Washing Characterization of Compression Socks

1.1. Compression pressure mechanism

The mechanism of action is the lowering of pressure exertion from the ankle to the calf. The value of exertion of pressure should be highest at the ankle and must gradually decrease along the direction of the leg, as shown in Figure 1. This varying degree of compression pressure propagates and regulates blood flow by stopping its reverse flow, as shown in Figure 2, keeps the muscles in-line in the right position to mitigate the risk of injury, gives relief to many of chronic venous disease patients, and is used for therapeutic purposes [3, 4].

Figure 1.

Mechanism of action of compression socks [5]

Figure 2.

Comparison of reversible and irreversible blood flow [6]

1.2. Standard used for the evaluation of compression socks

There are various standard descriptions of the procedure for evaluating the compression pressure (kPa) of compression socks developed by various authorities and countries. These national standards for compression stocking have been developed mainly as procedural requisites must be fulfilled for the evaluation of compression socks. These standards cover the testing methods, yarn specification, stocking manufacturing methodology, compression gradient, and durability. There are few European national standards for compression hosiery, e.g., the British standard BS 6612:1985 [7], the French standard ASQUAL [8], and the German standard RAL-GZ 387:2000 [9]. RAL-387:2005 is currently being implemented as European standard ENV 12718, and ENV 12719 has been obsolete because consensus could not be reached and the standard was canceled in 2005 and modified to RAL GZ-387:2000. All compression socks are classified on the basis of compression pressure at ankle level (B point; minimum girth circumference). The sock characteristics include fiber content, yarn type, denier, knit type, class of compression level, and size of socks, and all related parameters are evaluated with the RAL-GZ 387/1 method.

This research followed the German classification of compression socks and selected socks of manufacturers who follow the same RAL-GZ:2000 standard shown in Table 2.

1.3. Classification of compression socks

The intensity of compression pressure used for various diseases is categorized as moderate up to (20–30 mmHg) and firm compression (30–40 mmHg). This level of pressure has been decided upon and recommended to treat circulatory and vascular medical conditions as well tired, sore, swollen, or aching legs [10,11,12,13,14], conditions that are given below in Table 1. Table 1 shows that the various countries and regions of the world classify their compression socks in different ranges depending on the intensity of the type of disease.

1.4. Washing compression socks

On the basis of a review of the scientific literature, we observed that few studies exist in which performance of compression socks had been analyzed after multiple cycles of wearing and machine-washing the socks and their influence on compression pressure (Ps) [9, 22,23,24,25,26,27,28,29]. Performance includes pressure changes and dimensional deterioration of compression socks made up of various combinations of main and inlaid yarns. It is usually recommended that commercial elastic compression socks are routinely washed to eliminate hysteresis and refresh their mechanical performance.

More than 200 of the brands exist around the globe that develop and sell their products and ultimately recommend hand washing of socks at 40°C rather than machine washing them. Drying is done by placing these sock samples between two layers of towels avoiding any of external force that could lead to deterioration of the compression. A few brand manuals are mentioned here for reference purposes [15,16,17,18,19,20,21].

The RAL-GZ 387/1 standard quality evaluation protocol of compression socks recommends that prior to testing, compression socks should be washed once according to DIN EN 26 330/6 A. The test samples must be spun dry for two minutes and dried flat according to DIN EN 26 330, method C; after the socks are washed, the sock samples should be conditioned by spreading them out after drying them for minimum of 12 hours in a standard atmosphere according to DIN EN 20139 at room temperature [9].

According to guidelines for the use and prescribing of compression hosiery recommended by NHS subjected to care of compression socks. Most manufacturers have instructed users to follow the proper washing procedure to prolong working performance for three to six months. According to the manual, compression socks should be hand-washed at 40ºC, but some garments may be suitable for gentle machine washing with mild detergent. Compression socks should not be wrung out, twisted, or tumble dried. They should be dried flat (not hung from a washing line) away from direct heat and when dry should not be ironed [22].

R. Liu, T. T. Lao, T. J. Little, X. Wu, and X. Ke have developed heterogeneous hybrid knitted structures to enhance the stability of knitted panels by increasing the stiffness value. Thermoplastic polymer threads were plated with ground threads, which was then heated at 40ºC and reset to cool. Ke has also recommended routinely washing commercial elastic compression socks to eliminate hysteresis and refresh their mechanical performance. In the end, he concluded that such heterogeneous fabrics can be washed 50 times with controlled tension loss of less than 6% and exhibit relatively balanced elasticity and shape retention in cycles of stretch loading at 70% of tensile strain [23].

