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Keywords:

  • anti-microbial;
  • carboxymethyl chitosan;
  • delivery system;
  • emulsion stabilizer;
  • moisture retention

Synopsis

  1. Top of page
  2. SynopsisRésumé
  3. Introduction
  4. Preparations and properties of carboxymethyl chitosan
  5. Use of carboxymethyl chitosan in cosmetics
  6. The safety issue of carboxymethyl chitosan
  7. Conclusion
  8. Acknowledgements
  9. References

Carboxymethyl chitosan is a chitosan derivative of the most intensively investigated due to its water solubility in wider pH range compared with the parent compound, thus extended its use in various applications. In this review, different preparation conditions, which resulting in the N- and O-carboxylated chitosan, diverse degree of substitution and water solubility are recapitulated. Five important features of carboxymethyl chitosan from recent studies, which are moisture absorption–retention, anti-microbial properties, antioxidant capacities, delivery system and emulsion stabilization, have been centred and emphasized for cosmetic utilization. Additionally, cytotoxicity information has been inclusively incorporated to ensure its safety in application.

Résumé

Le carboxyméthyl chitosan est un des plus étudiés dérivés de chitosan en raison de sa solubilité dans l'eau dans la gamme de pH la plus large par rapport à la molécule mère; ainsi son utilisation s'est étendue dans diverses applications. Dans cette revue, les différentes conditions de préparation qui en résultent dans le chitosan N et O-carboxylé, le degré divers de substitution et leur solubilités dans l'eau sont récapitulés. Cinq caractéristiques importantes de carboxyméthylchitosan d’études récentes, qui sont l'absorption ou rétention d'humidité, les propriétés antimicrobiennes, les capacités anti-oxydantes, le système de vectorisation et de stabilisation des émulsions, ont été évalués pour l'utilisation cosmétique. En outre, les informations de cytotoxicité ont été incorporées pour assurer sa sécurité dans les applications.


Introduction

  1. Top of page
  2. SynopsisRésumé
  3. Introduction
  4. Preparations and properties of carboxymethyl chitosan
  5. Use of carboxymethyl chitosan in cosmetics
  6. The safety issue of carboxymethyl chitosan
  7. Conclusion
  8. Acknowledgements
  9. References

Chitosan is a linear copolymer of β-(1-4)-linked 2-acetamido-2-deoxy-β-d-glucopyranose and 2-amino-2-deoxy-β-d-glycopyranose. It is a partially deacetylated derivative of a natural polysaccharide chitin, which is one of the most abundant carbohydrates in nature and is mostly derived from the exoskeleton of crustaceans [1, 2]. Chitins and chitosans contain high percentage of nitrogen (6.9%) compared with synthetically substituted cellulose (1.3%). They are examples of highly basic polysaccharides. Chitosans have three reactive groups in the molecule, that is, (i) a primary hydroxyl group, (ii) a secondary hydroxyl group and (iii) an amino group (Fig. 1), and accordingly they possess many unique properties that include polyoxysalt formation, ability to form films, chelate metal ions and optical structural characteristics [3-5]. Moreover, chitosans have many useful characteristics such as biorenewability, biodegradability, biocompatibility, bioadhesivity and non-toxicity, which make them are of commercial interest [6-8]. However, the poor solubility in neutral water and in common organic solvents has barred their potential applications. At relatively low pH (pH < 6.0), chitosans are positively charged (–NH3+) and tend to be soluble in dilute aqueous solutions, but at higher pH, they tend to lose their charge and may precipitate from solution due to deprotonation of the amino groups [7, 9]. By chemically modifying the amino group and hydroxyl groups, many water-soluble chitosan derivatives can be synthesized and the most investigated one can be achieved by means of carboxymethylation reaction to obtain a derivative known as carboxymethyl chitosan.

image

Figure 1. Structure of chitin, chitosan and carboxymethyl chitosan.

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Preparations and properties of carboxymethyl chitosan

  1. Top of page
  2. SynopsisRésumé
  3. Introduction
  4. Preparations and properties of carboxymethyl chitosan
  5. Use of carboxymethyl chitosan in cosmetics
  6. The safety issue of carboxymethyl chitosan
  7. Conclusion
  8. Acknowledgements
  9. References

Carboxymethyl chitosan (CMCS) is an amphiprotic ether derivative, which contains active hydroxyl (–OH), carboxyl (–COOH) and amine (–NH2) groups in the molecule (Fig. 1). CMCS is soluble in water at neutral pH [7]. It possesses high viscosity, large hydrodynamic volume, film- and gel-forming capabilities together with the other useful properties, such as biocompatibility, biodegradation, biological activity and low toxicity [10], all of which make it an attractive option in connection with its use in foods and cosmetics [11, 12].

