A novel technique to determine pressure in pressure garments for hypertrophic burn scars and comfort properties

1. Introduction 

Burns damage skin caused by contact with fire, heat, electricity, radiation or caustic chemicals. The population of burns is usually defined by the exten and depth of the burn [1]. Severe burns affect the epidermis, dermis and hypodermis at the same time. The underlying bones, muscles and tendons may also be damaged. If this causes nerve ending destruction, there is no sensation in the area. In cases of massive burns, the body may lose large quantities of fluid. This can disturb the blood circulation and cause problems with body's salt balance.

The theory behind the use of pressure garments is quite simple and relies on two things; firstly the restriction of blood flow to the scar area and secondly, constant compression to inhibit the growth of hypertrophic scar tissue. According to affected areas, different designs of elastic pressure garments are used like pressure pants, gloves, heads, etc. [2]. These garments should be used 24h a day, being removed only for bathing, during 12–18 months according to the severity of burn and the length of the treatment [3]. Pressure garments are designed to exert a pressure of approximately 25mmHg on the underlying tissue. According to the literature, pressures should be more than 24mmHg in order to exceed the inherent capillary pressure [5], [6]. This ‘ideal’ pressure has varied over the years and has never been scientifically established [4]. This investigation is a novel technique to determine pressure in pressure garments for hypertrophic burn scars. It is aimed to determine the exact pressure in pressure garments on the applied body part.

2. Materials and methods 

2.1. Characterisation of the composite fabric 

A trilaminate composite fabric was used as a material composed of a face, sandwich laminate and knit backing.

2.1.1. Face of the fabric 

The face of the composite fabric is a plush knitted fabric. The plush loops from earlier rows are tightened in the production technique and the produced plush yarn can also be called terrycloth (Fig. 1, Fig. 2, Fig. 3) [7].

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Fig. 1. Plush knitted fabric.

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Fig. 2. Plush loops from earlier rows are tightened.

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Fig. 3. Plush yarn in the technique for the production of plush (also denoted as terry) (1); ground face yarn (2).

2.1.2. Back of the fabric 

The back of the composite material is a plain single-jersey knitted fabric (Fig. 4a and b).

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Fig. 4. (a) Plain single-jersey face, (b) plain single-jersey back.

2.1.3. Sandwich laminate 

The sandwich laminate is a flame-bonded polychloroprene rubber material. The material has a cushion effect and shows high elasticity, it is preferred to be used in textile industry especially in coated fabrics. Including chlorine atoms in the structure, the molecule also has a flame retardant characteristic (Fig. 5) [8].

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Fig. 5. Chemical structure of polychloroprene.

2.2. The basic determinant characteristics and elementary properties of the composite fabric 

The basic determinant characteristics and elementary properties of the composite fabric were assessed using the following procedures (Table 1):

•Thickness (mm) was tested on the James Heal R&B Cloth Thickness Tester following BS 2544: 1967 using a pressure of 5g/cm2 applied to a sample area of 1cm2 [9].

•Area density of the fabric (mass per unit area) was established using five (10cm×10cm) samples of the composite fabric and a mean was calculated (in g/cm2).

•Bulk density (fabric density) (g/cm3) was calculated by dividing the mean mass per unit area of the fabric by its mean thickness.

•Percentage fibre composition of each fabric in the composite material was established.

Table 1. Determinant characteristics of laminates of composite fabric

	Layers of the composite material	p	s	n
	PA%	100	100	0
	PCP%	0	0	100

PA indicating polyamide, PCP indicating polychloroprene.

In the referencing system for laminates in the tri-laminate composite fabric, the laminate numbers were prefixed with a ‘p’ for the plush fabric on the face of the composite fabric, ‘s’ for the plain single-jersey fabric on the back of the composite fabric and ‘n’ for the flame-bonded neoprene sandwich laminate of the composite.

The thickness (mm), bulk density (g/cm3) and area density (g/m2) of the composite fabric were determined (Table 2).

Table 2. Elementary properties of the composite fabric

	Properties of the composite fabric	Composite fabric
	Thickness (mm)	3.81
	Bulk density (g/cm3)	0.1795
	Area density (g/m2)	684

2.3. Thermo-physiological properties of the pressure garment 

The thermo-physiological properties of a garment provides comfort by maintaining body temperature and moisture output close to their normal levels [10]. Most manufacturers and health professionals, who are guided by the advice of the manufacturers as well as their experience with patients, recommend that the garment has to be worn for up to 2 years. So it is essential that they should be comfortable to wear. They must not abrade the developing scar or adjacent skin, which is either covered by or in contact with the pressure garment. They should not cause physiological discomfort due to excess warmth or sweat production [4].

