Optimization of Heating Performance of the Rib-Knitted Wearable Heating Pad ^†

Abstract

Textile structures such as knitting, weaving, braiding, and nonwoven can be used to produce wearable heating pads. Knitted fabric has unique properties such as stretchability, flexibility, and comfort among these textile structures. However, traditional knitted heating pads are manufactured by employing a straightforward three-structure design comprised of a plain, rib, and interlock with yarn that is entirely conductive. The usage of fully conductive materials in industrial applications has been restricted primarily as a result of their more expensive price tag and higher power requirements. Herein, we reported a rib (knitting structure)-based wearable heater with localized conductive yarn. A 14-gauge V-bed knitting machine is used to prepare a localized heating pad with a slight variation in the loop length. The rib-knitted structure (R1) with the lowest loop length showed a 47.4 °C average surface temperature at 9 Volt DC power source. The laboratory-based prototype of the heating pad is also designed for alleviation of joint and muscle pain in the affected area of the body.

1. Introduction

Nowadays, conductive yarn or fabrics are utilized for smart wearable electronic textiles. Textile-based wearable heaters have drawn attention to the application of heat therapy for the reliving of joint and muscle pain. A higher skin temperature increases the heat circulation and blood flow in a specific area of our body. Heating devices can be divided into four categories: electrical-, phase change material-, chemical-, and fluid-based heating [1]. Electrical heating is the most preferred for wearable applications since it requires less time to reach an equilibrium temperature. Therefore, many researchers focused on textile-based electrical heating products such as gloves, belts, and pads for heat therapy applications [2,3,4]. Equation (1) can be used to calculate the amount of heat that is produced when an electric current flows through a conductor.

$$Heat dissipated \left(H\right)=I^2\times R$$

(1)

where I and R are the current flowing in the conductor and the resistance of the conductor, respectively.

Recently, textile-based wearable heaters have gained a lot of attention owing to their intrinsically flexible, soft, breathable properties and efficient production techniques. Various techniques for incorporating heating components into woven [5,6], knitted [7,8], nonwoven [9], and embroidered [10] fabrics have been found in the literature. The woven structure lacks flexibility due to the horizontal and vertical interlacement of yarns, limiting the yarn mobility in the structure. Heating fabric made up of nonwoven fabric has a limited use owing to the high electrical resistance of conductive fabric. The knitted-based heating fabric has a significant advantage over other methods in terms of its flexibility, stretchability, and comfortability. Liu et al. [11] designed and fabricated the three types of knitted heating fabrics (KHFs), plain, rib, and interlock fabric, using silver-plated conductive yarn and polyester spun yarn. They concluded that the interlock structure was superior to the plain and rib-knitted structures in terms of the heating performance. The structural elements of the knitted structure play an important role for optimizing the heating performance. Kexia sun et al. [12] reported that float and tuck stiches decrease the resistance value of conductive knitted fabric. Consequently, the surface temperature of the combination of a float and tuck knitted heating pad was higher than that of the 100% knitted structure.

The literature has given very little justification on the optimization of the heating performance of the knitted-based heating pad. Herein, we reported a rib (knitting structure)-based wearable heater with localized conductive yarn. This study investigated the surface temperature of the heating pad with a slight variation in the loop length in the knitted structure.

2. Materials and Methods

Low twisted cotton yarn (74 tex) and silver coated nylon yarn (30 tex) were used to fabricate the heating pad by using a V-bed knitting machine (14 gauge). Cotton yarn was used as a nonconductive material, while silver coated yarn was used as a conductive material. The rib-knitted structure was prepared with three courses of localized conductive yarn after six courses of non-conductive yarn, as shown in Figure 1a–e. Four samples of heating pads, such as R1, R2, R3, and R4, were prepared with a varying loop length. The specification of the samples is illustrated in

Table 1. Sample specifications.

Sample Code	Sample Appearance	Electrical Resistance (Ω)	Areal Density (g/m2)	Loop Length (mm)	Courses Per Inch (CPI)	Wales Per Inches (WPI)	Stich Density (loops/inch2)
R1		51 ± 2	414 ± 10	4.82	27	33	891
R2		63 ± 2	346 ± 10	5.12	22	30	660
R3		75 ± 2	330 ± 10	5.81	18	28	504
R4		80 ± 2	309 ± 10	6.21	16	26	416

.

3. Results and Discussion

Analysis of Surface Temperature of Knitted Structures

The average surface temperature of the rib-knitted heating pad was analyzed by a thermal infrared camera (Fluke ti450 pro) with an under specified voltage source. The DC source (IT6720-100W) was used to power the heating pad’s active component. The sample was kept at a distance of 40 cm from the camera, which was fixed on a tripod with 95% emissivity.

The surface temperature of the fabric was investigated at a 9-volt DC power source at room temperature. The rib structure (R4) with a loop length of 6.21 mm showed a 31.4 °C average surface temperature. At the same input power source, the heating performance of the rib structure increased by 51% with a loop length of 4.82 mm, as shown in Figure 2a. This can be explained by the tightness factor of the knitted structures. The tightness factor of the knitted fabric is calculated by the following Equation (2) [13].

$$Tightness factor \left(TF\right)=\frac{\sqrt{Tex}}{l}$$

(2)

where l is the loop length of the rib structure in cm and Tex is the linear density of the yarn.

The fabric sample (R1) with the shorter loop length exhibited a more tightness factor, i.e., 11.36 (

Table 2. Tightness factor of the rib-knitted structures.

Sample Code	R1	R2	R3	R4
Tightness factor	11.36	10.69	9.43	8.82

), resulting in a more compact and less conductive yarn being required on the active part of the sample. Thus, knitted structures (R1) exhibited a lower electrical resistance, i.e., 51 Ω, than the other samples. Consequently, the amount of current flowing through the knitted structures (R1) increases, leading to an increase in the fabric’s surface temperature. A sample (R1) is utilized to design the heating pad to test the viability of the knitted structure. The heating pad is applied to the elbow, wrist, and calf regions of the body part at a 9 V/2.5 A DC power source, as shown in Figure 2b–d.

4. Conclusions

The objective of this study was to optimize the surface temperature of the heating pad by varying the loop length of the knitted structure, a subtle alteration in the structure of the loop that has a significant impact on the surface temperature of the heating pad. The sample (R1) with the lowest loop length with a compact structure showed a good heating performance. The prototype of the heating pad is designed for the alleviation of joint and muscle pain in the affected area of the body. The study’s findings indicate that the loop length of the knitted fabric is a critical component in optimizing the surface temperature.