H. Maleki, M. Aghajani, A.H. Sadeghi, A. Asghar, and A. Jeddi have investigated the pressure change effect due to repeated washing of different knitted fabrics. For this purpose, two kinds of knitted fabric were tested for pressure measurements after repeated washing and repeated usage. Finally, the experimental pressure values were compared with the theoretical results obtained with Laplace's law. As the results of statistical analysis indicate, the repeated washing and repeated usage have a significant effect on interfacial pressure and pressure reduction of both fabrics [24].

R. Harpa, C. Piroi, and C. D. Radu (2010) introduced a new approach to determine the capacity of medical compression hosiery to retain its designated gradual compression after repeated wearing–washing cycles. For his research, two pairs of sock samples were used for evaluation for 15 and 30 days. All the sock samples were tested before washing and after 15 and 30 cycles of wearing and washing. It was concluded that after repeated washing there is a decrease in compression pressure due to different levels of wearing and washing [25].

Recently, H. F. Siddique, A. A. Mazari, A. Havelka and Z. Kus (2019) have investigated the performance of compression socks. He washed the same sock sample first at 30°C and then 50°C and last at 75°C as a small part of his scientific research work. He has concluded from his research that as the temperature level of washing increases, there is a significant increase in compression pressure due to a gradual increase in the percentage of shrinkage of compression socks made up of nylon/Lycra compositions [26].

L. Macintyre, H. Stewart, and M. Rae (2016) mentioned that washing samples delivered a significant improvement in the pressure by allowing the samples by simple dry relaxing. Furthermore, machine washing samples at 60°C resulted in marginally higher pressures exerted compared with reconditioning. Machine washing socks at 60°C restored one brand to „like new“ pressure, and enabled the other brand of socks to exert higher pressures after wear and wash than they did when new. This marginal increase in pressure could compensate slightly for the pressure lost during wear. Thus washing anti-embolism socks between periods of wear (at least daily) should be recommended [27].

L. Macintyre, H. Stewart, and M. Rae (2016) mentioned in their work various manufacturers recommend washing the compression socks from 40°C to 75°C. They studied anti-embolism socks at 40°C and 60°C of wear and washing cycles and concluded that compression socks retain their ability to exert compression pressure when washed at 40°C while if the same sock samples are machine washed at 60°C, their intensity of compression pressure is significantly improved more than the new compression socks because when the socks are washed at a high temperature, the fiber relaxes, ultimately causing shrinkage and a decrease in the circumferential decrement, causing an increase in pressure [28].

S. Ghosh, A. Mukhopadhyay, M. Sikka, and K. S. Nagla (2008) studied the effect of washing on warp-knitted compression bandages on compression pressure. This study claimed that after use a garment has a drop in the pressure value at all the three critical positions (ankle, calf, and thigh). This indicates that contrary to the common claim, washing does not improve the performance of the garments. Actually improvement from washing takes place only with respect to the value of the pressure that is generated after second or third wearing of the garment before it is washed. This range of pressure may not be the desired pressure range for the treatment of the disease [29].

The scientific literature described above indicates the shortcomings of the washing process on compresison pressure and other related parameters. The effect of multiple (up to 20) wearing and machine washing cycles at a moderate temperature (40°C) on compression pressure (Ps) is usually recommended by various manufacturers, but there is much less literature in which the influence of 20 washing and drying cycles on compression pressure (Ps) has ever been evaluated. Also, few studies still exist in which the effect of different levels of temperatures (°C) on compression pressure (Ps) is studied, but collectively commercial brands have never been evaluated in this way, at different intensities of temperature levels (40°C–60°C–80°C–100°C), because they all recommend washing socks up to a maximum of 40°C. But in view of the skin fat, oils, dust, dirt, body residuals and foreign extracts may affect the performance of compression socks, and their long-run pressure efficacy may deteriorate, so it was decided to wash them at different temperatures. At different levels of washing (up to 20), the influence of temperature on shrinkage percentages (%) has also been evaluated for all levels of compression classes (Class I, Class II, and Class III) along the course direction. The influence of marking and unmarking methodology on compression pressure after handwashing (kPa) has also been analyzed, and the results have been statistically verified.