Carboxymethyl chitosan can be obtained by direct alkylation of chitosan [10, 11]. The CMCS achieved can be in many types: N-carboxymethyl chitosan (N-CMCS), O-carboxymethyl chitosan (O-CMCS), N,O-carboxymethyl chitosan (N,O-CMCS) and N,N-carboxymethyl chitosan (N,N-CMCS). Different reaction conditions will result in the N vs. O selectivity and different degree of substitution (DS) [11, 13].

N-carboxymethyl chitosan can be prepared using chitosan, glyoxylic acid with sodium cyanoborohydride (NaBH3CN) solution and pH adjustment [11, 14-16], see Fig. 2. With the proper selection of the reactant ratio, the product is in part N-monocarboxymethylated (0.3), N,N-dicarboxymethylated (0.3) and N-acetylated depending on the starting chitosan [17]. N-CMCS can also be obtained by direct alkylation [18]. At relative mild alkaline pH values (pH 8–8.5), alkyl halides such as monochloroacetic acid reacts preferentially with the amine groups of chitosan rather than with the hydroxyl groups, thus forming N-carboxymethylated chitosan derivatives (Fig. 2).

image

Figure 2. Preparation of N-carboxymethyl chitosan.

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For the O-CMCS and N,O-CMCS, chitosans are soaked in alkaline solution (40–60%w/v NaOH) at freezing or up to mild heating temperatures (e.g. 50–60°C) for 1 to 24 h [11, 19]. The amount of chitosan used in the preparation can be quite varying in the range of 4–20%w/v in sodium hydroxide solution. The alkalized chitosans are then reacted with monochloroacetic acid at 0–60°C for 2–24 h. The amount of monochloroacetic acid is reported from 1 to 10 g to each gram of chitosan [20-23]. The carboxymethyl groups were mostly substituted on the –OH groups, with a small amount on the –NH2 groups. The 6-OH group had the highest DS when the reaction temperatures were at 0 and 10°C. At high alkali concentrations (>25% aqueous NaOH), however, alkylation with monochloroacetic acid gives mixed N- and O-alkyl chitosan derivatives with substitution at the C6 and C3-OH groups and also some substitution on the C2-NH2 groups (Fig. 3). The ease of substitution is in the order of 6-OH > 3-OH > 2-NH2 [11].

image

Figure 3. Preparation of O-carboxymethyl chitosan and N,O-carboxymethyl chitosan.

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The water solubility of CMCS had close relationships with the modifying conditions and the degree of carboxymethylation. Recently, Chen and Park [20] have studied the solubility of O-CMCS with varied DS and found that different reaction temperatures and solvents resulted in diverse water solubility of O-CMCS, see Table 1.

Table 1. Water solubility of O-carboxymethyl chitosan prepared at different conditions
SamplePreparation conditionYield (%)aSolubility or insolubility (pH)b
Water/isopropanol (v/v)Temperature (°C)
  1. CMCS, Carboxymethyl chitosan.

  2. a

    Yield = Water-soluble CMCS (g)/Raw product (g) × 100%.

  3. b

    Solid sample 10 mg in 50 mL water.

CMCS 11/901.2Soluble over pH range 1–13
CMCS 21/91012.8Soluble over pH range 1–13
CMCS 31/92066.7Insoluble over pH 5.5–7.0
CMCS 41/93072.7Insoluble over pH 5.0–5.5
CMCS 51/94071.8Insoluble over pH 4.5–5.0
CMCS 61/95081.0Insoluble over pH 4.0–5.5
CMCS 71/96082.3Insoluble over pH 4.0–4.5
CMCS 80/105049.6Insoluble over pH 4.5–6.5
CMCS 92/85099.8Insoluble over pH 3.0–5.0
CMCS 105/55098.6Insoluble over pH 4.6–5.8
CMCS 118/25013.6Insoluble over pH 6.0–6.5
CMCS 1210/0502.4Insoluble over pH 6.0–7.0
ChitosanInsoluble over pH 5.5–13

