Wound-healing and wound-care technologies are an ever-expanding field due to the advancement of materials science, biomedicine and tissue engineering. Out of the total amount of alginate fibres produced annually, only 10% is used in medical surgery and wound management. In addition, advanced wound-care products are more efficient in wound healing, which reduces the length of time a patient remains in hospital. Analysis of the wound-care market indicates the increased shift towards advanced wound dressings such as those containing alginates.
Alginic acid is an unbranched binary copolymer of L-guluronic acid (G) and D-mannuronic acid (M). It is a polysaccharide found in brown seaweed (Phaeophyceae:
Laminaria hyperbores, Macrocystis pyrifera and Ascophyllum nodosum). After cellulose, it is the most abundant biopolymer in the world. It exists in the kelp cell wall as an insoluble mixed salt of calcium, magnesium, sodium and potassium, and the salts of this polysaccharide are generally named alginates. The two monomers (G and M) are linked together in blocks of poly G and poly M, or randomly intermingled, which is dependent on its origin as shown in Figure 8.1. The proportion as well as the distribution of the two monomers determines, to a large extent, the physicoochemical properties of alginate. The first scientific studies on the extraction of alginates from brown seaweed were reported by a British chemist, Charles Stanford, at the end of the 19th century and were patented by him [1, 2]. He found that the extracted substance, which he named ‘algin’, possessed several interesting properties, including the ability to thicken solutions, make gels and form films, leading him to propose several industrial applications. The first structure of alginic acid was proposed by Cretcher and Nelson in 1928 [3]. The large-scale industrial production of alginate was not introduced until 50 years after the patent was approved. Alginates are also synthesised by some bacteria (e.g., Azotobacter and Pseudomonas species). The industrial manufacture of alginate is based on extracting the polymer from brown algae. The M/G ratio will vary from one species of brown algae to another as shown in Table 8.1. An excellent review on the history of alginate has been written by Booth [4].
OH OC
HO
OH O O
O
O OC
OC HO HO
OH
M M
M M M M
G OH
O
O O
OH OC
HO
OH O O
O
OC HO
HO
OH
O O O
OC HO
HO
OH O
O OC
O OC
HO
OH O O
O
O O
OC HO
G G G
OH
OH
O
O
O O O
OC
HO HO
G
OH
O
O O O
O OC
OH O
HO
Figure 8.1 Structure of alginates with β-(1→4)-linked D-mannuronic acid (M) and α-(1→4)-linked L-guluronic acid (G) residues. Consisting of blocks of similar
(MMMMMM and GGGGGG) and strictly alternating residues (GMGMGM)
Table 8.1 Typical M/G composition and structural sequences of various species of brown algae
Seaweed M/G M% G% MM% GG%
Laminaria hyperborea (stem) 0.45 30 70 18 58
Laminaria hyperborea (leaf) 1.28 56 44 43 31
Laminaria digitata 1.43 59 41 49 25
Laminaria japonica 2.26 69 31 36 14
Macrocystis pyrifera 1.56 61 39 40 20
Lessonia nigrescens 1.63 62 38 43 23
Lessonia trabeculata 0.18 15 85 25 49
Ascophylbum nodosum 1.5 60 40 38 21
Durvillea antarctica 2.45 71 29 58 16
Durvillea potarum 3.33 77 23 69 13
Undaria pinnatifida 1.45−2.65 − − − −
Ecklonia cava 1.39−2.91 − − − −
Ecklonia maxima 1.22 55 45 38 28
8.2.1 Physicochemical Properties of Alginate
Alginate is a natural and biodegradable biopolymer. Several studies have shown that cells immobilised in alginate maintain good morphology and metabolism during long-term culture. There are, however, numerous studies using uncharacterised commercial alginates to immobilise cells that have resulted in inconsistent outcomes.
