Polymers Used For Wound Dressings

Published Date: 02 Nov 2017

Contents

1. Introduction

1.1 Skin

The human body has two systems that protect it from the harmful organisms existing in the environment. The internal defence system destroys microorganisms and bacteria that have already attacked the body. The external defence system prevents microbial microorganisms to enter the body. Skin is biggest external defence system. Skin covers the outside of the body but has other functions beside the defence mechanism. It serves as a mechanical barrier between the inner part of the body and the external world (1). Skin primarily serves as a protective barrier against the environment; therefore, the loss of integrity of large portions of the skin due to an accident, trauma or illness may result in significant disability or even death (2). The temperature of skin varies in a range of 30 to 40 °C degree depending on the environmental conditions (3).

Figure 1.1: Anatomy of the skin illustrating structure pertinent to wound repair.(4)

1.2 What is a wound?

A wound is described as a break or a defect in the skin, which can result from thermal or physical damage or in the presence of an underlying physiological or medical condition (5). It is a disruption of normal anatomic structure and function (6). Acute wounds normally progress through a systematic and timely reparative process that results in continual restoration of anatomic and functional integrity. Chronic wounds have failed to proceed through an orderly and timely process to produce anatomic and functional integrity, or proceeded through the repair process without establishing a sustained anatomic and functional result (6).

1.3 Wound dressing

Watson and Hodgkin (7) define the ideal properties of a wound dressing and they are as follows: They should maintain a moist environment around the wound, removes excess exudate, but prevents saturation of the dressing to its outer surface, permits diffusion of gasses, protects wound from micro-organisms, provides mechanical protection, controls local temperature and pH, is easy and comfortable to remove/change, minimises pain from the wound, controls wound odour, is cosmetically acceptable, is non-allergenic, does not contaminate the wound with foreign particles and is cost effective. They also state that hydrogels are used primarily to donate fluid to dry necrotic and sloughing wounds, and their absorbency is limited.

Abdelrahman and Newton (8) examined wound dressings and their properties and the uses of different wound dressings. They state that hydrogels consist of insoluble polymers with a high water content making them ideal dressings to facilitate autolytic debridement (wound cleansing) of necrosis and slough.

1.4 Polymers used for wound dressings

1.4.1 Natural Polymers

1.4.1.1 Gelatin

Among the various natural polymeric materials, gelatin is available in abundance and, therefore, is comparatively cheap. It is a biopolymer that is readily obtained by a controlled hydrolysis of collagens, which are the most abundant structural fibrous insoluble proteins found in skin, tendons, cartilages, bones, and connective tissues. Its composition and biological properties are almost identical to its precursors. Gelatin has been widely used in pharmaceutical and medical fields as sealants for vascular prostheses, plasma expanders, ingredients in drug formulations, carriers for the delivery of drugs or other therapeutic substances,[17–19] and, in particular, as wound dressing materials.[20–22]

Gelatin has been chosen as the hydrogel polymer backbone because of the following physicochemical properties of gelatin: (i) great capacity for modification at the level of amino acids, (ii) low level of immunogenicity and cytotoxicity, (iii) FDA approval as a clotting agent and exudate absorbing construct, (iv) hydrogel formation by facile procedures, and (v) ability to biodegrade. (9)

1.4.1.2 Hyaluronic acid

Hyaluronic acid (HA) is a glycosaminoglycan that is found in extracellular tissue in many parts of the body. It is a material of increasing importance to biomaterials science and is finding applications in diverse areas ranging from tissue culture scaffolds to cosmetic materials. Its properties, both physical and biochemical, in solution or hydrogel form, are extremely attractive for various technologies concerned with body repair.(10)

Hyaluronic acid (HA) offers great practical potential as a scaffolding material, a range of crosslinking techniques are available, both to enhance the material residence time as well as to control its mechanical properties. The resulting materials offer advantageous properties such as bioresorbability, inhibition of scar formation and promotion of angiogenesis (11). Cell adhesion ligands and growth factors can also be incorporated in the HA based scaffold to enhance the rate of tissue regeneration.

Dialdehydes are believed to crosslink through formation of acetal or hemiacetal groups on neighbouring chains, with kinetic and spectroscopic evidence indicating a prevalence of the hemiacetal (Tomihata & Ikada, 1997). Glutaraldehyde (GTA) is believed to form either a hemiacetal or an ether link with HA under acidic conditions.

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Figure 1.2: Structure of Hyaluronic acid.

1.4.2 Synthetic polymers

1.4.3 Combined polymers

1.5 Cross-linkers

1.5.1 Glutaraldehyde

Among the chemical crosslinking agents, glutaraldehyde (GTA) is by far the most widely used, due to its high efficiency of collagenous materials stabilization. Crosslinking of collagenous samples with GTA involves the reaction of free amino groups of lysine or hydroxylysine amino acid residues of the polypeptide chains with the aldehyde groups of GTA. GTA is easily available, inexpensive and its aqueous solutions can effectively crosslink collagenous tissues in a relatively short period. The success of thousands of bioprosthetic implants demonstrated in the last tens of years indicates that glutaraldehyde crosslinking has been clinically acceptable and has many merits in spite of the reports on its cytotoxicity.(12)

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Figure 1.3: Structure of Glutaraldehyde.

1.5.2 Tannic acid

Tannic acid (TA) is used as a natural phenolic cross-linker to modify gelatin and improve its mechanical performance. It is a hydrolysable tannin with multiple phenolic functionalities due to its high molecular weight. Possible chemical reaction pathways between gelatin and natural phenolic compounds have been postulated. The amino functional groups of amino acid units in gelatin (such as lysine, arginine, and histidine) would react with phenolic reactive sites of TA under alkaline conditions to form covalent C-N bonds and generate cross-linked networks. Other reactive groups in gelatin (e.g., the hydroxyl group in serine units) may also promote such reactions via a similar mechanism. On the other hand, the phenolic hydroxyl and carboxyl groups of TA could associate with gelatin chains via hydrogen bonding. The extent of these reactions and interactions in gelatin/TA systems plays an important role in determining consequent material properties.(13)

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Figure 1.4: Structure of Tannic acid.