Also, it has been reported that the critical DS value at which CMCS becomes soluble in water could be in the range of 0.4–0.45 [24]. The study illustrated that concentration of NaOH in the preparation affected the DS of the resulting CMCS and suggested that a 50% NaOH solution seems to provide the optimum alkali concentration in the carboxymethylation process. The low NaOH concentration was not enough to break the rigid crystalline structure of chitosan, thus lowering the penetration of the monochloroacetic acid into the interlocking polymer chains. While high alkali concentrations above 60% promoted side reaction between NaOH and monochloroacetic acid, and the monochloroacetic acid concentration decreased accordingly [15], in another study by Bidgoli et al. [25], the ratio of NaOH to monochloroacetic acid in the reaction mixture showed high impact on the water solubility of CMCS. At very low NaOH to monochloroacetic acid ratios (0.28–0.40), the CMCSs were either insoluble or partially soluble over the entire range of pH 2–12. At higher NaOH to monochloroacetic acid ratios, however, products with generally improved water solubility were obtained at optimal reaction time, but the insoluble pH range of CMCS was extended when increasing the reaction time [25]. In addition, Ge and Luo [22] studied the effects of the mass ratio of chloroacetic acid to chitosan at 5 : 1, 8 : 1 and 10 : 1 under microwave irradiation conditions, and the results revealed that after 20 min, the DS increased as the chloroacetic acid/chitosan ratio is increased from 5 : 1 to 8 : 1, but that at ratio of 10 : 1, no further increase is observed [22].

Compared with chitosan, the solubility of CMCS in aqueous solution is improved remarkably because of the introduction of carboxymethyl group. The combined driving forces that make CMCS soluble in water include the H-bonding between water and the polymer and the presence of COO− on the CMCS chain. The water insolubility of CMCS at various pH values varied with the DS. The insoluble region may be due to either aggregation of highly acetylated chain segments or amide formation subsequent to thermal drying. The aggregation behaviour is observed in neutral dilute aqueous solution of CMCS, and the driving forces are the intermolecular H-bonding of CMCS, the electrostatic repulsion between COO groups on the molecules and the hydrophobic interaction between the hydrophobic moieties in the CMCS molecules such as acetyl groups and glucosidic rings [26]. The critical aggregation concentration (cac) of CMCS was determined to be between 0.042 and 0.05 mg mL−1 [26]. Moreover, it also has been pointed out by Thanou et al. [27] that monocarboxymethyl chitosan with 87–90% DS has polyampholytic character, which allows the formation of clear gels or solutions depending on the polymer concentration at neutral and alkaline pH values but aggregates under acidic conditions.

Use of carboxymethyl chitosan in cosmetics

  1. Top of page
  2. SynopsisRésumé
  3. Introduction
  4. Preparations and properties of carboxymethyl chitosan
  5. Use of carboxymethyl chitosan in cosmetics
  6. The safety issue of carboxymethyl chitosan
  7. Conclusion
  8. Acknowledgements
  9. References

The high water solubility characteristics of chitosan derivatives make them very attractive candidates for applications in the field of cosmetics [3, 10-12, 17, 28]. It has been earlier mentioned by Muzzarelli [11] that carboxymethylated chitins and chitosans have similarities with the extracellular matrix polysaccharides, that is, water solubility, anionic functionality, high viscosity, large hydrodynamic volumes, cation-binding characteristics, large osmotic pressures and gel-forming capabilities. Modified chitin/chitosan could also function as a selective permeability barrier, as is the case for hyaluronate, which forms a protective layer around the human ovum. In this review, the potential applications of carboxymethyl chitosan in cosmetics will be viewed into five major uses as: (i) moisture absorption–retention agent, (ii) anti-microbial agent, (iii) antioxidant agent, (iv) delivery system, and (v) emulsion stabilization.