This is mainly related to the different levels of fibroblastic overgrowth when used as an implantation device. It is known that alginates may contain small amounts of pyrogens, polyphenols, proteins and complex carbohydrates. The presence of polyphenols might possibly harm the immobilised cells, and the presence of pyrogens, proteins and complex carbohydrates may induce immunological reactions by the host. It has also been demonstrated that perfectly spherical and smooth alginate droplets, which are the most effective, may only be formed by using highly purified alginates; to avoid manufacturing and quality issues, it is important to use alginates of high quality. To obtain repeatable results in all respects, the chosen alginates should therefore also be well characterised with respect to all critical parameters (impurities, M/G content, molecular weight and so on). The primary functions of alginates are as thermally stable cold-setting gelling agents, in the presence of calcium ions, and gelling at far lower concentrations than gelatin. Such gels can be heat treated without melting, although they may eventually degrade. Gelling depends on the ion binding (Mg2+<<Ca2+< Sr2+< Ba2+), with control of the dication addition being important for the production of homogeneous gels (e.g., by ionic diffusion or controlled acidification of CaCO3). A high G content produces strong brittle gels with good heat stability (except if present in low molecular weight molecules), but are prone to water weepage (syneresis) on freeze-thaw, whereas a high M content produces weaker more elastic gels with good freeze-thaw behaviour. However, at low or very high Ca2+ concentrations, high M alginates produce stronger gels. As long as the average chain lengths are not particularly short, the gelling properties correlate with the average G block length (optimum block size ~12) and not necessarily with the M/G ratio, which may be primarily a result of alternating MGMG chains. The future prospects of alginate use in wound healing are excellent as recombinant epimerases, with different specificities, may be used to produce novel designer alginates. The solubility and water-holding capacity of alginates depend on pH (precipitating below about pH 3.5), ionic strength (low ionic strength increases the extended nature of the chains) and the nature of the ions present. Generally, alginates show high water absorption and may be used as low-viscosity emulsifiers and shear-thinning thickeners. They can be used to stabilise phase separation in low-fat fat substitutes, e.g., as alginate/caseinate blends in starch three-phase systems. Alginate is used in pet food chunks, onion rings, stuffed olives and pie fillings.
The use of alginate as an immobilising agent in most applications relies on its ability to form heat-stable strong gels, which can develop and set at room temperatures.
Alginate forms gels with most divalent and multivalent cations; however, gels formed with calcium ions are used in most applications. Monovalent cations and Mg2+ ions do not induce gelation, while ions such as Ba2+ and Sr2+ will produce stronger alginate gels than Ca2+. The gel strength will depend upon the guluronic acid content and also on the average number of G-units in the G-blocks. Alginate gelling occurs when divalent cations take part in the interchain binding between G-blocks, giving rise to a three-dimensional (3D) network in the form of a gel. The binding zone between the G-blocks is often described by the so-called ‘egg-box model’ as shown in Figure 8.2.
During gelling, the concentration of divalent cations has a large impact on the gel network and homogeneity. When the gelling takes place in the presence of excess calcium, a modified egg-box model has been suggested, involving more than two alginate chains in the gelling zone, which may have an impact on gel porosity. If no shrinking of the gel occurs, there may be more space in between the chains, leading to an increased porosity. Because alginates can form strong complexes with polycations, such as chitosan or polypeptides, or synthetic polymers, such as polyethylenimine, they may be used to stabilise the gel. When used as coating materials, such complexes may also reduce the porosity.
