Can heterogeneous compression textile design reshape skin pressures? A fundamental study

Abstract

Textile-based compression interventions (e.g. compression stockings and bandages), as an essential “source of pressure”, have impacted the effectiveness of pressure dosage delivery. The homogeneous structures of traditional compression shells generate highly uneven pressure distributions around leg geometrics in a passive mode, resulting in side effects and uncomfortable wearing perception. With this in mind, new heterogeneous compression sleeves with hybrid elastic properties were fabricated utilizing advanced three-dimensional seamless knitting technology and a unique laid-in structural design. PicoPress pressure assessment revealed in vivo that the developed heterogeneous compression shells with appropriate configurations for the lower limbs demonstrated the capability to proactively reshape skin pressures around leg cross-sections via calibrated proportions of segments with hybrid elastic moduli. The reduced anterior peak focal pressures and increased pressures at muscle-dominated posterior calves together provided a promising measure to enhance pressure function and user compliance in practice. The results will contribute to the development of a new generation of heterogeneous compression stockings with “bi-axial” pressure profiles for improved compression performance in extensive applications.

Keywords

heterogeneous compression textiles hybrid elastic properties skin pressure pressure reshaping compression therapy

The global compression hosiery market is expected to grow at a compound annual growth rate of 5.3% through 2024,¹ driven by an increasing prevalence of venous disorders, the growing adoption of compression therapy and a rising elderly population. Chronic venous insufficiency (CVI) is a long-term progressive condition in which venous pooling reduces venous return, resulting in pain, itching, tiredness, varicose veins and even ulceration in the lower limb,² and which affects approximately 25–40% of women and 10–20 % of men globally.³ Compression therapy has been used in CVI treatment for centuries,⁴ for which textile-based compression interventions, as the essential “source of pressure”, have impacted the effectiveness of pressure dosage delivery,⁵ although their exact mechanisms of action remain controversial. The “uniaxial gradient” pressure is a core principle of compression hosiery used in CVI treatment. Through the control of dimensions and densities in continuous fabric segments along “compression shells”, a degressive or progressive gradient fashion following certain “residual pressure ratios” has been demonstrated to reduce venous hypertension or increase the ejection fraction of the venous calf pump.^6–10 However, high non-compliance resulting from discomfort perceptions and ill-fitting negatively affects the effectiveness of compression hosieries in clinical practice, which is considered to have been much underestimated in previous studies.^11–13

Homogenous fabric structures were employed in the traditional compression stockings, even though structural densities may vary between the sequential gradient segments lengthwise (Figure 1(a)). The skin pressures were passively generated by stretching elastic shells to wrap over irregular lower limbs with larger volume. The magnitudes and distributions of interfacial pressures at any angular sites around the leg largely depend upon the geometric structures of the anatomical sites located, which can lead to highly uneven skin pressure distributions (Figure 1(b))^14–16 and may result in side effects,¹⁷ e.g. ischemia, necrosis and even ulcerations at bony prominences, especially in the elderly with thin and fragile skin.¹⁸ To date, few studies have looked at solutions aimed at reforming the compression shell itself.

Figure 1.

(a) Homogeneous structure of a traditional compression shell (stocking); (b) cross-sectional pressure profile of a homogeneous compression shell; (c) pressure reshaped by new heterogeneous compression shell.

Long-stretch stocking materials (static stiffness index (SSI) <10 mmHg/cm) generate sustained elastic compression, and short-stretch materials (SSI > 10 mmHg/cm) (e.g. bandage) deliver intermittent high working but low resting pressures with muscular contractions and relaxations, which was demonstrated to provide a more significant muscular pumping action, thus promoting venous return.¹⁹ However, the sustained excessive or uneven pressure induced by elastic (long-stretch) materials, and the short functional terms (e.g. 1 week) of rigid materials, have reduced their tolerance and usability in practice.²⁰ How to optimize fabric elasticity and structures to promote pressure delivery and user compliance remains to be resolved.