Moisture absorption–retention properties of carboxymethyl chitosan

The moisture-retention abilities of chitin and chitosan derivatives have been reported to be related with the molecular structure [7, 20, 25, 29, 30]. It has been noted that the water-holding capacity of chitosan was superior to that of conventional methylcellulose [31]. Chitosans and their derivatives are bearing positive electrical charge and possess relative high molecular weight (MW) that will make them adhere well and remain long on the skin, thus they can act as a moisturizer for the skin [32]. Muzzarelli [11] had pointed out that a 0.25% N-CMCS aqueous solution was comparable with a 20% aqueous solution of propylene glycol in terms of moisture-retention ability, and the viscosity was almost equal to that of hyaluronic acid (HA), a compound with known excellent moisture-retention properties. The moisture-absorption and moisture-retention abilities of CMCS with respect to the structural properties of the polymer were studied by Chen et al. [24, 33]. The moisture-retention ability (Rh) was calculated as the percentage of residual water of wet sample prepared by adding 10% water to samples pre-dried over P2O5 in vacuo for 24 h. The moisture-absorption ability (Ra) was calculated as the percentage of weight increase in the sample dried over P2O5 in vacuo for 24 h. The authors found that the Ra and Rh abilities of CMCS were closely related to the site of substitution, MW, degree of deacetylation (DD) and DS. CMCS is better in moisture absorption and moisture retention than the parent chitosan. Under high relative humidity (43 and 81% RH), the maximum Ra of CMCS was obtained at a DD value of 53%, and when the DD values were higher, Ra decreased rapidly. However, under silica gel conditions, the moisture-retention displayed in the opposite way as when the DD value was 53%, the Rh was the lowest and when the DD was higher, the Rh increased. When considering the DS value, it showed that with increasing DS, Ra and Rh of CMCS increased. However, further increase in the DS value from 0.6 to 1.2 reduced the increasing tendency of Ra and Rh, and CMCS with the DS value from 0.6 to 1.0 showed equal or better Ra and Rh than that of HA, thus it has potential to be used as moisture absorption–retention ingredient in cosmetics [24]. Moisture-absorption and moisture-retention of CMCS with different substitution sites (6-O-CMCS, 3,6-O-CMCS, N-CMCS) and different MWs were also investigated [33], and HA was used as a control. The results indicated that at 81%RH, moisture-absorption abilities of 6-O-CMCS are a little less than that of HA but higher than that of 3,6-O-CMCS and N-CMCS. However, when study at 43%RH, the moisture-absorption abilities are in the following sequence: 6-O-CMCS > 3,6-O-CMCS > HA > N-CMCS and this sequence is also applied for the moisture-retention abilities at the same condition (43%RH). The study indicated that the introduction of carboxymethyl group in the C6 position resulting in a major active site for moisture absorption and moisture retention, although carboxymethylation at 3-OH and 2-NH2 position was not essential, it still contributed to the ability. Furthermore, higher MW also plays an important role in helping to improve moisture-retention ability. 6-O-CMCS with a DS higher than 0.8 and MW higher than 2.48 × 105 showed better moisture-retention ability than HA [33]. It was proposed that the intermolecular hydrogen bonds of molecular chains, which are the driving force for aggregates, may be an important factor to regulate moisture-absorption and moisture-retention ability of CMCS [24]. Also, CMCS was found to be able to form gel that assists in imparting a smoothness feeling to the skin and in protecting the skin from adverse environmental conditions and consequences of contact with detergents [34]. Based on the reviewed properties along with an economic point of view comparing with the well-known HA, thus, it would be literally proper to state that CMCS can be a superior candidate as natural-derived hydrating agent in cosmetics.

Anti-microbial activities of carboxymethyl chitosan

Anti-microbial activities of chitosan have been widely explored [35-41]. Chitosan derivatives of different modifications have also been prepared to improve the anti-bacterial activities [42-45]. Jayakumar et al. [31] have reported that derivatives of chitosan were found to be more effective than chitosan. Chitosan exhibits its anti-bacterial activity only in an acidic medium because of its poor solubility above pH 6.5. Thus, water-soluble chitosan derivatives soluble in both acidic and basic physiological circumstances may be good candidates for the polycationic biocide [46]. When chitosan is changed to O-CMCS, its anti-bacterial activity becomes stronger [47]. In addition, as O-CMCS is soluble in a wide pH scale, it has a much broader application as an anti-bacterial agent than chitosan does. Though, N,O-CMCS is found to be less effective in anti-bacterial activity compared with chitosan, which may be due to the decrease in the anti-bacterial effective-amino group compared with the parent chitosan and O-CMCS. In the work studied by Anitha et al. [48], O-CMCS and N,O-CMCS nanoparticles were found to exhibit higher anti-bacterial activity compared with chitosan nanoparticles. This anti-bacterial effect was increased with an increase in the nanoparticles’ concentration, and N,O-CMCS nanoparticles showed maximum anti-bacterial activity. In addition, anti-fungal effect of high- and low-molecular-weight chitosan hydrochloride, CMCS, chitosan oligosaccharide and N-acetyl-d-glucosamine against Candida albicans, Candida krusei and Candida glabrata has been investigated by Seyfarth et al. [49]. The studies revealed that C. albicans and C. krusei were the most sensitive species. Rather than the anti-microbial activity of chitosans and their derivatives in the original state or nanoparticle form, their metal complexes were found to process remarkable anti-microbial activities. The N,O-CMCS–zinc complex exhibited better anti-microbial activity than chitosan–zinc complex [50]. It was noted that even if the exact mechanism of anti-microbial action could not be clarified, the N,O-CMCS must possess multifaceted action due to the high effective outcome. It is essential to continually mention that chitosans have been spotted for their excellent metal adsorption characteristics, which were due to the high hydrophilicity owing to large number of hydroxyl groups. The large number of primary amino groups with high activity as adsorption sites and the flexible structure of the polymer chain of chitosan that enables to adopt the suitable configuration for complexation with metal ions also enhanced its metal sorption properties [51, 52]. Interestingly, CMCS was reported to enhance metal ions adsorption capacity or exhibit higher chelating ability compared with chitosan. The presence of CH2COOH group on N of glucosamine unit of chitosan makes it similar to glycine, and the lone pairs from the N-C-C-O sequence of glycine contribute towards superior chelation properties [53-55]. Thus, it is much more attractive to broaden the anti-microbial activity applications of chitosans and their derivatives by focusing on the potential use of their coordination complexes with metal ions, especially those with known anti-microbial activity. The use of CMCS for the complexation is even more attractive due to its water solubility, thus easy to utilize and will help to elicit broad range of biological activities of CMCS in pharmaceutical and cosmetic applications.