So the thermo-physiological properties of the pressure garment were assessed by the thermo-physiological tests.

All tests were performed on conditioned samples in their relaxed state under standard laboratory conditions and the tests are as follows:

•The air permeability of the fabric (mm/s) was tested on the Shirley Air Permeability Tester in accordance with BS 3217: 1960. The test area was 5.07cm2 and five samples of each fabric were measured at a pressure of 10cm of water [11].

•The dry and wet thermal resistance (r) were measured on the Sensora Alambeta (this instrument developed at the Technical College of Mechanical and Textile Engineering in Liberec, Czech Republic).

•The dry and wet thermal absorptivity (b) were measured on the Sensora Alambeta.

•Dry and wet thickness (h) of the fabrics were measured on the Sensora Alambeta.

•The relative water vapour permeability (Pwv)% of the fabric was tested on the Permatest.

•The resistance to evaporative heat loss (Er) was tested on the Permatest.

•The water absorption (g/g) was measured following method based on BS 3449: 1961 and the mean water absorption percentage was calculated [12].

•The absorption capacity (g/g) was measured following EDANA: ‘Method for Determining the Absorption Capacity’ and the mean absorption capacity (g/g) was calculated [13], [17].

The test results for the thermo-physiological properties of the pressure garment were compared with a standard sportwear garment which is a single-jersey knitted fabric (Table 3).

Table 3. Thermo-physiological properties of the pressure garment compared with a sportwool

	Properties of the fabrics	Sportwool	Pressure garment
	Dry thermal resistance (W−1Km2)×10−3 (Alambeta)	24	61.06
	Wet thermal resistance (W−1Km2)×10−3 (Alambeta)	10	59.72
	Dry thermal absorptivity (Wm−2s1/2K−1)	83	113
	Wet thermal absorptivity (Wm−2s1/2K−1)	328	117.4
	Water vapour permeability (%) (Permatest)	36.5	7.50
	Resistance to evaporative heat loss (m2PaW−1) (Permatest)	0.0155	0.1265
	Air permeability (mms−1 for 10mm water pressure)	The sportwool fabric is too permeable to be tested	201.6
	Absorption (g/g)	3.59	0.160
	Water absorption (%)	367.2	46.27

2.4. Method 

2.4.1. Calculation the load change in length relationship by Instron 

According to BS 29073: 1992 Section 3, tensile strength tests were applied to the five (10cm×5cm) specimens in width direction (WD) [14]. And a load change in length graph was obtained (Fig. 6).

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Fig. 6. Relationship between load and change in length in width direction.

2.4.2. Regression analysis of pressure garment 

The polynomial regression equation (y) and correlation coefficient (R) were calculated and a graph of polynomial regression equation was obtained (Fig. 7).

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Fig. 7. Relationship between load and change in length in width direction.

2.4.3. Calculation of theoretical pressures using Laplace equation 

The Laplace equation based on circumference was used for the calculation of theoretical pressures (Eq. (1)). For each load value using the predetermined circumference values, theoretical pressures were obtained (Table 4).

(1)

where P is the pressure (mmHg); T the tension (kgf); Lc the Laplace constant (4620); C the limb circumference (cm); and W is the fabric width (cm).

Table 4. Theoretical pressure vs. circumference using Laplace equation

	Circumference (cm)	Pressure (mmHg)
		2N	4N	6N	8N	10N
	10	18.48	37	55.44	74	92.4
	20	9.24	18.48	27.72	37	46.2
	30	6.16	12.33	18.48	24.66	30.8
	40	4.62	9.25	13.86	18.48	23.1
	50	3.7	7.4	11	14.8	18.48

A graph was created to illustrate that pressure is inversely proportional to the circumference (Fig. 8) using the results of theoretical pressure versus circumference in Laplace's equation for a 5cm fabric width.

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Fig. 8. Theoretical pressure vs. circumference.

2.4.4. Calculation of circumference–change in length relationship theoretically for the optimum pressure of 20mmHg 

The tensions (N) were calculated for the optimum pressure of 20mmHg (Table 5).

Table 5. Tension vs. circumference for optimum pressure (20mmHg)

	Circumference (cm)	Tension (N)
	10	(2×20)/18.48	2.16
	20	(4×20)/18.48	4.32
	30	(6×20)/18.48	6.5
	40	(8×20)/18.48	8.64
	50	(10×20)/18.48	10.82

For optimum pressure of 20mmHg, the change in length versus circumference were calculated using tensile strength test results (Table 6).