The available studies on heterogeneous properties of compression textiles are mainly focused on four aspects: (a) yarn multi-components and core-sheath-covering structures,²¹ (b) anisotropic and non-linear behaviors of elastic fabrics,²² (c) integrative elastic or inelastic layers in bandages,²³ and (d) tension variations created by “inserting or sewing” additional rigid or supportive elements into elastic fabrics.²⁴ The creation of heterogeneous elasticity in one piece of compression shell to raise the effectiveness of pressure dosage delivery is rarely reported. Stolk et al.²⁵ attempted to “insert” firm material at the posterior side of compression stockings against the expanding posterior leg muscles. However, they did not clearly depict the fabrication method (e.g. seamless knitting or sewing), and also did not provide adequate illustrations regarding cross-sectional skin pressure variations of the heterogeneous shell. The integration of firm and elastic materials into a monolayer to achieve both medical function and tactile comfort is most challenging in the fabrication of compression modalities, since any protruding seams or overlaps may potentially alter local pressure and chafe the skin in long-term wear.

In addition, the morphological irregularity and heterogeneous tissue properties of human legs significantly influence the evenness of skin pressure created by compression shells.²⁶ For example, a build-up of skin pressure in the anterior tibial area cannot decompress to the surrounding tissues due to the tibia’s stiffness, and the tibia’s large surface curvature commonly results in focal increased peak pressure,²⁷ which is more prone to causing skin damage and pressure sores in sustained compression therapy according to Laplace’s law. In contrast, the posterior elastic muscular regions, which have less surface curvature, can absorb and dissipate increased compression caused by external interventions due to less tissue stiffness, but deficient skin pressures²⁸ commonly occur when the tissue becomes flattened or pressure sensors are sunk into elastic tissues by the squeezing forces of compression shells. Therefore, an ability to proactively control and reshape skin pressures delivered by compression shells is critically important to improve pressure delivery effects in compression therapy.

In this study, heterogeneous compression shells with calibrated hybrid elastic properties have been developed to reshape cross-sectional skin pressure, to reduce peak focal pressure for more even pressure exertion around cross-sections in the lower limbs, and we also explored the possibility of elevating surface pressure over the posterior calf to augment muscular pumping action in dynamic wear (Figure 1(c)). The determined heterogeneous compression properties and wearing configurations in this study will provide valuable evidence to guide the design of new compression stockings with “bi-axial” pressure profiles for improved biomechanical function and user compliance in practice.

Experimental work

Sample preparation

Footless heterogeneous compression stockings (HCSs) with hybrid elastic properties were fabricated using advanced three-dimensional (3D) seamless circular knitting technology. The nine tubular HCSs, 25 cm in length and 21 cm in circumference, were knitted using three types of interlaced Lycra-based covered polyamide elastomers, including inlay threads with linear densities of 210D/40D/40F, ground knitting threads with linear densities of 40D/40D/40F and thermoplastic threads with 70D/10F (D indicates denier, 1 denier means the mass in grams per 9000 meters of the fibers; F indicates the number of filaments). Laid-in stitches are the key knitting elements in the fabrication of compression textiles, and they play a critically important role in managing and controlling pressure magnitudes in dynamic wear. Employing our specially designed heterogeneous knitting structures, each HCS shell consisted of eight seamlessly knitted panels fabricated by complex full knit, laid-in knit and plating knit stitches. As shown in Figure 2, the ground knitting threads (black) were positioned in interval courses to provide the heterogeneous shell with a basic layer. In the panels with lower elastic moduli (Figure 2(a)), the polyamide double-covered Lycra yarn as inlay threads (red) were incorporated into the basic layer in the form of integrative miss and tuck stitches in the same knitting cycle, to control the tension and compression of the knitted shells. In the panels with higher elastic moduli (Figure 2(b)), the inlay threads (red) participated in the loop formation of the interval knitting cycles to provide a more compact and stably shaped knitting structure.

Figure 2.

Nine heterogeneous compression stockings with different calibrated elastic panels; each tubular shell was evenly portioned by eight seamlessly knitted panels using advanced three-dimension seamless knitting technology. (a) technical face and back of panel with lower elastic modulus; (b) technical face and back of panel with higher elastic modulus; (c) nine specimens; and technical face (d) and technical back (e) of panel with hybrid elastic moduli.