Antioxidant activities of carboxymethyl chitosan

Chitosans were reported to possess antioxidant effects both in vitro and in vivo [56-58]. Low-molecular-weight chitosans and their derivatives were more pronounced in antioxidant capacity than high-molecular-weight ones [59]. Tomida et al. [60] have detailed that low-molecular-weight chitosans were more effective in preventing the formation of carbonyl groups in plasma protein exposed to peroxyl radicals and also found to effectively prevent the formation of carbonyl groups in human serum albumin when exposed to peroxyl radicals. Moreover, they were also good scavengers of N-centred radicals, but high-molecular-weight chitosans were much less effective. Chitosans have two hydroxyl groups and one amino group in each of their monosaccharide construction units, and the hydroxyl groups in the polysaccharide units can react with free radicals by the typical H-abstraction reaction [61]. In addition, according to free radical theory, the amino groups in chitosan can react with free radicals to form additional stable macroradicals. Therefore, the active hydroxyl and amino groups in the polymer chains are the origin of the scavenging ability of chitosan. High-molecular-weight chitosans have a compact structure, and the effect of intramolecular hydrogen bond is stronger. The strong effect of the intramolecular hydrogen bond weakens the activities of the hydroxyl and amino groups, and the chance of exposure of their hydroxyl and amine groups might be restricted, which would account for less radical-scavenging activities [62]. Antioxidant activities of various chitosan derivatives were also reported. Modification of chitosan by adding quaternium on amino groups found to help increase the antioxidant activity [63]. Ying et al. [64] prepared various water-soluble Schiff base typed chitosan saccharide derivatives and investigated for their antioxidant activities, and these water-soluble chitosan derivatives exhibited higher ability of scavenging 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical compared with chitosan. The superoxide anion scavenging activity of CMCS was studied by Sun et al. [65], and the results demonstrated that the scavenging rate increases with the increase in concentration. At the maximum concentration of about 1200 mg mL−1, it has scavenging rate of 13.4%. Moreover, antioxidant activities of N-carboxymethyl chitosan oligosaccharides with different degrees of substitution (DS 0.28, 0.41, and 0.54) were evaluated by the scavenging of DPPH radical, superoxide anion and the determination of reducing power. With the increase in substituting degree, the scavenging activity of N-CMCS against DPPH radical decreased and reducing power increased. As for superoxide anion scavenging, the N-CMCS of 0.41 DS is higher in activity than those of 0.54 and 0.28 DS. The unrelated results obtained may interrelate to the different radical-scavenging mechanisms and donating effect of substituting carboxymethyl group [16]. In another study by Yang et al. [66], the chitosan, hyaluronan and starch-carboxymethylated derivatives showed lower scavenging ability on hydroxyl radicals than the parent compounds. Decrease in scavenging ability of the carboxymethylated polysaccharides could be rationalized by the fact that part of the hydroxyl groups was substituted by carboxyl groups in the molecules. For the three kinds of polysaccharides, scavenging ability on hydroxyl radicals was found to be in the order of chitosan > hyaluronan > starch. The scavenging ability of carboxymethylated polysaccharides had the same order as related to its corresponding polysaccharides at higher concentrations (≥0.8 mg mL−1). There were not only hydroxyl groups but also amino or acetamino (CH3CONH–) groups in the molecules of chitosan and hyaluronan, but only hydroxyl group for starch. Thus, it was suggested that the influence of the scavenging activity against hydroxyl radicals might be in the sequence of amino group > acetamide group > hydroxyl group. However, in another work reported by Zhao et al. [67], N,O-CMCS from squid cartilage showed a better antioxidant than chitosan, especially in terms of its reducing power, scavenging ability towards DPPH and superoxide radicals, and chelating ability of ferrous ions. The data on the antioxidant capacities obtained may be due to the type of β-chitin in squid cartilage, which possessed weak intermolecular forces and exhibited higher reactivity under various modification conditions as well as higher affinity for solvents than α-chitin. The introduction of hydrophilic carboxymethyl groups on squid cartilage chitosan decreased the intramolecular and intermolecular hydrogen bonds, resulting in the exposure of more hydroxyl groups, thus the scavenging abilities of N,O-CMCS are better than that of chitosan. It is worth mentioning at this point again that the antioxidant activities of antioxidant compounds are attributed to various mechanisms, among which are prevention of chain initiation, binding of transition metal ion catalysts, decomposition of peroxides, reductive capacity and radical scavenging. The data on activities of chitosan and carboxymethyl chitosan suggested that these various mechanisms likely to significantly contribute towards the observed antioxidant effects. Oligochitosan and N,O-CMCS also have been reported on inhibiting 50% of ABTS radical formation. Oligochitosans displayed IC50 values of 3.5 to 4.6 mg mL−1, whereas N,O-CMCS is a more effective free radical scavenger as it possessed the IC50 value of 0.98 ± 0.07 mg mL−1 [68]. On the basis of the data obtained from all the mentioned works, it may be reasonable to claim that the antioxidant activities of water-soluble chitosan derivatives are medium and would be rather useful option to expand their applications in foods, pharmaceuticals and cosmetics [59, 64, 67, 69].

