Elsevier

Biomaterials

Volume 75, January 2016, Pages 174-181
Biomaterials

Smart medical stocking using memory polymer for chronic venous disorders

https://doi.org/10.1016/j.biomaterials.2015.10.032Get rights and content

Abstract

Proper level of pressure or compression generated by medical stocking or hosiery is the key element for successful treatment or management of chronic venous disorders such as oedema, leg ulcers, etc. However achieving the recommended compression level and, more importantly, sustaining it using stockings has been a major challenge to the health practitioners supervising the treatment. This work aims to investigate and design a smart compression stocking using shape-memory polymer that allows externally controlling the pressure level in the wrapped position on the leg. Based on thermodynamical rubber theories, we first derived several criteria that have to be satisfied simultaneously in order to achieve the controlled pressure adjustment using external heat stimuli. We then presented a case where such a stocking is developed using a blend yarn consists of selected shape-memory polyurethane and nylon filaments. Extensive experimental work has also been conducted to demonstrate the feasibility and explore the influencing factors involved.

Introduction

Patients suffering from chronic venous disorders, such as leg ulcers, oedema, venous stasis, venous hypertension, etc., are known to have poor quality of life due to continuous discomfort or pain, limited mobility and long recovery time, in addition to a rigorous management plan and thus the financial cost involved [1], [2]. Compression therapy is the cornerstone in the conservative treatment of such disorders since ancient times. Herein, external compression is provided to the affected leg by applying medical stockings or bandages to accelerate venous blood circulation, and finally decrease the venous pressure. The success of this treatment depends to a great degree on the level of pressure at the affected portion on the limb, and the sustenance of this pressure during the course of treatment [3], [4]. This interface pressure has to be applied quite accurately within certain limits and should not be either below or above the prescribed level, or certain complications will occur [5], [6].

In practice, selection of the stockings with proper sizing and fitting has always been a contest for both health practitioners and manufactures, for better patient's compliance and more effective treatment [7]. Different class of stockings are required to provide light (Class I, 14–17 mmHg), medium (Class II, 18–24 mmHg) and strong (Class III, 25–35 mmHg) levels of compression depending on the severity of the disease [8]. However in practice it is difficult to achieve the targeted pressure level due to various reasons, including mainly the different leg attributes (shape or size) among patients, and difference in material (including both stockings and the legs) properties (time and temperature dependence). Moreover, pressure drop over time is also a major concern due to the time dependence of the system behaviours [9]. For instance, experimental studies have showed that the pressure decreases over time due to reduction in swelling [10]. Also, many compression products displays initial pressure drop just after their application [4], [11], as they are made of polymeric materials (cotton, viscose, PET, etc.), to which stress relaxation is an inherent attribute, although stockings containing elastomeric yarns can alleviate the problem. As pressure drop is inevitable for almost all available stockings, and re-instalment or replacement of the stocking is needed once the pressure falls below the targeted level. Another reason for changing the stocking is for patient comfort, as there is often the need during night the stocking be removed so it will not interfere with the sleep [12]. Clearly, the aforesaid inadequacies of the conventional approaches demonstrate a compelling demand for a novel smart stocking system in compression management that allows modulating the compression level via external control to easily change or readjust pressure whenever needed.

In this work, based on some theoretical considerations, we first derived several criteria that have to be satisfied simultaneously in order to achieve the controlled pressure adjustment using external heat stimuli. We then proposed the use of shape memory polymers for the development of a smart compression stocking. Shape memory polymers (SMPs) are the smart material that can memorize the original shape so that they can recover from a temporary deformed shape, upon exposure to an external stimulus, e.g., heat, light, water, etc. [13]. SMPs have gained practical significance over the last 10 years in developing many potentially innovative products for biomedical applications such as clot removal devices, aneurysm occlusion devices, vascular stents, orthodontics, tissue engineering, etc. [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28]. In the field of compression management, shape-memory polymer based film actuator has been proposed by Ahmad et al. (2012) for pressure bandage application. They have used temperature–responsive SMP strips attached to fabrics to control the compression by an external heat source. The use of SMP film actuators may significantly obstruct the moisture transmission and the permeability of the compression system, and therefore not a viable solution for providing improved comfort to the patients. Moreover, the use of shape memory alloy based compression bandage as proposed by Moein and Menon (2014) is also not an effective method as there exists challenges in the integration of shape memory alloy wires with the textile yarns.