The main purpose of current research is to investigate the influence of different washing parameters and methodologies on compression pressure (kPa) and the percentage of shrinkage of compression socks at the ankle. It includes the influence of frequent wearing and machine washes and their effect on compression pressure (Ps). The main objectives of the current research are as follows:

1. To find the effect of multiple wearing-washes (up to 20) on compression pressure (classes I, II, and III)

2. To find the effect of temperature (up to 100°C) on compression pressure (classes I, II, and III)

3. To find the effect of multiple wearing-washes on shrinkage percentage

4. To find the effect of marking methodology on compression pressure (Ps)

2.1. Preliminary testing of fabrics to obtain preliminary data of fabrics

2.2. Knitting type used for compression socks

The knitting construction became renowned after the invention of the compression garments. There are two main types of knitting: warp knitting and weft knitting. For weft knitting, the flat knitting machine and the circular knitting machine consist of a single jersey, interlock, a double jersey, and rib machines, which are always used. In terms of warp knitting, Tricot, Raschel, and double-needle-bar Raschel machines are used.

All of the compression socks were visually analyzed are comprised of two type of designs named as 1 × 1 laid-in knit-miss stitches as well as 1 × 1 laid-in mesh-knit stitches as shown in Figure 3.

Figure 3.

Microscopic knit-type of compression socks used (a) 1×1 laid-in knit-miss (b) 1×1 laid-in Mesh-knit

A total of 13 sock samples were purchased from three different countries (Czechia, Turkey, and Switzerland) exhibiting different compression levels (class I, 2.399–2.799 kPa; class II, 3.06–4.266 kPa and class III, 4.532–6.132 kPa) where 1 kPa =7,500 mmHg). While selecting the sock samples it was ensured that the type of knit/structure should be same (plain/single jersey, 1 × 1 laid-in, knit-miss and 1 × 1 laid-in mesh-knit. All sock samples were selected for a fixed-sized standard wooden leg (240 mm circumference at the ankle) exhibiting three compression levels and a circumference range between (14.5–21.4 cm) at a lower part of the leg (the ankle). Selected sock samples were made up only of Polyamide and Polyurethane contents.

Sock samples were evaluated for their built-in physical and technical specifications as shown in Table 1 and Table 2 with great precision and accuracy under standard atmospheric conditions RH, 65 ± 5%, temperature, 20 ± 2°C) according to standard testing protocol provided by RAL-GZ 387/1 (medical compression hosiery quality assurance) and CEN 15831:2009.

Table 3 shows the results of quantitative analysis of the compression socks as well the categorization of the different classes of compression socks on the basis of their intensity of compression pressure at the ankle. Per the scientific literature, classification of the compression socks is based on the range of compression pressure at the ankle because of the contour surfaces and most of the bone of the leg being such a complex part of the leg.

Fiber analysis of the compression socks at the ankle portion was evaluated using the standard procedure mentioned in AATCC-20A-2013 by dissolving the nylon in formic acid and finally measuring the weight of the Lycra under standard atmospheric conditions.

The classification of the compression socks in classes I, II, and III was finalized on the basis of the German quality assurance method for the evaluation of the compression socks (RAL GZ-387/1) with more precision and accuracy.

Table 8 shows the technical specifications of compression socks measured at the ankle, which includes the weight of the fabric at the ankle, also called the “gram per meter square,” abbreviated as GSM, the unit of the measurement of the weight of the fabric used here is gram per meter square (g/m²). The weight of compression socks at ankle portion was measured by cutting two square cut strips, each 5 cm², and averaging their weight as given below in Table 4. The wales and course density were measured with the pick glass method as mentioned in the RAL GZ-387/1 standard of the quality assurance for the evaluation of the compression socks; the stitch density was measured by multiplying the course density, also known as the number of the courses in the wale direction, measured in “number of loops” in the vertical direction along the length of the socks by 1 cm while the “course density” was measured along the width of the socks at the ankle, also known as the number of the horizontal lines passing across the width of the compression socks at the ankle by 1 cm. The course density was measured by multiplying the wale density and course density in units of stitches per cm². All of the 13 sock sample cut strips were unraveled by detaching the inlaid yarn from the complex structure and untwisting them to analyze the type of yarns used. Most of the sock samples were made up of double-covered yarn instead of single-covered yarns as inlaid yarn while the main yarn was air-covered multifilament nylon and a combination of these threads depending on the requisite compression pressure (kPa).