To enhance the stability of panels with higher elastic moduli, the thermoplastic polymer threads were plated with ground threads (blue), which were heated at 40℃ and reset upon cooling. Such heterogeneous fabrics can be washed 50 times with controlled tension loss of less than 6%. The eight panels were constructed by nine types of calibrated hybrid elastic proportions (Figure 2(c)). They were: (a) specimen A: HCS shell with lower elastic modulus (8:0), (b) specimen I: HCS shell with higher elastic modulus (0:8) and (c) HCSs (specimens B, C, D, E, F, G and H) with hybrid elastic moduli (Figure 2(d) to (e)), in which the proportions of panels with lower and higher elastic moduli were 7:1 (B), 6:2 (C), 5:3 (D), 4:4 (E), 3:5 (F), 2:6 (G) and 1:7 (H). The prepared specimens laid down an important foundation to determine the effects of heterogeneous compression and its spatial configuration on pressure reshaping at lower extremities.

Mechanical assessment of heterogeneous elasticity

The mechanical properties of the compression tubular specimens were tested in controlled conditions (temperature of 21 ± 2℃ and a relative humidity of 65 ± 2%) (ASTM D1776) following a minimum of 24 hours of equilibrium.²⁹ The mechanical behavior at large deformations (0–120% stretch) for the nine HCS shells studied were measured via an Instron Universal Tensile Testing Unit (Model-4411) (Figure 3), with two opposite directional stretching clamps (gauge length of 11.5 cm and actual stretch width of 8 cm) at a constant speed of 300 mm.min^-1. The stress–strain curves under the three cycles of loading were recorded to represent interactive relationships between tensile forces in Newtons (N) and elongations (strain) of the tested specimens in percentage (%). The elastic moduli of the specimens (megapascal, MPa) were automatically recorded by the Instron testing system. The overall tension–stretch performances of the HCS shells in horizontal (circumference) directions were determined, which dominate encircling pressures around the lower limbs. To standardize testing conditions, the panels with higher elastic moduli of specimens B, C, D, E, F, G and H were positioned at the balanced center between the two stretching clamps, as shown in Figure 3(c) and (d).

Figure 3.

Measurement settings of mechanical behavior of the studied compression tubes. (a) tube with homogeneous structure and lower elastic modulus (e.g. specimen A, 8:0); (b) tube with homogeneous structure and higher elastic modulus (e.g. specimen I, 0:8); tubes with hybrid elastic properties (e.g. (c) specimen B, 7:1 and (d) specimen D (5:3)).

Interfacial pressure measurements in vitro and in vivo

According to Laplace’s law (P = T/R), sub-bandage pressure (P) is directly proportional to fabric tension (T) but inversely proportional to the radius of curvature (R) where the compression shell is applied to the lower limb. The soft tissue properties and surface curvatures of leg geometries synergistically influence pressure performances. In this study, three conditions of pressure measurement were adopted, including: (a) in vitro pressure measurements as a control condition and (b) in vivo pressure measurements under two configurations of HCSs and lower limbs (i.e. conditions I and II).

In vitro pressure measurements

In controlled conditions, the amount of interfacial pressure was determined by using an air-filled pneumatic pressure testing system (PicoPress, Microlab Elettronica Sas, Italy; pressure range: 0–189 mmHg; deviation within ±1 mmHg) (Figure 4(a)). When the tested HCSs were mounted onto the wooden leg model with rigid textures and round cross-sections (Figure 4(b)), a flexible circular plastic bladder with a diameter of 5 cm was respectively inserted at the four typical directions (anterior (P1), medial (P2), posterior (P3) and lateral (P4)) around the calf cross-section beneath the HCS shell. In each test, the operator pushed on an embedded syringe to introduce 2 cm³ of air to the bladder. The resultant expansion in thickness was retained by the compression shells and the manometer recorded the pressure values in mmHg. After data collection, the probe was deflated and left in position until further readings needed to be taken. In the vitro test, the surface stiffness and radius of curvature at any testing point around the same cross-section of the leg model was maintained as identical. The amount of interfacial pressure was dominated by the variations in the hybrid elastic moduli of the HCS shells. The calf circumference of the wooden leg was 35.6 cm and all the nine tested HCS shells produced approximately 70.3% of horizontal stretch when they were being mounted onto the wooden leg model.

Figure 4.

(a) PicoPress pressure measuring unit and transducer; (b) control condition: pressure test in vitro on wooden leg model; (c) condition I: pressure test in vivo when panel(s) with higher elastic moduli were mounted onto anterior leg (P1) and (d) condition II: pressure test in vivo when panel(s) with higher elastic moduli were mounted onto posterior leg (P3).

