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Chitosan and Aloe Vera: Two Gifts of NatureMohmmad Younus Wani a , Nazim Hasan b & Maqsood Ahmad Malik ca Centre for Interdisciplinary Research in Basic Sciences , Jamia Millia Islamia (CentralUniversity) , New Delhi, Indiab Department of Chemistry , National Sun Yat-sen University , Kaohsiung, Taiwanc Department of Chemistry , Jamia Millia Islamia (Central University) , New Delhi, IndiaPublished online: 13 May 2010.

To cite this article: Mohmmad Younus Wani , Nazim Hasan & Maqsood Ahmad Malik (2010) Chitosan and Aloe Vera: Two Giftsof Nature, Journal of Dispersion Science and Technology, 31:6, 799-811, DOI: 10.1080/01932690903333606

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Chitosan and Aloe Vera: Two Gifts of Nature

Mohmmad Younus Wani,1 Nazim Hasan,2 and Maqsood Ahmad Malik31Centre for Interdisciplinary Research in Basic Sciences, Jamia Millia Islamia (Central University),New Delhi, India2Department of Chemistry, National Sun Yat-sen University, Kaohsiung, Taiwan3Department of Chemistry, Jamia Millia Islamia (Central University), New Delhi, India

The present review discusses the preparation of chitosan, chemical composition, and constituentsof aloe vera, their physiochemical characteristics, and their applications, particularly their woundhealing properties. Since chitosan and aloe vera are two natural polymers having the requiredproperties for wound healing these have been used since antiquity as wound healers and are sincethen considered as the nature’s two gifts to mankind. Chitosan based wound dressings have beenprepared and at the same time some reports about the preparation of aloe vera based wounddressings are there, however till now no wound dressing based on both chitosan and aloe veratogether has been reported and therefore, an effort has been made to review the literature, dealingwith properties, processing, and applications, with an emphasis on wound healing.

Keywords Aloe vera, chitosan, wound dressings, wound healing

1. INTRODUCTION

Biopolymers receive more and more attention as possiblealternatives for synthetic polymers in many technologicalprocesses involving separation or stabilization of dispersedsystems, such as in drinking water and wastewater treat-ment, sludge dewatering, or downstream processing. Theterm chitosan refers to the whole family of acidic solublelinear heteropolysaccharides. Chitosan and aloe vera arethe two gifts of nature used by man since antiquity for dif-ferent purposes. These are natural polymers, while chitosanis a natural nontoxic biopolymer derived by deacetylationof chitin, a major component of the shells of crustacea suchas crab, shrimp, and crawfish, it also occurs naturally insome fungi but its occurrence is much less widespread thanthat of chitin. Chitosan is the N-deacetylated derivative ofchitin, although this N-deacetylation is almost never com-plete. A sharp nomenclature border has not been definedbetween chitin and Chitosan based on the degree ofN-deacetylation.[1,2] Its copolymer structure is depicted inFigure 1.

Aloe vera on the other hand is also a natural polysac-charide, comprising of both mono- and polysaccharides.By far the most important are the long chain polysac-charides, comprising glucose and mannose, known as glyco-mannans (beta-(1, 4)-linked acetylated mannan). Chitosanand aloe vera are of great interest because of their alround

properties and applications in almost every field. Muchattention has been paid currently to chitosan as a potentialpolysaccharide resource.[3] Although several efforts havebeen reported to prepare functional derivatives of chitosanby chemical modifications,[4–6] only a few examples attainedsolubility in general organic solvents.[7,8] Chitosan is solublein aqueous solutions of some acids; some selectiveN-alkylidenations[4] and N-acylations[5,6] have also beenattempted. Although several water-soluble[9–10] or highlyswellable[6–11] derivatives were obtained, development ofsolubility in common organic solvents was found to be dif-ficult by these methods. Chemically modified chitosan struc-tures, resulting in improved solubility in general organicsolvents, have been reported by many workers.[12–23] How-ever, no comprehensive review has yet been published thatcovers the entire range of applications.

2. PREPARATION OF CHITOSAN

2.1. Purification of Chitin

The main sources of material for the laboratorypreparation of chitosan are the exoskeletons of variouscrustacea particularly crab and shrimp. In these chitin isclosely associated with proteins, inorganic material whichis mainly CaCO3 pigments and lipids. Various procedureshave been adopted to remove these impurities. Deminerali-zation is most frequently carried out by treatment with HCland deproteinization by treatment with NaOH, but othermethods may be used and the order in which these twosteps are carried has varied with different workers,

Received 15 January 2009; accepted 26 February 2009.Address correspondence to Maqsood Ahmad Malik, Depart-

ment of Chemistry, Jamia Millia Islamia (Central University),New Delhi-110025, India. E-mail: maqsoodchem@gmail.com

Journal of Dispersion Science and Technology, 31:799–811, 2010

Copyright # Taylor & Francis Group, LLC

ISSN: 0193-2691 print=1532-2351 online

DOI: 10.1080/01932690903333606

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although in most instances deproteinization has been carriedout prior to demineralization. The use of enzymes for pro-tein removal has been examined by a number of workers.The use of enzymes such as pepsin and trypsin has beensuggested, if the chitin is required to be as fully N-acetylatedas possible, but no experimental details were given.