Ca2+
more Ca2+
Figure 8.2 Egg-box model for alginate gel formation. The divalent calcium cation, Ca2+, fits into the guluronate block structure like eggs an in egg box
8.2.2 Alginate Wound Dressings
The role of wound dressings, based on alginic material, in wound management are well- known in the literature, as well as from a commercial point-of-view [5−8]. Alginate is highly absorbent, gel forming, haemostatic and biocompatible, making alginate a good candidate for burns and wound management. Cytotoxic effects on the proliferation of fibroblasts and epidermal cells in culture, and residual debris along with inflammation in healing wounds have been reported recently with commercial alginate dressings;
however, alginate is well known for its haemostatic properties. Calcium alginate has been shown to increase the proliferation of fibroblasts but decrease the proliferation of microvascular endothelial cell and keratinocytes; it also decreased fibroblast motility but had no effect on keratinocyte motility [9]. In a controlled clinical trial, split skin graft donor site patients who were treated using calcium alginate were compared with patients who were treated using paraffin gauze. A significant number of patients dressed with calcium alginate was completely healed at day 10 compared with members of the paraffin gauze group. Thus, calcium alginate dressings provide a significant improvement in healing split skin graft donor sites [10]. In another study with burn patients, calcium alginate significantly reduced the severity of pain and was favoured by nursing personnel because of its ease of care; however, there was no clinical advantage as a dressing for skin graft donor sites. The combined use of calcium sodium alginate and a bioocclusive membrane dressing, in the management of split-thickness skin graft donor sites, eliminated the pain and the problem of seroma formation and leakage seen routinely with the use of a bioocclusive dressing alone [11]. It has been recommended that calcium alginate should be used as the dressing of choice for split skin graft donor sites. As an alternative to standard alginate dressings, new alginate dressings have also been investigated, resulting in claims of superior properties. Different alginate wound dressing materials have been commercially utilised as shown in Table 8.2, and have been reviewed widely in the literature.
Alginate dressings are highly absorbent and biodegradable. The high absorption efficiency is achieved by the formation of a strong hydrophilic gel which limits wound secretions and minimises bacterial contamination; therefore, alginate dressings are particularly useful for moderate to heavily exudating sloughy wounds. They maintain a physiologically moist microenvironment which promotes healing and the formation of granulation tissue. Alginates are not indicated for dry sloughy wounds or those covered with hard necrotic tissue. For leg ulcers that are heavily exudating and shallow, fibrous sheet dressings may be used while for cavity wounds, alginate fibre packing (in rope form) can be used. Calcium alginate dressings are often used for burns and ulcers, including diabetic skin ulcers which can be very difficult to treat with other types of dressings. Alginates can be easily removed by saline irrigation. Dressing removal does not cause any interference to the healing granulation tissue and makes it painless, resulting in high patient compliance. This is a beneficial effect compared with cellulose dressings, particularly for epithelializing wounds.
Table 8.2 Alginate-based wound dressings commercially available on the market
Product Manufacturer
ACTICOAT Flex/Barrier Smith & Nephew, Inc.
ALGISITE M
AlgiDERM Bard
ALGICELL® Derma Sciences, Inc.
Algivon Advancis Medical
Algosteril® Johnson & Johnson
ApinateTM Comvita
AQUACEL® Ag Extra ConvaTec
Biatain® Ag Coloplast Corp.
Biatain® Soft
Calcium alginate rope and sheet McKesson Medical-Surgical
CarraSorbTM H Carrington
Curasorb Kendall
Curasorb Zinc
DermaGinate® DermaRite Industries, LLC
DermaceaTM Sherwood-Davis & Geck
ExcelGinate® MPM Medical, Inc.
FyBronTM B. Braun Medical Ltd.
Gentell® Gentell Wound and Skin Care
Hyperion Advanced Alginate Dressing® Hyperion Medical, Inc.
Kalginate® DeRoyal
Kaltogel ConvaTec
Kaltostat®
KendallTM Covidien Ltd
Maxorb® Medline Industries, Inc.
Maxorb® Extra Ag MediPlus®
Melgisorb® Mölnlycke Health Care US, LLC
MEDIHONEY® Derma Sciences, Inc.
MEDIHONEY® gel sheet Comvita
Opticell® Medline Industries, Inc.
Opticell® Ag
PolyMem® Ferris Mfg.
Restore Hollister Wound Care, LLC
Restore Ag
SorbsanTM Dow Hickam
SorbsanTM Plus
Sofsorb® Ag DeRoyal Industries Inc.
Sorbalgon® Hartmann USA Inc.