In vivo pressure measurements

Compared with pressure testing in vitro, the subjects’ lower legs, with highly uneven curvatures and tissue stiffness around the anterior and posterior regions, allow the interactions between HCS shells and leg morphologies to be more complicated. The hybrid elastic materials acting synergistically with rigid anterior bones that have greater local curvature, or acting with elastic posterior muscular compartments that have less local curvatures, change the amount of pressure delivered by the HCS shells. Therefore, the two measurement configurations of HCSs for the lower limbs were protocolled to determine skin pressure profiles in vivo, including: (a) condition I, mounting the segments of HCS shells with higher elastic moduli (shorter stretch) at the anterior region and the segments with lower elastic moduli (longer stretch) at the posterior leg (Figure 4(c)) and (b) condition II, mounting the segments of HCSs with lower elastic moduli at the anterior rigid tibia region and the segments with higher elastic moduli at the posterior elastic muscular calf (Figure 4(d)). A total of 20 healthy female subjects aged 24.0 ± 1.6 years (weight: 59.2 ± 10.4 kg, height: 165.5 ± 10.5 cm and body mass index 21.6 kg/m²) participated in the pressure assessment in vivo.

Ethical approval was obtained from the Human Subjects Ethics Sub-Committee of Hong Kong Polytechnic University. The inclusion criteria were that the participants were: (a) >20 years old and (b) had no open wounds or venous disorders (e.g. varicose veins or swelling) on their lower limbs so that they could smoothly put HCSs onto legs as required in the experiments. In general, the circumferences of the subjects’ calves were 34.3 ± 2.7 cm at the widest calf level. HCS shells produced 51.9–76.2% of horizontal stretch in pressure tests in vivo. Each condition of measurement was conducted three times and the average pressure values were recorded for data analysis.

Statistical analysis

Statistical analysis was conducted based on interface pressure data tested on a wooden leg in vitro and on subjects’ legs in vivo. Mean values and SDs were given in quantitative measurements. Comparative analysis of interfacial pressures induced by HCSs and under different measurement conditions were analyzed using an analysis of variance (ANOVA) model. Post hoc analysis was performed by utilizing Bonferroni’s test to identify if any particular differences in interfacial pressures at the tested anatomical sites existed among the nine HCS specimens, and a two-tailed Student’s t-test was employed to examine differences in interface pressures between conditions I and II in vivo. Evidence of statistical significance was considered to be p < 0.05. All of the statistical analyses were conducted using SPSS for Windows (v.23.0, IBM SPSS).

Results and discussion

Mechanical behavior of the heterogeneous compression shells

Figure 5 illustrates the stress–strain curves of the nine types of HCS specimen studied. It can be seen that the overall tensile forces (N) increased with an increasing proportions of panels with higher elastic moduli. When the tensile strain was up to 70.0%, the tensions of specimens B, C, D, E, F, G and H achieved 29.8 N, 33.9 N, 34.6 N, 34.9 N, 37.4 N, 44.3 N and 52.8 N during the third loading cycle, respectively. Specimen I presented approximately 2.5-fold greater tensile forces than those of specimen A. The 70% stretch simulated the stretching condition when the tubular specimen was mounted onto the leg model or the subject’s lower limb in the pressure test. The elastic modulus can measure a fabric’s resistance to being deformed elastically when stress is applied to it. A higher elastic modulus reflects a stiffer or shorter stretch material property. It can be seen that the elastic moduli of the tested specimens ranged from 0.78–4.25 MPa and that almost all of the specimens presented linear elastic properties under a tensile strain of 70%.

Figure 5.

Representative graphics of stress–strain curves of the nine tested heterogeneous compression stockings with different elastic designs.

Compared with the conventional compression stockings with homogeneous elastic properties (Figure 5(a)), the designed heterogeneous compression shells presented superior capabilities regarding the generation of controllable multi-level tensile forces under the same tensile strain in the horizontal fabric direction, which demonstrate the possibility of flexibly delivering multi-class (level) pressure dosages to treat CVI symptoms³⁰ via the calibration of hybrid ratios of lower and higher elastic moduli in the knitted panels of compression shells.