2.2. Deacetylation of Chitin

One of the main reactions carried out on chitin isdeacetylation, most commonly by using aqueous alkali.The most frequently used alkali is NaOH. The extent ofdeacetylation is governed by the alkali concentration, timeof reaction, temperature, particle size and density. Whiletreatment with 50wt% NaOH at 100�C for 1 hour givesa product having 82% deacetylation (Scheme 1), extendingthe reaction time to 48 hours enables almost 100% deacety-lation but at the expense of a considerable decrease insolution viscosity indicating chain degradation.[24]

3. PHYSIOCHEMICAL CHARACTERISTICS OFCHITOSAN

3.1. Degree of N-Acetylation

Chitosan is characterized by either the degree of acety-lation (DA) which corresponds to the N-acetylaminegroups or the degree of deacetylation DDA (DDA¼100DA), of D-glucosamine groups. Thus the degree ofN-acetylation is the ratio of 2-acetamido-2-deoxy-D-glucopyranose to 2-amino-2-deoxy-D-glucopyranose struc-tural units. This ratio has a striking effect on chitinsolubility and solution properties. Chitosan is the univer-sally accepted nontoxic N-deacetylated derivative of chitinwith the chitin N-deacetylated to such an extent that itbecomes soluble in dilute aqueous acetic and formic acids.To define this ratio, attempts have been made with infrared(IR) spectroscopy,[25,26] pyrolysis gas chromatography,[27]

gel permeation chromatography and ultraviolet (UV)spectrophotometry,[28] first derivative of UV spectro-photometry,[29] 1H nuclear magnetic resonance (NMR)spectroscopy,[30] 13C solid-state NMR,[31,32] thermal analy-sis,[33] various titration schemes,[32–34] acid hydrolysis andhigh-performance liquid chromatography (HPLC),[35] sep-aration spectrometry methods,[36] and, more recently,near-infrared spectroscopy;[37] most of these methods havebeen reviewed thoroughly in the literature.[29]

3.2. Molecular Weight

The definition of average molecular weight of polysac-charides and the understanding of its consequences on theirphysiochemical behaviour have presented a real challengeto chemists for a number of years. The weight-averagemolecular weight Mw of chitosan has been determined bylight scattering.[38] Viscometry is a simple and rapidmethod for the determination of molecular weight; theconstants a and K in the Mark-Houwink equation havebeen determined in 0.1M acetic acid and 0.2M sodiumchloride solution. The intrinsic viscosity is expressed as:

½g� ¼ KMav ¼ 1:81� 10�3M0:93

v :

The charged nature of chitosan in acid solvents and thepropensity of chitosan to form aggregation complexes

FIG. 1. Chemical structure of chitosan consisting of (a) deacetylated and (b) acetylated unites.

SCH. 1.

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require care when applying these constants. Furthermore,converting chitin into chitosan lowers the molecular weight,changes the degree of deacetylation and thereby alters thecharge distribution, which in turn influences the agglomer-ation. The weight-average molecular weight of chitin is1.03� 106 to 2.5� 106 kDa, but the N-deacetylation reac-tion reduces this to 1� 105 to 5� 105 kDa.[39,40]

3.3. Solubility

Chitosan in the form of free amine is insoluble in waterto pH near neutrality, in concentrated acids with the excep-tion of sulphuric acid, the bases and organic solvents. Theiracid-base properties allow easy dissolution. Therefore chit-osan is soluble in dilute HCl, HBr, HI, HNO3, and HClO4.Chitosan is also slightly soluble in dilute H3PO4 but isinsoluble in dilute H2SO4 at room temperature, althoughchitosan sulphate dissolves in water on heating and reformson cooling. In fact chitosan is soluble in dilute acids onaccount of protonation of free amine group. The dis-sociation constant Kb of an amine group is obtained fromthe equilibrium

Kb ¼½�NHþ

3 �½OH��½�NH2�

½2�

and

pKb ¼ � logKb ½3�

While the dissociation constant of the conjugated acid isobtained from the equilibrium

Ka ¼½�NH2�½H3O

þ�½�NHþ

3 �½5�

and

pKa ¼ � logKb ½6�

As in all polyelectrolytes the dissociation constant ofchitosan is not, infact constant, but depends on the degreeof dissociation at which it is determined. The variationof the pKa value can be calculated using Katchalsky’sequation.

pKa ¼ pHþ logðð1þ aÞ=aÞ ¼ pK0 � eDwðaÞ=KT ½7�

where DW is the difference in electrostatic potential betweenthe surface of polyion and the reference, a is the degree of

dissociation, KT is the Boltzman’s constant and E is theelectron charge. Extrapolation of the pKa value to a¼ 1,where the polymer becomes uncharged and hence the elec-trostatic potential becomes zero, enables the value of theintrinsic dissociation constant of the ionizable groups,pKo, to be estimated. The value obtained �6.5, is inde-pendent of the degree of N acetylation, whereas the pKa

value is highly dependent on this factor. The pKo, is calledthe intrinsic pKa of the chitosan. Therefore the solubility ofchitosan depends on its degree of dissociation and themethod of deacetylation used.

3.4. Crystallinity

The three structural forms of chitosan (hydrated andanhydrous crystal and non crystal) could be easily exam-ined by measuring the x-ray powder diffraction of a chito-san sample. The hydrated crystal shows a strong reflectionat an angle (2h) of 10.4� and the other peaks more weaklyat 20� and 22�. The anhydrous crystal exhibit a strongpeak at 2h of 15� and a peak supplementar at 20�. Anamorphous chitosan does not show any reflection but itexhibits a broad halo at 2h of 20�. The dissolution of chit-osan involves the progressive disappearance of the peak at2h� 20� and the polymer swelling destroys the residualCrystallinity, increasing the accessibility of solute to thesorption sites. It has been observed that substitution ofnonaoyl groups on chitosan causes the crystallinity todecreases at low substitution level while increases at highsubstitution level.