A thermal sensitive SMP has the potential to adjust the internal stress in its structure via external heating [29], [30], [31] and this characteristic is the key for the present case to obtain smart compression management as suggested by Laplace's law [32], [33], [34]. We will investigate in this paper the fundamental relationship between the extra pressure and the recovery stress generated by the stocking. We will then present a case where such a stocking is actually developed using a blend yarn consists of selected shape-memory polyurethane and nylon filaments. Extensive experimental work has also been conducted to demonstrate the feasibility and explore the influencing factors involved.

Section snippets

Working principle and material determination

Before proceeding with the stocking design, there are a couple of theoretical issues to be examined to provide guidance in selection of desirable material.

Preparation of SMP chips

The shape memory polyurethane (SMPU) chips was prepared by bulk polymerization method using polytetramethylene ether glycol (PTMEG; Mn = 650; Aldrich Chemical Company, USA) as the soft segment, and 4,4′-methylene diphenyl diisocyanate (MDI; Aldrich Chemical Company, USA) and 1,4-butanediol (BDO; Acros Organics) as hard segment. The weight ratio of soft segment and hard segment was 12:13. Extra pure-grade MDI was used for the synthesis without further treatment. BDO (1,4-butanediol) was dried by

Results and discussion

Clearly, giving the nature of both the SMPU itself and the stocking-leg system, and when the patient leg and stocking design and size are fixed, the pressure level actually exerted to the leg by the stockings is determined by the following major factors.

Conclusions

First in smart stocking material selection, the following criteria have to be met simultaneously, based on the methodology developed in this paper:

  • 1.

    Its mechanical properties are sensitive enough to temperature changes of a narrow range above the human body temperature;

  • 2.

    It obeys the rubber elasticity theories and, more specifically, follows Eq. (6): during use of the stocking, an increase in temperature ΔT will lead to a sufficient increase in internal stress Δσ;

  • 3.

    Also the selected material should

Acknowledgements

The authors of this project would like to acknowledge the funding support from University Research Grants Council, project numbers: PolyU 5162/12E, PolyU 5161/11E for this research work. This work has been partly supported by US NIFA projects CA-D*-TXC-6426-RR and CA-D*-TXC-7694-H.

References (42)

  • B. Kumar et al.

    Analysis of sub-bandage pressure of compression bandages during exercise

    J. Tissue Viability

    (2012)
  • H. Partsch

    Compression therapy for deep vein thrombosis

    Vasa Eur. J. Vasc. Med.

    (2014)
  • S. Reich-Schupke et al.

    Quality of life and patients' view of compression therapy

    Int. Angiol. J. Int. Union Angiol.

    (2009)
  • S. Thomas

    The production and measurement of sub-bandage pressure: Laplace's Law revisited

    J. Wound Care

    (2014)
  • B. Kumar et al.

    Effect of material and structure of compression bandage on interface pressure variation over time

    Phlebology

    (2013)
  • S. O'Meara et al.

    Compression for venous leg ulcers

    Cochrane Database Syst. Rev.

    (2012)
  • J. Hafner et al.

    Instruction of compression therapy by means of interface pressure measurement

    Dermatol. Surg.

    (2000)
  • M. Dennis et al.

    Effectiveness of thigh-length graduated compression stockings to reduce the risk of deep vein thrombosis after stroke (CLOTS trial 1): a multicentre, randomised controlled trial

    Lancet

    (2009)
  • J. Sue

    Compression hosiery in the prevention and treatment of venous leg ulcers

    J. Tissue Viability

    (2002)
  • B. Kumar et al.

    Study on interface pressure generated by a bandage using in vitro pressure measurement system

    J. Text. I

    (2013)
  • G. Mosti et al.

    Compression stockings with moderate pressure are able to reduce chronic leg oedema

    Phlebology

    (2012)
  • Cited by (0)

    View full text