The hysteresis effect of elastic compression shells is largely dependent on the polymer material characteristics, knitting structures and tension settings. The elastic hysteresis of compression shells determines the sustained pressure function during ambulatory wear. Elastic shells with higher elastic hysteresis are unable to maintain pressure with circumferential changes of the leg during muscular contraction and relaxation in motion. Conversely, elastic shells with lower elastic hysteresis provide sustained dynamic recoiling forces to squeeze the muscular tissues to facilitate venous return after repeated elongation and recovery processes in use. In this study, specimen A (8:0) presented lower elastic modulus and hysteresis, and specimen I (0:8) showed higher elastic modulus and hysteresis, indicating that increasing the number of short-stretch panels can potentially raise the elastic hysteresis but reduce the shape recovery of a compression shell during repeated trials in vivo, which would decrease the corresponding interfacial pressure data, thus potentially influencing squeezing forces to augment muscular pumping action and their transmission into the deeper venous system. It is usually recommended that commercial elastic compression stockings are routinely washed to eliminate hysteresis and refresh their mechanical performance. The newly developed heterogeneous compression shells (e.g. specimens B, C, D, E, F, G and H) can reduce tension loss to less than 6% after 50 washing cycles, and present relatively balanced elasticity and shape retention in cycles of stretch loading at 70% of tensile strain (Figure 5). To further reduce the elastic hysteresis of the compression shells with higher elastic moduli, more material studies, structure design and mechanical experiments, and dynamic wear trials need to be carried out in future studies to promote sustainable pressure function in long-term use.

Interfatial pressure in vitro assessment

Under in vitro conditions, the influences of curvatures and soft tissue properties on pressure magnitudes were excluded since the cross-sectional morphology and texture of the wooden leg were round and rigid, respectively. Figure 6 depicts the average interfacial pressures and SDs (in the table) on the testing points by the tested nine specimens. It can be seen that the interfacial pressures exerted by HCS shells with higher elastic moduli are significantly greater than those on opposite side of the wooden leg model exerted by the panels with lower elastic moduli (p < 0.05). The most significant differences in the interfacial pressures between the test points P1 and P3 were found at specimen D (5:3) (p < 0.001). Compared with specimen A, consistent increases in average pressures were found for specimens B to I: 17.9 % (B), 21.6 % (C), 29.5 % (D), 35.8% (E), 39.5% (F), 43.7% (G), 53.2% (H) and 57.9% (I). The presence of segments with higher elastic moduli generally increased the tension forces of the HCS tubes, resulting in increased overall interfatial pressures around the calf in vitro.

Figure 6.

Interfacial pressure beneath heterogeneous compression stocking shells with hybrid elastic moduli tested on a rigid wooden leg model.

Interfacial pressure in vivo assessment

Figures 7 and 8 illustrate the skin pressure measurements in vivo (means and SDs are shown in the table) under the two configurations of HCSs and lower limbs. Compared with the in vitro pressure test, calf surface curvatures and soft tissue elasticities were involved in interface contacts between skin and the HCS shells under the assumption that no frictional component occurred between compression shells and the biological skin surface. That is, the influence of frictional forces generated on interfacial pressures was not considered in this static test in vivo. In condition I, the panels with higher elastic moduli (shorter stretch) were placed at the anterior tibia crest region and the panels with lower elastic moduli (longer stretch) at the opposite posterior elastic muscular region (Figure 7). For specimens A and I, resembling traditional long-stretch compression stockings and short-stretch bandages, the pressures at the anterior tibia crest (P1) were greater than those at the posterior muscular calf (P3) by 30.0–42.1%. It can be observed that the intervention of biological lower limbs in the condition I configuration further increased the unevenness of cross-sectional pressures. In general, the anterior skin pressures were significantly greater than the posterior pressures by approximately 30.6–50.9 % (p < 0.001), especially for specimens E and F.

Figure 7.

Condition I: interfacial pressures in vivo when the panel(s) of heterogeneous compression stockings with higher elastic moduli were positioned at the anterior tibia, and panel(s) with lower elastic moduli were positioned at the posterior calves.

Figure 8.

Condition II: interfacial pressures in vivo when panels of heterogeneous compression stockings with higher elastic moduli were positioned at the posterior calves and panels with lower elastic moduli were placed at the anterior tibia crest region.