3.5. Solvent and Solution Properties

Chitosan degrades before melting, which is typical forpolysaccharides with extensive hydrogen bonding. Thismakes it necessary to dissolve chitosan in an appropriatesolvent system to impart functionality. For each solventsystem, polymer concentration, pH, counterion concen-tration, and temperature effects on the solution viscositymust be known. The comparative data from solvent tosolvent are not available. As a rule, researchers dissolvethe maximum amount of polymer in a given solvent thatstill retained homogeneity and then regenerated in therequired form. A coagulant is required for polymer regen-eration of solidification. The nature of the coagulant is alsohighly dependent on the solvent and solution properties aswell as the polymer used.

4. APPLICATIONS OF CHITOSAN

4.1. Photography

Chitosan has important applications in photographydue to its resistance to abrasion, optical characteristics,and film-forming ability. Silver complexes are not appreci-ably retained by chitosan and therefore can easily penetrate

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from one layer to another of a film by the diffusion transferreversal process.[41]

4.2. Cosmetics

For cosmetic applications, usually organic acids aregood solvents; chitosan has fungicidal and fungistaticproperties. Chitosan is the only natural cationic gum thatbecomes viscous on being neutralized with acid. Thesematerials are used in creams, lotions, and permanentwaving lotions; several derivatives have also been reportedas nail lacquers.[42]

4.3. Food and Nutrition

N-Acetylglucosamine (NAG) moiety present in humanmilk promotes the growth of bifidobacteria, which blocksother types of microorganisms and generates the lactaserequired for digestion of milk lactose. Cow’s milk containsonly a limited amount of NAG moiety; hence, some infantsfed cow’s milk may have indigestion. Many animals andsome humans (including elderly people) have similarlactose intolerances.[43–46] Animal nutritional studies haveshown that the utilization of whey may be improved ifthe diet contains small amounts of chitinous material. Thisimprovement is attributed to the change in the intestinalmicroflora brought about by the chitinous supplement.[47]

Chicken fed a commercial broiler diet containing 20% driedwhey and 2% chitin improved weight gain significantlycompared to the control.[46,48] Chicken fed a commercialbroiler diet containing 0.5% chitin showed significantweight gain (10%) compared to the control. The feed intakeincreased by 5%, and the feed efficiency ratio shifted from2.5 to 2.38 due to incorporation of chitin in the feed.[49,50]

4.4. Chromatography

The presence of free amino and hydroxyl groups in chit-osan makes it a useful chromatographic support. Chitosanhas been used in thin-layer chromatography for separationof nucleic acids.[51–54] It was found that the ability of thechitin layer to separate mixtures of phenols, amino acids,nucleic acids, and their derivatives or inorganic ions (thecopper group) was almost equal or superior to that ofroutinely used crystalline cellulose, silica gel, and polyam-ide layers. Muzzarelli and Rocchetti[41] proposed a methodfor the determination of molybdenum and vanadium inseawater using chitosan columns followed by determi-nation by atomic absorption. Other workers have also usedchitosan in chromatographic separation and obtainedinteresting results in their investigations.[55–57]

4.5. Wound Healing

The merit of attempting to isolate a wound fromenvironment has long been appreciated, at least in the senseof providing physical and aesthetic protection. Chitosan as

a wound healer has been used in wound dressings,discussed in Section 8.1.

5. ALOE VERA

5.1. Chemical Composition and Active Constituents

The word ‘‘aloe’’ has its roots in the Arabic word‘‘alloeh,’’ which means ‘‘radiance.’’ A native plant ofSomalia with a history dating back to the fourth centuryB.C, aloe vera also figures prominently in Egyptian,Chinese, Greek, Indian and Christian literature. Aloe verais the colorless mucilaginous gel obtained from the paren-chymatous cells in the fresh leaves of aloe vera.[58,59] Thealoe plant is grown in warm tropical areas and cannot sur-vive freezing temperatures. The aloe plant is the source oftwo herbal preparations: aloe gel (AG) and aloe latex. Aloegel is often called aloe vera and refers to the clear gel ormucilaginous substance produced by parenchymal cellslocated in the central region of the leaf. Diluted aloe gelis commonly referred to as aloe vera extract. The gel iscomposed mainly of water (99%) and mono- and polysac-charides (25% of the dry weight of the gel). The mostprominent monosaccharide in AG is mannose-6-phosphate, and the most common polysaccharides arecalled gluco-mannans (beta-(1, 4) acetylated mannan).[60]

They are long-chain sugars containing glucose and man-nose. A prominent gluco-mannan named acemannan hasbeen isolated and is being marketed as carrisyn. A glyco-protein with antiallergic properties, called alprogen, hasbeen isolated from aloe vera.[61] In addition, a novel antiin-flammatory compound, C-glucosyl chromone, has alsobeen isolated from aloe vera.[62] Aloe gel also contains lig-nan, salicylic acid, saponins, sterols, and triterpenoids. Thefresh gel contains the proteolytic enzyme carboxypeptidase(which breaks down bradykinin), glutathione peroxidase,as well as several isozymes of superoxide dismutase.[63–65]

The gel also contains vitamins A, C, E, B12, thiamine,niacin, and folic acid, as well as the minerals sodium,potassium, calcium, magnesium, manganese, copper, zinc,chromium, and iron.[60]

5.2. Physical and Chemical Properties of Aloe Vera

Aloe gel has long been used both externally and intern-ally for its beneficial effects in the wound healing process. Itis most often included in tropical formulations (ointmentcream or lotion) but evidence also supports its effectivenesswhen taken orally. At least part of AG’s beneficial effect onthe skin likely is due to its moisturizing effect. Also, it mayleave a protective layer on the skin after drying possiblyproviding some protection to the wound. The list of differ-ent illnesses and conditions, aided by the use of aloe vera isindeed impressive, covering everything from burns andslight infections to very serious conditions.

