics (the insulation value of S1 and that of S2 increases 15 and 14 times after inflation, respectively) and is much higher than that of traditional materials (the insulation value of 3M G series increased by 1.3 times on average after the shell is added). According to previous studies,⁴,⁵,²⁵,⁴⁹ within a certain temperature range, a thicker air layer generates greater thermal and evaporative resistance. This means that the combination of SRF and a shell changes the thermal insulation performance of the structure. Generally, SRF has good thermal insulation performance. With an additional shell, the inflated SRF composite fabric has a thermal insulation performance similar to G150, three times as high as that of wool blends and polar fleece; SRF with an additional shell has a thermal insulation performance equivalent to that of G60. Previous research estimated that if the setpoints are expanded by 2.2°C (4°F) on both hot and cold sides, at least 15% of HVAC energy use was saved (equivalent to 4% of energy consumed).²,⁶ PMV–PPD results reveal that SRF series has a great potential to achieve thermal comfort in an expanded ambient temperature range. The objective of the ARPA-E program is to achieve thermal comfort in an expanded ambient temperature range of 18.9°C to 26.1°C. S1DK and S2DK completely cover and even exceed this range, which is about three to eight times wider than those of other specimens. This, therefore, indicates that SRFs have a great potential for energy saving.

SRF specimens with shell surface meet the thermal comfort requirements of the human body in most working conditions (80%) in the Yangtze River Delta region. Furthermore, SRF composite specimens have nearly the same thermal insulation performance as Arctic clothing systems. From the vapor permeability test, we have found that SRF with less silicone has higher permeability. This finding provides a strong reference for further improvement in SRF structures. This paper states that SRF single-layer or composite fabric specimens meet thermal comfort requirements of the human body when ambient temperatures and metabolic rates vary. Single-layer SRF meet the thermal insulation performance required for moderate, slightly heavy, and heavy work intensities at a temperature from –40°C to 30°C, and warmer environments, or lower work intensity. Meanwhile, composite SRF is more suitable for cold environments, which means that the wearer does not need additional heating to maintain thermal comfort.

The thickness of traditional insulation materials, such as down and cotton, is greatly reduced under external pressure. This leads to a reduction in the air layer space, thereby badly affecting the insulation performance. In contrast, the inflatable structure developed by our research team demonstrates excellent compression strength; that is, it can carry a load 270 times as heavy as its own weight after inflation. This superior compressive strength makes up for the defects of traditional materials. Despite far exceeding the requirements of daily clothing, this feature paves the way for the application of this material in high-pressure scenarios, such as diving, tunnel, caisson, and aerospace. This performance also broadens the application scope of inflatable structures. Adjustable insulation materials can be applied not only in clothing but in building surface thermoregulation and other industries.

The inflatable structure can also be combined with other textile techniques, such as reflective metal coating, to further enhance its thermal performance. The FTIR spectrum test revealed that the transmittance of infrared is reduced by 9.3% when the structure is covered with a layer of infrared-reflect coating, providing space for further design progress.

Footnotes

Acknowledgements

Yan Cui acknowledges the financial support (No. 201606280243) from China Scholarship Council (CSC) for her visit to Cornell University.

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the Research Grants Council of Hong Kong (Project No.: PolyU 252029/19E), the Innovation and Technology Fund of Hong Kong (Project No.: ITS/093/19), the PolyU-Industry Collaborative Research Project (Project No.: ZDCH), and the PolyU GRF Project (Project No.: 1-BE1F), and the Shanghai Style Fashion Design & Value Creation Collaborative Innovation Center (Project No.: X11071904).

ORCID iDs

Yan Cui

Jintu Fan

Note

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