SeaSorb® Coloplast Sween Corp.
SILVERCELTM Johnson & Johnson
Silverlon® Argentum Medical, LLC
3M TM Tegaderm HGTM 3M Health Care
3MTM Tegaderm HITM 3M TM TegagelTM 3M TM TegadermTM Ag
It has been demonstrated in a study that calcium alginate increased the proliferation of fibroblasts, but decreased the proliferation of human microvascular endothelial cells and keratinocytes. However, the calcium alginate decreased fibroblast motility but had no effect on keratinocyte motility. The effects of calcium alginate on cell proliferation and migration have been reported to be mediated by released calcium ions. Some alginate-containing dressings activate macrophages within the chronic wound bed and generate a pro-inflammatory signal, which may initiate an inflammatory response characteristic of healing wounds [12, 13]. However, there are significant differences in characteristics, such as fluid retention, adherence and degradation, between different brands of alginate dressing [14].
Calcium alginate wound dressings have been indicated for leg and pressure ulcers, burn wounds and donor sites, surgical wounds and as haemostatic dressings; hence products, such as SorbsanTM, TegagelTM, Kaltostat® and so on, have been subjected to many controlled clinical trials, which indicated their significant and beneficial effect on leg ulcers, SorbsanTM, Kaltostat®, Seasorb® on burn and donor sites, Algosteril®, SorbsanTM, Kaltostat® and so on, for pressure ulcers and surgical wounds.
8.2.3 Calcium Alginate as a Haemostat
Calcium alginates, on contact with blood, release calcium ions in exchange for sodium ions. These migrating calcium ions activate coagulation both by stimulating platelet aggregation and working as a cofactor (clotting factor IV) in the coagulation cascade [15−17]; thus, calcium alginate acts as a haemostatic agent as shown in Figure 8.3.
A recent study demonstrated the effectiveness of calcium alginates as a haemostatic material for hepatic bleeding, however, intense fibrosis caused inraperitoneal adhesions [18]. The extent of platelet aggregation and coagulation activation was influenced by the composition of M and/or G groups in the alginates. It has also been demonstrated that zinc alginates had the greatest effect on prothrombotic coagulation and platelet activation [19].
In a randomised controlled study, haemostasis during an adenoidectomy was shown to be more effective and the operating time shortened by the use of calcium alginate compared with gauze dressings [20]. A study using a Kaltostat® haemostatic wound dressing showed no difference to Surgicel®, i.e., oxidised regenerated cellulose [21], in healing tooth sockets. However, it was effective in achieving haemostasis at the buccal mucosa compared with standard gauze [22]. Foreign body giant cell reaction was reported to be elicited by Kaltostat® when used to obtain haemostasis in an apicectomy cavity [23]. In another randomised clinical study, calcium alginate swabs were not found to produce any clinical or statistical advantage over traditional cotton swabs [24]. A foreign body reaction was also shown on application of Kaltostat® to control
bleeding from the left submandibular fossa of a patient [25]. Experimental studies on rats proved that calcium alginates can be used as a haemostatic agent in a splenic injury [26]. A combination of alginate and chitosan (Alchite) has been investigated to form fibres which could be developed for wound-care applications; a study on its antibacterial property suggested its suitability in the development of wound dressings [27]. This may aid in the development of a haemostatic wound dressing, as chitosan possesses an inherent haemostatic action.
Na+
b) a)
Ca2+
Ca2+
Ca2+
Na+ Na+ Na+
SorbsanTM dressing
Platelet Red blood cell
Blood clot
Fibrin strands SorbsanTM dressing
Activated platelet
Blood vessel
Figure 8.3 The Na+ ions present in wound fluids and around damaged tissues are absorbed a) and exchanged for Ca2+ ions in the alginate dressing, and b) Ca2+ ions then support the coagulation cascade. Adapted from the SorbsanTM
Wound Dressing Brochure, Aspen Medical Europe Ltd., Worcestershire, UK.
http://www.aspenmedicaleurope.com