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1. Acne, aching joints and muscles, asthama, athlete’sfoot, abscesses, arthritis, allergy rashes, age spots, acidindigestion.

2. Brown skin spots, burns, boils, blood pressure, bruis-ing, bad breath, bleeding, bowl problems=conditions,blisters, bronchitis.

3. Cancer treatment (i.e., helps ease the radiation effects),cuts and wounds, colon cleansing, constipation,calcium, chapping, cataracts, cradle cap, candida, cir-culation, colitis, colic.

4. Digestive problems, diarrhoea, dermatitis, dandruff,diabetes, detoxification, duodenal ulcer, diaper(nappy) rash, denture sores, depression.

5. Eye and ear problems (inflammation, infection),eczema, energy loss.

6. Gum disease, bleeding gums.7. Hair and scalp, heat rash haemorrhoids, headache.8. Infection, inflammation, itching, irritable bowel

syndrome, indigestion, insomnia, influenza, insect bite.9. Jaundice.

10. Kidney ailments.11. Liver ailments, laryngitis.12. Moisturizes, mouth ulcers, muscle cramps.13. Nasal congestion, nutrition, cracked nipples, nausea.14. Operation recovery.15. Psoriasis, prickly heat, pimple, peptic ulcer, pain relief.16. Radiation burns, razor burn, rheumatism, rashes.17. Sear removal, scalp problems, sinusitis, sore throat,

scalding, stomach disorders, sciatica.18. strains, sprains, skin problems, stress shingles, stings,

styles, sunburns.19. Tonsillitis, thrush, teething, tennis elbow.20. Ulcers (all kinds).21. Varicose veins, veterinary treatments, venereal sores.22. Warts, wind chapping.

The aloe plant, being a cactus plant, is between 99% and99.5% water, with an average pH of 4.5. The remainingsolid material contains over 75 different ingredients includ-ing vitamins, minerals, enzymes, sugars, anthraquinones orphenolic compounds, lignin, saponins, sterols, amino acidsand salicylic acid. These are described in more detail below.

5.2.1. Vitamins

The plant contains many vitamins, excluding vitamin Dbut including the important antioxidant vitamins A, C, andF. Vitamins B (thiamine), niacin, vitamin B2(riboflavin),choline, and folic acid are also present. Some authoritiessuggest that there is also a trace of vitamin B12.

[66]

5.2.2. Enzymes

When taken orally, several of these biochemicalcatalysts, such as amylase and lipase, can aid digestion bybreaking down fats and sugars. One important enzyme, a

carboxy-peptidase, inactivates bradykinins and producesan antiinflammatory effect. During the inflammatoryprocess, bradykinin produces pain associated withvasodilation and, therefore, its hydrolysis reduces thesetwo components and produces an analgesic effect.[67]

5.2.3. Minerals

Sodium, potassium, calcium, magnesium, manganese,copper, zinc, chromium, and iron are all found in the aloeplant. Magnesium lactate inhibits histidin decarboxylaseand prevents the formation of histamine from the amino acid,histadine. Histamine is released in many allergic reactionsand causes intense itching and pain. The prevention of itsformation may explain the antipuritic effect of aloe vera.[68]

5.2.4. Sugars

Sugars are derived from the mucilage layer of the plantunder the rind, surrounding the inner parenchyma or gel.They form 25% of the solid fraction and comprise bothmono- and polysaccharides. By far the most importantare the long chain polysaccharides, comprising glucoseand mannose, known as the gluco-mannans (beta-(1, 4)-linked acetylated mannan). When taken orally, some ofthese bind to receptor sites that line the gut and form a bar-rier, possibly helping to prevent ‘‘leaky gut syndrome.’’Others are ingested whole by a method of cellular absorp-tion known as pinocytosis. Unlike other sugars which arebroken down prior to absorption, the polysaccharides areabsorbed complete and appear in the blood streamunchanged. Here, they act as immuno-modulators-capableof enhancing and retarding the immune response.[69–71]

5.2.5. Anthraquinones

These phenolic compounds are found in the sap. The bit-ter aloes consist of free anthraquinones and their derivatives:

. barbaloin-lO-(1151-anhydroglucosyl)-aloe-emodin-9-anthrone)

. lsobarbaloin

. anthrone-C-glycosides and chromones.

In large amounts, these compounds exert a powerfulpurgative effect, but when smaller they appear to aidabsorption from the gut, are potent antimicrobial agentsand possess powerful analgesic effects. Topically, theycan absorb ultra violet light, inhibit tyronase activity,reduce the formation of melanin and any tendency to hyperpigmentation. Lignin, this woody substance, inert in itself,endows topical aloe preparations with their singularpenetrative ability to carry other active ingredients deepinto the skin to nourish the dermis.[72,73]

5.2.6. Saponins

These soapy substances form 3% of the gel and aregeneral cleansers, having antiseptic properties.[74]

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5.2.7. Plant Sterols

These include campesterol, f3 sitosterol, and lupeol,which is an aspirin-like compound possessing antiinflam-matory and antibacterial properties. Topically, it has akerolytic effect which helps to debride a wound of necrotictissue.[66]

5.2.8. Aminoacids

These are the building blocks of proteins. Aloe vera gelprovides 20 of the 22 necessary amino acids required by thehuman body and seven of the eight essential amino acidswhich the body cannot synthesize like lysine, threonine,valine, methionine, leucine, isoleucine, phenylalanine,tryptophane, histidine, arginine, hydroxy proline, asparticacid, serine, glutamic acid, proline, glycerine, alanine,cystine, and tyrosine.