In condition II (Figure 8), the higher elastic moduli panels interacted with the anterior rigid tibia area and the less elastic moduli panels interacted with the posterior elastic calves. It can be seen that this new configuration significantly reduced the anterior–posterior pressure differences by around 41.7–57.1% compared with those in condition I. Furthermore, under similar pressure ranges, the peak focal pressures at the anterior calf were generally lowered but the posterior calf pressures were elevated for all the tested HCSs with hybrid elastic moduli in condition II. This meaningful result indicated that knitted heterogeneous compression shells with appropriate elastic moduli and configurations could not only reshape pressure profiles around limb cross-sections, but could also reduce peak tibia pressures and potentially augment posterior muscle squeezing to facilitate venous return. In traditional compression shells with homogenous structures, skin pressures are passively generated depending upon the geometric structures of the anatomical location. Bony prominence regions (e.g. the anterior tibia, tarsus and patella bones) with greater curvatures are the common sites resulting in high peak focal pressure in compression therapy. Monotonous tensile strains and tensile forces are generated when homogenous compression shells are stretched to conform to body volumes. Constant tension forces could be produced between adjacent testing points within the compression shells following Hooke’s law. In the 3D homogeneous compression shells, radiuses of curvatures (R) (i.e. a derivative of curvature) of the applied anatomical position become the dominant determinant of pressure dosage exerted at a local region according to Laplace’s law (Figure 1).³⁰

In comparison, the underlying mechanisms of the designed heterogeneous compression shells are fundamentally different, as the segments with lower elastic moduli follow the mechanical behaviors of long-stretch properties, and the segments with higher elastic moduli follow short-stretch mechanical behaviors (Figure 5). An approximately 5.4-fold difference is found between specimen A, with the minimum elastic modulus, and specimen B, which has the maximum elastic modulus under the same strain ratio of 70%. The horizontal fabric tensions varied under different panel contacts. During stretching, interactions of tension forces were produced between segments with lower and higher elastic moduli, and frictional forces were produced between hybrid elastic panels and the skin surface accordingly, which together cause fabric tensions to become inconsistent around irregular lower limbs. Under the setting of condition II, such new hybrid elastic compression shells were demonstrated to be capable of redressing pressure profiles on biological legs by proactively adjusting heterogeneous structures and elastic moduli rather than by varying local curvatures via traditional auxiliary accessories (e.g. by inserted padding or foams).

Comparison of cross-sectional pressures among different testing conditions

Heterogeneous internal tissue properties and distributions influence the surface contours of biological legs. From a cross-sectional anatomical view, the bony structures are at the anterior half section, including mainly the tibia, tibial tuberosity and fibula, and abundant elastic muscle compartments are at the posterior half section, e.g. the gastrocnemius, soleus and tibialis posterior. The anterior tibial region most sensitively reflects the influence of heterogeneous structures and configuration variations on skin pressures in vivo. In general, the anterior skin pressures (at P1) in condition II are significantly lower than those in condition I (10.7–14.2%; Figure 9(a)); and the posterior skin pressures (at P3) in condition II are significantly higher than those in condition I (4.3–13.3%; Figure 9(c)). In contrast, skin pressures at the medial (P2) and lateral (P4) lower limbs were not significantly influenced by hybrid elastic moduli design (Figure 9(b) and (d)).

Figure 9.

Average interfacial pressure distributions around cross-sections of lower limbs.

Figure 10(a) further highlights the increased anterior–posterior pressure differences (ΔPanter-poster) in condition I caused by the biological properties of lower limbs. In contrast, similar ΔPanter-poster values were found between condition II and the control condition in vitro, especially for specimens B, C, F and G. The results imply that condition II would be the preferred setting to lessen the impact of the irregularity of lower limbs on skin pressure distributions and to contribute to the equalization of cross-sectional skin pressures in compression product design. In general, the average interfacial pressures were 20–40% greater in vitro than those in vivo for all the studied HCS specimens (Figure 10(b)), which indicated that the application of the traditional wooden leg model with a lack of anatomical attributions to monitor or control the qualities of heterogeneous compression fabrics during manufacture may cause pressure levels to be overestimated, thus resulting in deficient pressure dosages in application. Therefore, a more realistic leg model that involves elements of bioproperties is required for more accurate pressure assessment of heterogeneous compression shells in practice.

Figure 10.

(a) Anterior–posterior pressure differences (ΔP _anter-poster ) tested in vitro and in vivo; (b) Comparison on average interfacial pressures between in vitro and in vivo test conditions.