Application of carboxymethyl chitosan in delivery system

Lately, chitosans and their derivatives have been widely investigated for their use as delivery system for pharmaceuticals and cosmetics [70-73]. The use of chitosans for the encapsulation of active components has gained interest due to their mucous adhesiveness, non-toxicity, biocompatibility and biodegradability. The benefits of encapsulating active agents in a polymer matrix include their protection from the surrounding medium or processing conditions and their controlled release. Chitosan nanoparticles and microspheres were prepared by ionic gelation of chitosan hydrochloride and sodium tripolyphosphate for the encapsulation of an antioxidant, yerba mate extracts [74]. Spray-drying method was used to prepare microspheres and the resulting products allowed controlling the release of natural antioxidants, thus it is a promising system for nutraceutical and cosmetic applications. It has been earlier mentioned by Zhu et al. [26] that O-CMCS demonstrated aggregation behaviour in dilute solution. The glucose backbones of O-CMCS form the hydrophobic domains, and the dissociated carboxylic groups as well as the hydrophilic groups around the backbone engage the hydrophilic ones. This structure of O-CMCS aggregate provides an ideal system to load the hydrophobic active substances. Furthermore, Zhu et al. [23] have detailed that the aggregation behaviour was strongly affected by the salt and its valence. In a saltfree aqueous solution, O-CMCS aggregates are formed at concentrations above critical aggregation concentration (cac, ~0.042 mg mL−1) and show a swollen microgel-like structure. The cac value of O-CMCS showed no obvious change in different salt solutions as compared with that in the saltfree solution. However, the relative viscosity of the O-CMCS salt solutions is lower than that of the saltfree solution. The decrease in viscosity is more pronounced in the system containing CaCl2 than in the one with NaCl, and it implies that the smaller and more compact aggregates of CMCS are formed in the CMCS/CaCl2 system [23]. Moreover, the study demonstrated that in the presence of NaCl, the aggregate size increases with NaCl concentration, and the driving forces for inducing O-CMCS aggregation are intermolecular hydrogen bond, hydrophobic interaction and electronic repulsion. However, the effect of Ca2+ or Cr3+ on the CMCS aggregation is different. At low concentrations of Ca2+ or Cr3+, the aggregates are more compact in structure than that in a saltfree solution, which due to the interaction between metal cations and the -COO or amino groups of CMCS within an aggregate (intra-aggregate). On the contrary, at higher concentration of Ca2+ or Cr3+, the interaggregates are dominated by the interaction between cations and the -COO or amino groups of CMCS from different primary aggregates; thus, different types of aggregation complex can form by varying either O-CMCS or Ca2+ (or Cr3+) concentration. The intra-aggregate complexation makes the formation of small, compact and spherical aggregate, whereas the interaggregate complexation makes the aggregate big, compact and spherical. The effect of Cr3+ is much stronger than that of Ca2+ [75]. All these properties are important for developing O-CMCS aggregates as drug and cosmetic delivery systems. One motivating example for this application can be seen in the aggregates of O-CMCS containing a well-known anti-cancer drug, camptothecin [23]. The study demonstrated that not only the aggregates but also the unimers of O-CMCS can help to enhance the solubility of camptothecin. After camptothecin is loaded in O-CMCS, the release of camptothecin is significantly sustained due to the interactions between O-CMCS and lipophilic camptothecin. The result of O-CMCS unimers showing as good drug-loading and controlled-release capability as its aggregates indicates that this novel release system has a great potential application in pharmaceuticals and may be extended in the field of cosmetics as well. The ability to form visco-elastic gels of N-carboxymethylated chitosan in aqueous environments or with anionic macromolecules at neutral pH values is also another important property for the application as it was found to help increase the permeation and absorption of low MW heparin; an anionic polysaccharide across intestinal epithelia without provoking any damage of the cell membrane [27]. Another work studied by Chen et al. [21] showed that hydrogels base of CMCSs of various degree of deacetylation (DD) and substitution (DS) possessed amphoteric character responding to pH of the external medium. At the isoelectric point (IEP), the hydrogel shrunk most; when the pH deviated from IEP, the swelling degree increased. In addition, chitosans and their derivatives can also be used to prepare the nanoparticles delivery system as reported by Sayin et al. [76]. The study demonstrated that the negatively charged mono-N-CMCS nanoparticles loaded with tetanus toxoid antigen were found to enhance mucosal immune responses. Mono-N-CMCS nanoparticles induced relatively lower immune responses for the antigen when compared with the negatively charged N-trimethyl chitosan nanoparticles, yet it produced the smallest nanoparticles with much narrower size distribution and high loading capacity. With these desired features, mono-N-CMCS nanoparticles can be suggested as promising delivery systems for diverse range of drugs as well as a gene/protein delivery. In another recent study by Ji et al. [77], novel composite nanocarriers comprised of O-CMCS/β-cyclodextrin nanoparticles were prepared and used to improve the bioavailability of hydrophobic drugs. The nanoparticles were spherical in shape with average particle sizes of 166 nm. It was suggested that O-CMCS/β-cyclodextrin nanoparticles were more suitable for the oral delivery of hydrophobic drugs compared with the chitosan/β-cyclodextrin nanoparticles. In addition, the conjugates of O-CMCS with methotrexate were synthesized and found to possess amphiphilic properties and self-aggregation characteristics. Drug releases of O-CMCS-methotrexate nanoparticles were studied, and methotrexate exhibited significant sustained release behaviours in PBS buffer solutions, indicating that these nanoparticles had good in vitro stability and the potential to be used as a novel drug-carrier system [78]. Curcumin loaded O-CMCS nanoparticles are also reported with a mean diameter of about 120–180 nm with spherical shape. Curcumin was entrapped in O-CMCS nanoparticles with an efficiency of 87%, and in vitro drug release profile studies indicated that the O-CMCS nanoparticles are promising nanomatrix for hydrophobic substances delivery system [79]. Another interesting system is calcium carbonate/carboxymethyl chitosan (CaCO3/CMCS) hybrid microspheres and nanospheres, prepared by the precipitation of CaCO3 in an aqueous solution containing CMCS and loaded with anti-cancer drug, doxorubicin hydrochloride, with high encapsulation efficiency. The in vitro release showed that the release of the drug from the microspheres and nanospheres could be effectively sustained [80]. It can be seen that all the properties of carboxymethyl chitosan, which are the abilities to form aggregates, hydrogels, micro- and nanospheres, and nanoparticles, have made carboxymethyl chitosan as a great entrant for the further investigation of its prospect as delivery system for cosmetics.