6. APPLICATIONS OF ALOE VERA

6.1. Medical Uses and Pharmacological Applications

Aloe vera gel is widely used for the external treatment ofminor wounds and inflammatory skin disorders. The gel isused in the treatment of minor skin irritations, includingburns, bruises, and abrasions. The gel is further used inthe cosmetics industry as a hydrating ingredient in liquids,creams, sun lotions, shaving creams, lip balms, healing oint-ments, and face packs. Aloe vera gel has been traditionallyused as a natural remedy for burns. Aloe vera gel has beenclaimed to be effectively used in the treatment of first andsecond degree thermal burns and radiation burns. Boththermal and radiation burns healed faster with less necrosiswhen treated with preparations containing aloe vera gel. Inmost cases the gel must be freshly prepared because of itssensitivity to enzymatic, oxidative, or microbial degradati-on. Aloe vera gel is not approved as an internal medication,and internal administration of the gel has not been shown toexert any consistent therapeutic effect. The uses described infolklore medicine not supported by experimental or clinicaldata include, treatment of acne, hemorrhoids, psoriasis,anemia, glaucoma, petit ulcer, tuberculosis, blindness,seborrhoeic dermatitis, and fungal infections.

6.2. Antiinflammatory Effects

Several animal studies have been undertaken since 1989,clearly demonstrating the antiinflammatory activity of aloevera gel. One study found that an aqueous extract of aloevera decreased carrageenan-induced edema in a rathind-paw.[75] Further, the aloe vera extract reduced prosta-glandin E2 production from 14C arachidonic acid viainhibition of cyclooxygenase. In a series of experimentsconducted from 1989 to 1994, Davis and colleaguesdemonstrated the antiinflammatory action of oral andtopical Aloe Vera preparations in various animal models

of inflammation. In their earliest study, they reported a47% reduction in swelling in the croton oil-induced edemaassay in rats after topical administration of aloe vera.[76]

In a later study they demonstrated an antiinflammatoryresponse to aloe vera in an inflamed synovial pouch modelin rats.[77] Aloe vera reduced the vascularity and swelling inthe inflamed pouch by 50%. The investigators also noted a48% reduction in the number of mast cells in the synovialfluid of the pouch. Also of interest, they found an increasednumber of fibroblasts following treatment with aloe gel.C-glucosyl chromone is the antiinflammatory compoundrecently isolated from aloe vera extracts.[78] The substancewas shown to be similar in potency to hydrocortisone whentested in a mouse ear bioassay.

6.3. Effects on Burns

Several animal studies and a clinical trial have assessedthe effectiveness of aloe vera in the treatment of skin burns.One study looked at full-thickness burns in guinea pigs.[79]

Aloe gel promoted complete healing of burn woundswithin 30 days, compared to 50 days in the control group.In contrast, a similar study in guinea pigs published thesame year showed that aloe vera was less effective in treat-ing second-degree burns when compared to standard 1%silver sulfadiazine cream.[80] Wound re-epithelialization,wound contraction, and formation of granulation tissueoccurred more slowly in the aloe vera-treated animals. Inanother study, aloe vera was found ineffective in treatinghydrofluoric-acid induced burns in rats.[81] In a humanstudy, 27 patients with partial thickness burn wounds weretreated with topical AG or standard Vaseline gauze[82] theaverage healing time was 18.19 days in the Vaseline-gauzetreated wounds and 11.89 days in the aloe vera-treatedwounds. Histological examination showed early epitheliali-zation in the aloe Vera-treated skin areas.

6.4. Effects on Skin Exposure to UVand Gamma Radiation

Some of the first scientific studies on the effectiveness ofaloe vera were performed during the 1930s and involvedprotection of the skin against radiation damage. For themost part, these studies were inconclusive. Interestingly,recent evidence has supported a protective benefit of aloevera gel against several forms of radiation damage to theskin. An acemannan-containing topical gel was demon-strated to reduce skin damage following exposure to gammaradiation in mice.[83] The results were best in animals whoreceived the gel treatment for at least two weeks beginningimmediately after irradiation. A protective effect also wasdocumented in mouse skin exposed to soft x-irradiation.[84]

Investigators found that an antioxidant protein, metal-lothionein, was induced in the skin and liver within 24 hrsof AG administration, following x-ray exposure, AGwas found to scavenge hydroxyl radicals and prevent

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suppression of superoxide dismutase and glutathioneperoxidase in the skin. Several additional studies in miceand in epidermal cell culture have demonstrated an immu-nomodulatory effect of AG in protecting skin cells fromthe damaging effects of UVB radiation. UVB radiation isknown to suppress the ability of Langerhans cells in theepidermis to support antibody primed T-cell mitogenesis.In one study, aloe gel prevented this UVB-mediated sup-pression within the first 24 hours of irradiation in murineepidermal cell culture.[85] Immunomodulatory activity wasfound to reside in a number of low molecular weight com-pounds present in AG. A more recent study reports the iso-lation of these small immunomodulatory substances fromAG.[86] Topical application of these compounds preventedUVB-induced immune suppression in mouse skin. Furtherwork in this area has confirmed these observations andhas demonstrated how AG-derived immunoprotective fac-tors likely work. Presumably, UV-induced suppression ofdelayed type hypersensitivity is prevented by reducing theproduction and release of skin keratinocyte derived immu-nosuppressive cytokines, such as interleukin-10 (IL-10).[87]

Another study demonstrated that AG’s prevention ofUV-induced immune suppression did not involve preven-tion of UVinduced DNA damage or an acceleration ofthe repair of DNA.[88]