Another purpose of unraveling was to know type of knit which was mainly found to be (plain/single jersey, 1 × 1 laid-in knit-miss and 1 × 1 laid-in Mesh-knit as shown in Figure 3.

2.3. Washing evaluation of compression socks

All of the 13 sock samples were evaluated for the compression pressure using a Salzmann compression pressure measuring device, and then all of the sock samples were hand-washed at a normal temperature and then dried by placing the set of sock samples between towels for 24 hours under standard environmental conditions (RH%, 65 ± 5%, temperature, 20 ± 2°C), after all of the sock samples were machine-washed and then dried up to 20 times. The compression pressure of the machine-washed sock samples was measured after every 5 cycles of washing and drying them for four times after completing the of total 20 cycles of dry-machine-washes. Another set of the compression socks were also machine washed but in this set of the socks samples were washed at successively high temperatures ranging from (30°C, 50°C, 70°C, and 100°C). At each temperature, the sock samples were later dried for 24 hours under standard atmospheric conditions and continued the same up to 100 °C. For washing all of the sock samples, the combined concept of the manufacturers RAL-387/1 and ISO 6330 was followed.

Here are the details of the handwashing and machine washing methodology used for the compression stockings.

2.3.2. Machine washing (20 wearing-washing cycles)

All hand-washed compression socks after pressure measurement, were machine washed as shown in Figure 4(a) as per ISO 6330 standard method recommended by RAL-GZ 387/1 and CEN 15831 compression socks testing manuals. At first, we selected the most delicate washing procedure of the type B washing machines to launder the socks samples for 5, 10, 15, and 20 machine washes on a daily basis according to aforementioned standard method. All the individual sock samples were weighed and measured the ballast amount of type II (Polyester 100%) to complete the total wash load of 2 kg. The specimen and ballast were mixed thoroughly before putting them into the machine.

Figure 4.

(a) Washing machine (b) Flat cabinet dryer

In a type B washing machine, as shown in Figure 3(a), hot water at a temperature 37 ± 3°C had 10 g of reference detergent of type 3 added directly into the dispenser. Reference detergent 3 is a nonphosphate powder detergent without an optical brightener and without enzymes. Another designation of reference detergent 3 is ECE 98. The main specifications of the machine washing procedure are given in Table 3.

The reference washing machine type B selected was a vertical axis, top loading agitator-adjusted machine per the ISO 6330 standard test method. Drum speed was selected to slow spin ranges from 6–7 revolutions per second while the stroke rate was selected as a gentle one, two strokes per second. Total wash time was 8 minutes, rinse time was 3 minutes, and spin time was 4 minutes.

After the rinsing, the hydro-extraction cycle was completed and all of the sock samples were taken out for flat drying. There are various procedures for drying, but the most prompt method selected was flat cabinet drying, usually recommended for drying. In this method, socks were removed from the machines and spread out in a flat dryer ensuring no wrinkles and no stretching. We allowed the sock samples to dry for 24 hours in still air in ambient conditions as shown in Figure 4(b).

2.4. Strip slicing and pressure measurement from the ankle

Strip slicing is very important for evaluating the compression pressure at the ankle after each time donning the socks, relaxing, and then donning them accurately. After each turn, the arrangement of each inlaid yarn at the same grooved line of the wooden leg at the ankle was aligned to measure the compression pressure. There were two methods: method 1 (without marking) and method 2 (with marking) were performed to get accurate results with minimal errors. Both of the methods’ standard deviations were compared and fixed; method 2 was evaluated for compression pressure.

2.5. Slicing of compression socks at ankle

Slicing of the cut strips for tensile analysis was done after marking a square of 50 mm² on the face of the compression socks, and pressure measurement ensured the main groove line (−) on the groove made on the face of the wooden leg at the ankle, as shown in Figure 5(e). Circular strips having widths of almost 50 mm were sliced to loop strips as shown in Figure 5(d). The slicing can be made at any position of the leg up to the thighs and arms in unstretched form. The sliced loop strips of all 13 sock samples were donned to measure the deformed width (wf), as shown in Figure 5(d) and were measured and used for theoretical modeling.