Table 6. Change in length vs. circumference

	Circumference (cm)	Tension (N)	Change in length (mm)
	10	2.16	(1.35×10)/3.7	3
	20	4	(3.7×10)/3.7	10
	30	6.5	(6.7×10)/3.7	18
	40	8.5	(9.6×10)/3.7	26
	50	10	(12.3×10)/3.7	33

2.4.5. Calculation of pressure experimentally using the circumference–change in length relationship 

Two specimens were prepared in 5 and 10cm width in fabric width direction and grid was printed on the specimens in fabric width direction with an interval of 2cm.

2.4.6. Preparation of a change in length measure ruler 

Five circumference values were determined and a change in length measure ruler was prepared for each circumference value using change in length versus circumference for optimum pressure of 20mmHg. A 4N tension must be applied to the fabric in width direction to get a 1cm change in length for a circumference value of 20cm.

2.4.7. Test procedure on the prototype instrument pressure testing device 

Prototype Instrument Pressure Testing Device developed at Bolton Institute was used [15]. The principle of the prototype instrument centred around a mannequin leg and eight foil type strain gauge devices. The leg was used to simulate a real lower limb and has definable tibia, calf and ankle regions so that pressure profiles can be obtained. The pressures were detected by means of pressure pins which are connected to each strain gauge device [16].

2.4.8. Applying the novel technique using circumference–change in length relationship 

The ruler was put on the grid part of the fabric by taking a reference point from left and a compression applied to the specimen at width direction around the pressure sensors assembled on prototype electronic instrument until a circumference value of 10cm was obtained. When a required change in length value was obtained, the pressures were measured on the Prototype Pressure Testing Device (Table 7).

Table 7. Experimental pressures using mannequin leg

		Initial reading	Final reading	Avg. final reading	Lineer regression [{(Avg final−initial reading)+2.66}/1.06]	Pressure (mmHg)
	Fabric width 5cm
	Ankle (Sensor 1)	4915	4942	4942	27.6	28.61
			4945
			4941
	Calf (Sensor 5)	760	777	780	20	21.37
			780
			785
	Fabric width 10cm
	Ankle (Sensor 1)	4914	4933	4934	21.7	27.98
			4936
			4934
	Calf (Sensor 5)	759	781	781	23.3	21.37
			781
			781

3. Results and discussion 

The material used was a trilaminate composite fabric composed of a face, sandwich laminate and knitting backing. It was observed that the fabric was soft, flexible and extensible with good elastic recovery and the thickness of the fabric was calculated as 3.81mm which was beneficial, providing a cushioning effect to the scar areas (Table 2).

The thermo-physiological test results revealed that the fabric is warm next to the skin, even when wet, it has extremely low water vapour permeability (7.5%). Normally, this value should be >30% for a comfortable fabric. The fabric also showed high resistance to evaporative heat loss (0.1265m2PaW−1). This demonstrates that eventually the fabric will be too hot to wear. The water absorption percentage of the fabric was observed as 46.27 and the absorption capacity (g/g) was observed as 0.160 which were too low when compared with a control fabric (Table 3). These results reveal that the fabric absorbs very little water and thus will feel warm and wet next to the skin after prolonged wear.

The theory developed in this work relies on the hypothesis that the fabric width does not alter when extended or stretched in the width direction. Normally, this is the case in all bandages constructed with Lycra. In this fabric the fabric width changed significantly upon stretching, therefore the correct pressure is difficult to predict and hence calculate. This is the major drawback of this material.

This novel technique was studied for an optimum pressure of 20mmHg with a 5 and a 10cm width of the fabric. A graph was created to illustrate that pressure is inversely proportional to the circumference (Fig. 8) using the results of theoretical pressure versus circumference using Laplace equation (Eq. (1)). While the circumference of ankle is smaller than the circumference of calf, the pressure applied to the ankle is greater than calf (Table 7).

The relationship between circumference and change in length was calculated theoretically for the optimum pressure of 20mmHg (Table 6). The pressure values for ankle and calf were measured on the Prototype Pressure Testing Device experimentally and were calculated as 28.61, 21.37, 27.98 and 21.37mmHg (Table 7). The mean of four pressure values was calculated and the resultant pressure applied to the body was found as 24mmHg. Satisfactory pressure results were obtained using 3mm change in length values versus circumference (Table 6), these values were preferred to be used at the beginning of this novel technique.

A new burn fabric was invesigated which was introduced by one of our industrial partners, Vernon-Carus Ltd., UK. In this novel technique, it was aimed to determine the exact pressure on pressure garments on the applied body part and the comfort properties of a neoprene (perforated) composite fabric for use as a burn pressure garment were investigated.

It was found that the new fabric was not suitable for the intended use. It was much too uncomfortable and did not maintain a constant width when extended in the width or length direction.

We have developed a novel technique in this work which is easy to use in any burn centre. Rectangles have to be marked on the fabric and to obtain the desired compression of around 20mm Hg the stretch required have to be indicated.