6.5. Treatment of Frostbite and Psoriasis

Several animal studies support the clinical use of AG intreating frostbite tissue damage. Heggars and associatesutilized an experimental rabbit ear model to demonstratethe effectiveness of AG, as well as that of several inhibitorsof arachidonic acid metabolism (e.g., aspirin and methyl-prednisolone).[89] In control animals, no tissue survivalwas seen. In contrast, AG treatment resulted in 28.2%tissue survival compared to 22.5% and 12.5% with aspirinand steroid, respectively. The investigators concluded thatthe progressive dermal ischemia occurring during frostbitecould be reduced by inhibiting the production of prosta-glandins and thromboxanes from arachidonic acid. A morerecent study supports these observations. Systemic pentox-ifylline and topical AG cream were both found to improvetissue survival in the frostbitten ears of New Zealandrabbits.[90] Using both agents together further increasedtissue survival. A double-blind, placebo-controlled studyin 60 psoriasis patients evaluated the efficacy of treatmentwith topical AG.[91] PG extract (0.5% in a hydrophiliccream) was administered three times daily for five consecu-tive days each week for 16 weeks. At the end of the studyAG had significantly reduced lesions, decreased erythema,and lowered PASI (psoriasis area and severity index)scores in 25 out of the 30 patients in the treatment group.In comparison, two out of 30 patients in the placebogroup improved.

6.6. Effects on the Immune System

Many claims have been made throughout the yearsregarding AG’s ability to support and enhance the immunesystem. Experimental evidence is now accumulating docu-menting immune-stimulating constituents present in AG.One of the first studies in the early 1980s demonstrated thata partially purified AG extract from Aloe vahombe acted asa nonspecific immunostimulant, protecting mice againstinfection from various bacteria and fungi.[92] The AGextract had to be administered two days before exposureof the mice to the pathogenic agent to be effective. Laterin the 1980s, acemannan isolated from AG was shown toincrease the response of lymphocytes to antigens in anin vitro study.[93] This helped explain the many reports ofacemannan’s apparent antiviral effect. In later studies, ahighly purified form of acemannan derived from AGstimulated the synthesis and release of interleukin-1 (IL-1)and tumor necrosis factor from peritoneal macrophages inmice that had previously been implanted with murine sar-coma cells.[94] These cytokines in turn initiated an immuneattack on the sarcoma cells that resulted in necrosis andregression of the cancerous cells. These effects resulted inan increased survival of the sarcoma-implanted mice. Inanother study, acemannan stimulated the production ofnitric oxide in cultures of chicken macrophages.[95] Stillanother study demonstrated that several low molecularweight compounds isolated from AG are capable of inhibit-ing the release of reactive oxygen free radicals fromactivated human neutrophils.[96] This inhibition does notappear to affect the phagocytic activity of neutrophils butmay protect tissues from excessive oxidative damage fromfree radicals.

6.7. Antiviral and Antitumor Activity

Most of the reported antiviral and antitumor effects ofAG likely are due indirectly to the stimulation of theimmune system, as discussed previously. However, onestudy reports that anthraquinones, which are present in aloelatex, have direct virucidal effects. The anthraquinone aloinwas shown to inactivate various enveloped viruses, such asherpes simplex, varicella-zoster, and influenza. Althoughanthraquinones only appear in AG as a contaminant[97]

low concentrations present in some preparations could havesignificant antiviral activity. Several studies have demon-strated direct inhibitory effects of AG on both tumorinitiation and promotion. A polysaccharide fraction ofAG inhibited the binding of benzopyrene to primary rathepatocytes and thus prevented the formation of potentiallycancer-initiating benzopyrene-DNA adducts,[98] this effectwas also demonstrated in vivo, where adduct formationwas reduced in various organs. Several other plant-derivedpolysaccharides were also able to block benzopyrene-DNAadducts,[99] they also reported in this study an induction of

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glutathione S-transferase and an inhibition of the tumor-promoting effects of phorbol myristic acetate by AG. Thesetwo studies suggest a possible benefit of using aloe gel incancer chemoprevention.

6.8. Laxative Effects of Aloe Vera

Anthraquinones present in aloe vera function as potentstimulant laxatives. Aloe latex is typically sold as anncapsulated dried powder. The substance is still listed inthe U.S. Pharmacopoeia and is recognized by the U.S.Food and Drug Administration (FDA), as well as inseveral other European countries, as an effective laxative.Studies in rats have shown that aloe latex increasesintestinal water content, stimulates mucus secretion, andincreases intestinal peristalsis.[100] Long-term use of thesubstance could result in electrolyte imbalances, especiallydepletion of potassium salts.

6.9. Wound Healing

Aloe gel has long been used both externally and intern-ally for its beneficial effects in the wound-healing process.It is most often included in topical formulations (ointment,cream, or lotion), but evidence also supports its effective-ness when taken orally. At least part of AG’s beneficialeffect on the skin likely is due to its moisturizing effect.Also, it may leave a protective layer on the skin after drying,possibly providing some protection to the wound. Both top-ical and oral AG has been shown to significantly stimulatecollagen synthesis in experimental dermal wounds inrats.[101] Aloe gel not only increased collagen content ofthe wound but also changed collagen composition (moretype III). In addition, it increased the degree of collagencrosslinking. In an earlier study, the investigators alsodemonstrated an increased synthesis of hyaluronic acidand dermatan sulphate in the granulation tissue of a healingwound following oral or topical AG treatment.[102] Bothstudies support an earlier trial demonstrating that both oral(100mg=kg=day) and topical (25% AG) treatment (twomonths) of biopsy punch wounds in mice resulted in asignificant (50%� 3%) reduction in wound diameter.[103]

Similar beneficial effects of topical AG have been demon-strated in a skin-wound rat model. Aloe gel treatment accel-erated wound contraction and increased the breakingstrength of resulting scar tissue, due to increased collagencontent and degree of crosslinking.[104] In order to identifywhich constituents of AG are responsible for wound-healing effects, Davis and colleagues tested the effects ofmannose-6-phosphate in a mouse-wound model system.[105]