Carboxymethyl chitosan as an emulsion stabilizer

Chitosans and their derivatives still have numbers of potential use in cosmetics especially water-soluble chitosan derivatives. Chitosan can be used as an emulsion stabilizer as can be seen from work reported by Speiciene et al. [81]. The studies showed that the presence of chitosan in a continuous phase during the formation of oil-in-water emulsions leads to an increase in stability of acidic (pH 3) oil-in-water emulsions containing whey protein isolates. This effect has been confirmed by the increased viscosity, the creaming stability, and the smaller droplet size formation in emulsions. The phenomenon of depleted flocculation can explain this behaviour of acidic model emulsions containing whey protein isolates and chitosans. Another study by Mun et al. [82] also proved that chitosans have great potential application in preparing emulsion stabilized by multiple interfacial layers. CMCS is an amphiprotic ether derivative as the molecule contains hydrophobic glucose backbone and hydrophilic carboxylic group, and it showed aggregation in dilute solution. These are of ideal properties for use as stabilizer in emulsion preparation. Recently, Tzoumaki et al. [83] measured the surface tension of chitin nanocrystals’ aqueous dispersions of various concentrations to further understand the ability of the nanocrystals on stabilization of the oil droplets, and it was found that the values are close to some proteins that usually stabilize emulsions and indicate that chitin nanocrystals are in fact surface-active nanoparticles. Nanoparticles of chitosan and its derivatives also possessed surface-active properties that make them able to form micelles and aggregates [84]. The resulting aggregates may have enormous importance in the stabilization of Pickering emulsions.