In addition, it is worth noting that heterogeneous compression shells present the capability to reshape cross-sectional pressures while maintaining similar pressure ranges under the two in vivo conditions investigated (Figure 10(a)), that is, the skin pressures were redistributed in four directions of a leg cross-section by HCSs shells without influencing the overall pressure level. For example, for specimens A, B, C and D with the same light pressure level (16–18 mmHg), the HCSs B, C and D in condition II can present more rational cross-sectional skin pressures than the traditional homogeneous elastic shells (i.e. specimen A).

Medical significance of heterogeneous compression design

According to Stolk et al.’s study, on average only 35% of the leg perimeter near the calf expands posteriorly, while the other 65% (including the anterior region) does not expand at all during kinematic movement.²⁵ The new HCS profiles explored in this study provide an evidence-based design approach to cater for this unique biostructure. By applying higher elastic moduli panels (shorter stretch) at the posterior leg, the increased skin pressure increases the counteracting forces to restrict the enlarged volume of the elastic muscles, thus potentially augmenting pumping action to facilitate venous return. The activity of the calf muscle pump is the motive force that affects venous circulation in the lower extremities. It causes the streaming of venous blood in both vertical and horizontal directions. The vertical flow has two components: centripetal flow during calf muscle contraction and centrifugal flow during muscle relaxation. Calf muscle contraction elevates the internal hydrostatic pressure to approximately 140 mmHg and expels venous blood into the popliteal and femoral vein, as well as transmitting blood from the deep lower leg veins into the saphenous system through calf perforating veins; the pressure situation is reversed during muscle relaxation.^31–33 Such calf pump activity evokes a physiological decrease in pressure to around 25 mmHg in the veins of the lower legs and foot,³⁴ which is critically important in ensuring the health of the lower extremities. In our HCS design in condition II, the external squeezing forces caused by shorter stretch compression shells set at posterior muscle regions would enhance internal ambulatory pressure gradient distributions and improve venous hemodynamics, which can be expected based on a number of existing clinical studies on the use of short-stretch bandages.^35,36

In addition, our study demonstrates that increasing the proportions of panels with higher elastic moduli from 12.8–87.5% can elevate skin pressures at the posterior calves by 2.0–37.2 %. However, the degree of elevation in skin pressures under the present conditions are still limited due to the lack of muscle volume variation while standing still. Stolk et al. reported²⁵ that the dynamic stiffness index (DSI) could increase by 40% or more within a mere 0.7–1.8 cm variations in leg perimeters when muscles contract during walking (DSI is a critical indicator reflecting pressure pulsation capability and therapeutic effectiveness in dynamic wear³⁷). Therefore, the larger increase of skin pressures by the designed heterogeneous shells is highly promising in kinematic movement, when friction forces synergistically interact with contact pressure under panels with hybrid elastic moduli. Further mechanical assessment and theoretical analysis will be performed in future dynamic studies.

Different pressure dosages can be applied for prophylaxis or treatment of different CVI symptoms. Normally, lower pressure dosages (e.g. <20 mmHg) are used for daily care to relieve leg discomfort (heaviness and fatigue), mild leg swelling and slight varicosity; moderate pressure dosages (e.g. 20–30 mmHg) are for the treatment of spider veins or pronounced varicosis; and higher pressure dosages (e.g. 30–40 mmHg) are for more severe venous stasis (e.g. severe varices with edema, the prevention of ulceration reoccurrence and mild lymphoedema). In this study, the designed heterogeneous compression shell presented controllable pressure levels via the adjustment of hybrid elastic moduli. Pressures ranging from 15–23 mmHg in the calf could be used for daily leg care and primary venous disorders (e.g. varices); meanwhile, the designed heterogeneous compression shell can redress anterior–posterior pressures to be more even in leg cross-sections under the defined pressure levels. This would contribute to the promotion of the effectiveness of pressure dosage delivery by elastic compression textiles in practice.