An oral dose of 300mg=kg resulted in significant woundhealing, similar to that seen with AG. However, not allanimal studies have shown positive results. In one study,various topical agents were tested for their effects on woundcontraction and rate of re-epithelialization in full-thicknessexcisions in a porcine mode; AG failed to show any

beneficial effects.[106] AG studies in humans are morelimited, and the results generally are not as positive asthose from the aforementioned animal studies. In one posi-tive study, AG (when added to a polyethylene oxide gelwound dressing) was shown to accelerate wound healingfollowing full-face dermabrasions.[107] By day six,re-epithelialization was complete at the AG-treated sites.However, soon after this study a report emerged of fourpatients who experienced severe burning sensations anddermatitis upon application of topical aloe gel followingdermabrasions.[108] An acemannan-containing gel (carrisyngel wound dressing) was recently shown to be no moreeffective than standard saline gauze dressing in the treat-ment of pressure ulcers.[109] In another study, a similarAG dermal wound gel was actually shown to significantlydelay wound healing in surgical wounds following cesareandelivery or laparotomy.[110]

6.10. Comparison Between the Properties of Aloe Veraand Requirements for Wound Healing

Wound healing Aloe veraNutrients 75 essential nutrientsMoisture MoisturizesClot formation Decreases bleedingMaintain wound tempIncrease bloodperfusion V=D

Increase blood flow tothe skin

OxygenationDecrease inflammation AntiInflammatoryStimulation of immunity Immuno modulatingKill germs Cleanser and antimicrobialEpithelialization and Increases cell proliferationFibroblast multiplication

7. BIOCHEMICAL PROCESS IN WOUND HEALING

The science of wound healing is recorded as ‘‘threehealing gestures’’ on a clay tablet, one of the oldestmedical texts dated 2200 BC. It describes the three ges-tures as; washing the wound; making plasters; and ban-daging the wound. Although there has been a significantadvancement in today’s Science of wound healing, thebasic theme seems to be similar. The work of JosephLister and Louis Pasteur established a sound basis forthe management of infection by identifying the causeand developing methods for preventing it.[111] LouisPasteur proved that bacteria did not spontaneously gen-erate but were introduced into wounds from a foreignsource. These findings encouraged Lister’s advocacy offrequent washing with soap and water and fueled hissearch for ways to kill bacteria, or the antiseptic techni-ques – a major advance in the field of wound healing.The antiseptic technique was followed shortly by the

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‘‘aseptic technique’’ in which a sterile environment wasused to prevent the onset of infection.

Wounds are generally classified as wounds withouttissue loss (e.g., in surgery), and wounds with tissue loss,such as burn wounds, wounds caused as a result oftrauma, abrasions or as secondary events in chronicailments, for example, venous stasis, diabetic ulcers orpressure sores and iatrogenic wounds such as skin graftdonor sites and dermabrasions. Wounds are also classifiedby the layers involved, superficial wounds involve only theepidermis, Partial thickness wounds involve only epider-mis and dermis, and full thickness wounds involve thesubcutaneous fat or deeper tissue. Although restorationof tissue continuity after injury is a natural phenomenon,infection, quality of healing, speed of healing, fluid lossand other complications that enhance the healing timerepresents a major clinical challenge. However the overallWound healing process may be divided into four continu-ous phases, namely haemostasis, inflammation, prolifer-ation and maturation or remodeling.[112] The plateletspresent in the exposed blood aggregates and a temporaryplug is formed reducing bleeding. The phagocytes act toclear debris and destroy the ingested material. New vesselsare formed and carry oxygenated blood to the woundbed. The fibroblast cells lay down a network of collagenfibers surrounding the neo-vasculature of the wound.Finally the process of remodeling of the collagen fiberslaid down in the proliferation phase occurs which maytake years.

8. WOUND DRESSINGS

Wound dressings are generally classified as

1. Passive products,2. Interactive products, and3. Bioactive products based on its nature of action.

Traditional dressings like gauze and tulle dressings thataccount for the largest market segment are passive products,Interactive products comprise of polymeric films and formswhich are mostly transparent, Permeable to water vapourand oxygen but impermeable to bacteria. These films arerecommended for low exuding wounds. Bioactive dressingis which delivers substances active in wound healing; eitherby delivery of bioactive compounds or dressings is con-structed from materials having endogenous activity. Thesematerials include proteoglycans, Collagen, non collagenousproteins, alginates or chitosan. In November 1999, the FDAreclassified the dressing categories as

1. Non-resorbable gauze=sponge dressing for external use2. Hydrophilic wound dressing3. Occlusive wound dressing4. Hydro gel wound and burn dressing5. Interactive wound and burn dressings.

Recent years have witnessed significant advance inthe era of wound dressing. However an ideal wounddressing is one which fulfills all of the followingrequirements.

. Be capable of maintaining high humidity atwound dressing interface whilst removing throughabsorption of excess wound exudes and associatedtoxic compounds.

. Permit the exchange of gases whilst maintainingan impermeable layer to micro organism sopreventing secondary infections.

. Provide thermal insulation

. All components of the dressing including theadhesive must be biocompatible and not provokeany allergic reaction through their prolongedcontact with tissues.

. There must be minimal adhesion to the wound sothat the dressing can, when required be removedwithout trauma.

. The dressing must be physically strong evenwhen wet.

. Be produced in sterile form

. Easy to dispose of when removed at the endof use.