The safety issue of carboxymethyl chitosan

  1. Top of page
  2. SynopsisRésumé
  3. Introduction
  4. Preparations and properties of carboxymethyl chitosan
  5. Use of carboxymethyl chitosan in cosmetics
  6. The safety issue of carboxymethyl chitosan
  7. Conclusion
  8. Acknowledgements
  9. References

Almost every substance that placed on human skin has the potential to produce physiological changes in the skin [85]. Even with the outlook of natural basis, biodegradability and biocompatibility, it is still important to concern about safety when utilizing CMCS as cosmetic ingredient. Muzzarelli had earlier stated that no abnormality was observed on human patch tests performed with the liquid undiluted preparation of N-CMCS [11]. Along with the intense research on the usefulness of chitosan derivatives in the last ten years, the safety issues are regularly included in the study to ensure their safety, as will be noticed in the following published works. The water-soluble chitosan derivatives and chitosan nanoparticles were reported to possess high-cytotoxicity activity towards tumour cells, whereas low toxicity against normal human cells [86, 87]. Recent paper reviewed by Baldrick [88] suggested that chitosans have the potential to be safe pharmaceutical excipient for non-parenteral, non-blood contact use as shown by publicly available data. In another work, cytotoxicity studied by MTT assay indicated that O-CMCS was safe both on normal cell L02 and three tumour cell lines: Bel-7402, SGC-7901 and Hela in vitro. CMCS also improved the TGF-α secretion of L02 cell, whereas decreased levels of TGF-α and VEGF secreted by Bel- 7402 cell, which are compatible with the observations at cell levels. In vivo, transplant tumour model of sarcoma 180 was established in mice and O-CMCS was administered through intraperitoneal injection. Experimental data indicated that O-CMCS was safe in vivo and slightly inhibited growth of sarcoma 180 and enhanced body immunity via the elevation of serum IL-2 and TNF-α level in treated mice. These results suggest that O-CMCS is safe in tumour application as biomedical material [89]. Moreover, biocompatibility of N-CMCS and its acetylated derivative has been studied on L929 mouse fibroblasts cell by MTT assay, and the results indicated that cell viability was not affected by contact with high concentration of biopolymers, indicating their biocompatible properties [14]. In addition, cytotoxicity of the chitosan, O-CMCS and N,O-CMCS nanoparticles was determined by MTT assay using breast cancer MCF-7 cells. The results showed that compared with the negative control (normal tissue-cultured wells) almost 98% cells were viable in all types of nanoparticles, which indicated that the prepared nanoparticles are low toxic [48]. In another study, cytotoxicity testing of low-molecular O-CMCS (5, 10, 15, 20, and 25%) was determined by performing an MTT assay with 3T3-L1 cells fibroblasts, and the survival ratios of cells incubated with each solution relative to the control were 100. These results proved that low-molecular O-CMCS was non-cytotoxic [90]. Furthermore, the cytotoxicity of oligochitosan, N,O-CMCS and N-CMCS derivatives in sheet-like and paste forms were evaluated using primary normal human dermal fibroblast cultures and hypertrophic scars, a fibrotic condition representing a model of altered wound healing with overproduction of extracellular matrix and fibroblast hyperproliferative activity. Cytotoxicities of these chitosan derivatives were assessed using MTT assay, and the results indicated that both chitosan derivative sheets and pastes have appropriate cytocompatibility and appear promising as safe biomaterials with potential wound-healing applications [91].

Conclusion

  1. Top of page
  2. SynopsisRésumé
  3. Introduction
  4. Preparations and properties of carboxymethyl chitosan
  5. Use of carboxymethyl chitosan in cosmetics
  6. The safety issue of carboxymethyl chitosan
  7. Conclusion
  8. Acknowledgements
  9. References

This review summarizes the water-soluble properties of carboxymethyl chitosan. The recent research on carboxymethyl chitosan with potential application in cosmetics has been emphasized. Different aspects of carboxymethyl chitosan have been discussed into five major directions: moisture-retention agent, anti-microbial agent, antioxidant agent, delivery system and natural-derived emulsion stabilizer. To finish, cytotoxicity information of carboxymethyl chitosan is included in the review to exclusively ensure its safety. It is optimistically expected that the contents will provide conclusive insights on the utilization of this biocompatibility, biodegradation, biological activity and low-toxicity carboxymethyl chitosan in cosmetic industry and hope to boost up its application as multifunctional cosmetic ingredient.

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  2. SynopsisRésumé
  3. Introduction
  4. Preparations and properties of carboxymethyl chitosan
  5. Use of carboxymethyl chitosan in cosmetics
  6. The safety issue of carboxymethyl chitosan
  7. Conclusion
  8. Acknowledgements
  9. References
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