Sustained peak pressures by homogeneous elastic materials (e.g. traditional compression stockings) and quick pressure loss by rigid or short-stretch materials (e.g. bandages) after application have long been blamed for causing low user compliance and limited treatment periods. The present hybrid elastic moduli design combined the advantages of long-stretch and short-stretch properties in a monolayered continuum, achieving a balance of biomechanical function, comfort and durability in pressure application. An advanced laid-in knitting layout fabricated by 3D seamless knitting techniques integrating adhesive enhancement materials provided a smooth surface, and a reliable and easily controlled compression shell through digitally adjusting panel morphologies and knitting parameters, including the feeding amount of elastomers, loop densities and dimensions, to fit the specific requirements of individual users. Compared with the existing additional inserted or sewn rigid panel materials in elastic stocking bodies, such a seamless knitted compression design avoids skin chafing and increases usability in daily wear, which has not previously been reported for the use of current compression therapy products.

Based on results of the present study (Figure 11(a)), a new type of compression stocking with “bi-axial” pressure design (Figure 11(b)), delivering both longitudinal gradient pressures from the ankle to calf and horizontal reshaped encircling pressures from the anterior tibia to the posterior muscles, has been developed. This new bi-axial pressure design simultaneously generates two biofunctions, i.e. (a) degressive vertical pressure from the distal to proximal lower limb counteracts gravitational force to reduce venous hypertension and facilitate venous return, and (b) the gradient of horizontal pressure from the posterior to anterior limb cross-section reduces the risk of anterior peak focal pressure and augments muscular pumping action. This basic design principle can be further applied in the design of a new generation of compression stockings involving more consideration of ergonomic and comfort fitting concerns (Figure 11(c)), e.g. lowered fabric tension at the posterior Achilles tendon and the tip point around the ankle bones, a wider welt at the top of the stocking to avoid a tourniquet effect and a shaped instep contour to reducing the risk of pain being caused during sustained use, etc. Ongoing research work is being conducted using 3D body scanning, 3D digital seamless knitting designs and dynamic pressure assessment to further optimize elastic moduli, hybrid ratios, panel structures and pattern effects to fulfill different pressure requirements of individuals for diverse end use purposes and wearing conditions. This new generation of compression stocking is showing promise in improving user compliance and biomechanical function, which largely determine the effectiveness of compression textiles in both medical and active sports applications.

Figure 11.

(a) Reshaped skin pressures around the lower limb; (b) medical significance of the heterogeneous compression design (took specimen G with hybrid elastic moduli as an example); and (c) a new compression stocking prototype with a bi-axial compression design based on calibrated hybrid elastic panel ratios of 2:6.

Conclusions

In this study, HCSs with hybrid elastic moduli have been created by applying specially designed laid-in structures, calibrated panel designs and complex knitting materials. The interfacial pressures between heterogeneous compression shells and leg models have been assessed using the PicoPress pressure testing system in vitro and in vivo. The results demonstrate that the HCSs have the capability of reshaping pressure distributions when arranged in appropriate locations (i.e. condition II in vivo) around the leg cross-section. The traditional rigid wooden leg model may cause the overestimation of pressure dosages in practical applications. The biological properties of lower limbs (e.g. surface curvatures and heterogeneous tissue materials) exert remarkable influences on the amounts and distributions of cross-sectional interfacial pressures exerted by heterogeneous compression shells. More realistic leg models involving biological geometrics and tissue properties are needed to effectively assess pressure performance in vitro in quality control testing during their production. The segments with lower elastic moduli of HCSs positioned at the anterior tibia crest and segments with higher elastic moduli positioned at the posterior muscular region generally improved the evenness of cross-sectional skin pressure distributions; that is, they yielded profile with reduced anterior peak focal pressure but increased surface pressures around muscle-dominated posterior calves, which would enhance pressure function for ergonomic comfort and the augmentation of muscular pumping action in dynamic wear. Such new heterogeneous compression shells, by controlling fabric tension and configuration, provide a proactive way to redress cross-sectional pressures on lower limbs rather than changing local curvatures via additional insertions (e.g. padding or foams). Further studies are needed to explore the underlying working mechanisms, including the influence of frictional forces produced between heterogeneous shells and biological legs in dynamic wear, on skin pressure performances. Through the optimization of further dimensions and tension properties, a new compression stocking prototype with bi-axial pressure design, with consideration of optimal longitudinal and transverse pressure, could be engineered to enhance the ergonomic comfort and hemodynamic improvement of compression textiles in practice.

Footnotes

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: We would like to thank Hong Kong Polytechnic University for supporting this study through block grants 1-ZE7K and 1-ZVLQ, and research projects PolyU 252064/17E and ITS/031/17.

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