8.1. Chitosan Based Wound Dressings

Biopolymers have generated finding considerable inter-est in the field of medical science. Chitosan is one of thesenovel polymers which are an application in drug deliverysystems, hydrogels and wound dressings. Chitosan gener-ally obtained from chitin, is the polymer second mostabundant in nature next only to cellulose. Chitosan isknown in the wound management field for its haemostaticproperties. Further it also possesses other biological activi-ties and effect macro phage function that helps in fasterwound healing.[113] It also has an aptitude to stimulate cellproliferation and histoarchitectural tissue organization.[114]

The biological properties including bacteriostatic andfungistatic properties are particularly useful for woundtreatment.[115,116]

The chemical structure of chitosan has been exploitedfor its innovative features in biotechnology and medicinalfield. The presence of free amino group within moleculemakes it highly interesting for immobilization of bioactivesubstances. This is where CS becomes highly attractive forwound dressing application.

Hydrogel material: Chitosan based wound dressingsbecause of its inherent properties like biodegradability,biocompatibility, antimicrobial nature, ability to healwounds, ability to reduce scar formation and ability toabsorb large amounts of wound exudates are mostcommonly used.[117,118]

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8.1.1. Antimicrobial Activity

The polycationic nature of chitosan and interactionbetween the positively charged chitosan and negativelycharged cell wall of the microorganism leads to the leakageof intracellular substances and ultimately death of theorganism.[119,120]

8.1.2. Biodegradability

Chitosan is considered biodegradable in nature as itcan be metabolized by certain human enzymes such aslysozyme,[121,122] Chitosan is also known for its biocom-patibility allowing its use in various biomedical applica-tions, such as implantations, injections or topical ocularapplications.

8.1.3. Wound Healing Ability

Chitosan containing wound dressings has been found togenerate optimal healing environment for the woundunderneath. The efficacy of chitosan for wound healingmay be summarized as follows.[123,124]

. Acceleration of the infiltration of polymorphnuclear leukocytes (PMN) cells into the woundarea.

. Increase in effusion which forms thick fibrin andactivates the migration of the fibroblast into thewound area.

. Stimulates migration of the macrophages.

. Stimulates proliferation of fibroblast andproduction of type 3 collagen.

. Scar less healing.

. Drug delivery.

Wound dressing based on chitosan material is wellknown in literature as well as from commercial point ofview[125] in wound management. More than 40 years havelapsed since this biopolymer had aroused the interest ofscientific community around the world for its potentialbio-medical applications.[126] In November 1999, FDA,legalized the use of these bioactive wound dressings,constructed from materials having endogenous activity.[127]

A Chitosan based membrane, applied for wound healing astopical drug delivery system were developed by graftco-polymerization of acrylic acid (AA) and 2-Hydro-xyethyl methacrylate (HEMA).[128] Polysaccharides-basedmembranes of chitosan and cellulose were prepared usingtrifluoroacetic acid as a co-solvent.[129] The chitosan-cellulose blend membrane demonstrated effective anti-microbial activities as examined by antimicrobial tests. Ina study a photo crosslinkable chitosan hydrogel wasapplied on full thickness skin incision made on the backsof mice, significantly induced wound contraction andaccelerated wound closure and healing compared with theuntreated controls.[130,131] A chitosan film containing

Fucoidan as a wound dressing has been prepared.[132] Ithas been found that the application of Fucoidan-chitosanfilm onto an open burn wound induces significant woundcontraction and accelerates the wound closure and healingprocess, thus, the Fucoidan-23. Chitosan film is a promis-ing new dressing for wound occlusion and tissue repairing.A yet another study focuses on the development ofchitosan-based films intended for wound dressingapplication.[133] Films of chitosan and their blends withcornstarch=dextran and propylene glycol were developedthat showed optimal properties for wound management.A chitosan containing PU=poly (NIPAAm), thermosensi-tive membrane for wound dressing has been reported andit has been found to exhibit very low cytotoxicity and highhealing power.[134] A short review on chitosan and alginatewound dressings has been reported by Paul and Sharma.Chitosan being a natural healer and calcium alginate anatural haemostat is a perfect for bleeding wounds.[135]

8.2. Aloe Vera Based Wound Dressings

A fascinating reference of aloe vera as a wound healingmaterial dates back to the times when Alexander the Great(333 BC) was persuaded by his mentor Aristotle to capturethe islands of Socotra in the Indian ocean for its famed aloevera, needed to treat his wounded soldiers.[136] There arefew reports in the literature on the effectiveness of compli-mentary therapies on chronic wounds, use of an aloe veragel resulted in full healing.[137] Several studies and a clinicaltrial have assessed the effectiveness of aloe vera in the treat-ment of skin burns.[138] Aloe vera promoted completehealing of burn wounds. Historical examination showedearly epithelialization in the Aloe Vera treated skin areas.Aloe vera has also been found to exhibit antiinflammatoryeffects.[139,140] A research work was carried to evaluate theeffectiveness of lyophilized aloe vera dressing on woundhealing process.[141] The treatments were applied on thedeep partial thickness burn wounds that were induced bymodified electric solder. It was observed that the lyophi-lized aloe vera showed faster healing properties, scarformation and epithelialization. A study, with good repli-cation, of healing after a precise skin hole punch demon-strated the antiinflammatory properties of the aloe gel asa dressing material, leading to more rapid healing.[142] Awound dressing has been prepared by adding aloe vera toa polyethylene gel wound dressing. This dressing showedto accelerate wound healing and re-epithelialization,following full-face derm abrasions.[143]

9. CONCLUSION

Chitosan and aloe vera have a wide range of applica-tions. They may be employed, for example, to solve numer-ous problems in environmental and biomedical engineering.It is clear from the applications of chitosan and Aloe vera

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that they are really the gifts of nature and it appears thatthis material can be the most interesting and beneficial,for the researchers in the near future. Thanks to the bioac-tivities of chitosan and aloe vera both, which have the char-acteristics and constituents necessary for wound healing.

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