Figure 5.

Marking (a) Locating exact grooved line on leg on face of socks (b) Square marking 5×5 cm (c) Coinciding mean line and main line over the sensor at ankle on leg surface (d) Deformed width after wearing loop strip (e) grooved line (ankle portion)

2.6. Wooden leg model

2.6.1. Specification of the wooden leg

Pressure measurement of each socks sample was performed on a standard sized wooden leg. This leg exhibits the specifications mentioned in Table 8 and was arranged from Swisslastic standard leg producing company, Switzerland. The standard test method for the evaluation of compression pressure was developed by the German Institute of Quality Assurance and Identification (RAL-GZ 387/1) and CEN 15831, as shown in Figure 6.

Figure 6.

Standard wooden leg model

Table 8.

Standard wooden leg specifications (RAL-GZ 387/1)

Sr.#	Parameters	symbols	Wooden Leg
	Size		9
1	b level girth circumference	cb	240 mm
2	c level girth Circumference	cc	375 mm
1	b level height from floor	lb	120 mm
2	c level height from the floor	lc	300 mm

1. cb-circumference at ankle(b), cc-circumference at calf(c), lb-length from sole of foot to ankle (b) and lc-length from sole of foot to calf (c)

2.7. Measurement of compression pressure

There are various devices used for the experimental measurement of compression pressure (Ps). Three types of compression measuring devices developed and used by different researchers in their publications include MST MKII, MKIII, and MKIV Salzmann (Salzmann AG, St Gallen, Switzerland); Talley Digital Skin Evaluator (Talley Group Ltd, Romsey, England), which has become obsolete nowadays; and Kikuhime (TT Medi Trade, Soro, Denmark).

The main technical characteristics of the three measuring devices provided by the manufacturers are listed below in Table 9.

The basic working principle of all devices is pneumatic, but the most popular and frequently used device for the analysis and measurement of the compression device is the Salzmann MST MKIV device. All devices other than Salzamn MST MKIV consist of a probe having variable shape and dimensions depending on the device, connected to a pressure sensor device by a stretchy tubular string impossible to deform. These probes are positioned between the backing and the compressive device. Then the probes are filled with air to transmit the pressure through the tubular strings to the pressure sensor. These measuring devices do not require any type of calibration of the measurements. All of them display the pressure in mmHg (1 mmHg = 133 Pa), which is the most commonly used unit in the medical world. Comparative studies of the performance of the three sensors mentioned above show that they have different advantages and limitations. Of the three sensors tested, the Salzmann gave the lowest systematic error in the model leg, and it is also reproducible. It allows the measurement of four points; its limitation is that it requires a regularly increasing pressure from the thigh to the ankle (or at least an equal pressure). The Talley system recorded performance similar to the Salzmann system but is sensitive to probe curvature [1].

With all of the above benefits of compression pressure measuring devices and their availability, it was great breakthrough in the history of the development of the compression pressure measuring devices when Salzmann invented and patented their equipment, with names such as MST professional 2. This has very recently been invented and is the latest equipment used for both the static and the dynamic measuring of compression pressure. For our research we have used this device for the measurement of static compression pressure (kPa) at the defined dimensions of the wooden leg.

2.8. Calculation of pressure measurement

2.8.1. Experimental measurement of compression

To measure the compression pressure of all of the thirteen sock samples of varying circumferences, we put the sock samples on a wooden leg with the MST Salzmann sensor's sleeve between the wooden leg and the sock samples. We ran the machine and measured the readings of the compression pressure values when the data were relatively stable. In this research, we noted five readings of compression pressure taken from the device at the ankle and averaged the data recorded as the final pressure as shown in Table 10.

Table 10.

Measurement of compression pressure

Sample code	Pressure [kPa]	Standard deviation
A1	2.24	0.25
A2	2.40	0.40
A3	3.07	0.27
B1	3.65	0.35
B2	3.75	0.31
B3	4.34	0.37
C1	4.71	0.10
C2	4.83	0.30
C3	5.29	0.30
C4	5.33	0.20
C5	5.46	0.15
C6	6.26	0.27
C7	6.46	0.35

The graphical representation of the measurement of the compression pressure values is given in Figure 7.

Figure 7.

Measurement of compression pressure