Comprehensive review in moisture retention mechanism of polysaccharides from algae, plants, bacteria and fungus

3.1 Moisturizing activity of polysaccharides from algae

As a biomass-rich and inexpensive resource, algae are generally divided into multicellular macroalgae and unicellular microalgae (microalgae). Macroalgae in the ocean include green algae, brown algae, and red algae, while microalgae include all ecosystems and phytoplankton found in ocean water (Ariede et al., 2017). Algae are often left on shore to decompose, causing waste problems. However, algae have fast growth rate, high photosynthetic efficiency, high CO₂ fixation potential and low lignin ratio. They are excellent renewable resources for production of polymers and other products (Noreen et al., 2016). Seaweed polysaccharides, including cell wall polysaccharides and mucopolysaccharides (Murata & Nakazoe, 2001), are the research focus because of their high yields, sustainability and special chemical components. In particular, they contain abundant sulfurized polysaccharides (SPS), which have antioxidant, antiviral, and blood coagulation effects (Jiao et al., 2011), and are widely used in food, cosmetics, biotechnology, and other fields (Renn, 1997).

Some algal polysaccharides have good moisturizing effects (Table 1). From the sources of moisturizing polysaccharides, there are mainly green algae and brown algae as well as some freshwater algae and microalgae. The moisture absorption of various algae polysaccharides is slower than that of glycerin, but they have better moisture retention, especially marine green algae. In vitro moisturizing assays show the moisturizing rate of polysaccharides in E. prolifera (Li et al., 2017) can reach about 98 % (25 °C, 10 % RH, 96 h), which is closely related to the moisturizing component HA, and the polysaccharide in Enteromorpha linza (Wang et al.,2014). The moisture retention rate was 49.3 % (25 °C,43 % RH,96 h), which was slightly better than that of chitosan. As for brown algae, fucose was mostly present in its monosaccharide composition (Table 1) (Ning et al.,2020). As a component of glycoprotein, fucose widely exists in the plasma membranes of various cells. In addition to a certain moisturizing effect, it also has anti-aging effects, such as increasing skin thickness, accelerating skin tissue regeneration, reducing wrinkles and increasing skin elasticity by applying fucoidin on the skin. The polysacchariedes from Sargassum horneri (Shao et al., 2015), Sargassum vachellianum (Jesumani et al., 2020), Sargassum hemiphyllum (Wang et al., 2013) belonging to brown algae, have moisturizing retention rate over 94 % (25 °C, 43 % RH, 48 h). Reportedly, polysaccharide extraction was performed on three different brown algae of Sargassum and the moisturization rate was determined to be around 65 % (20 °C, 43 % RH,72 h) under the same conditions (Jesumani et al., 2019). In the sulfated polysaccharides obtained from red algae, porphyra, these polysaccharides can improve the maturation of the keratinized envelope of the stratum corneum and enhance the dermal-epidermal junction. Therefore, algal polysaccharides may be a good choice to improve dry or aging facial skin features and prolong moisturization when applied topically. Some polysaccharides produced by microalgae, which are used to produce high-value active substances (Nicoletti, 2016), have better moisturizing properties. For instance, the moisturizing rates of polysaccharides extracted and isolated from Sarcinochrysis marina Geitler (Wang et al., 2012), and Nostoc commune (Li et al., 2011) can reach 60 %-70 %. Currently, algal polysaccharides were more used as cosmetic moisturizing ingredients, such as sheep sorrel polysaccharide serum, and cosmetic patch compounded with seaweed polysaccharides as functional ingredients that are effective on skin moisturizing (Byeon et al., 2017; Yang et al., 2017).

3.2 Moisturizing activity of polysaccharides from plants

Plant polysaccharides are derived from the flowers, stems, leaves, and fruits of plants. The polysaccharides in natural plant products have anti-oxidation, whitening, and anti-aging effects as well as good moisturizing activity, and are widely used in cosmetics (Feng et al., 2021; Li et al., 2012; Liao et al., 2017; Liu et al., 2018). Generally, products containing plant polysaccharides can relieve dryness and provide a soothing film covering human skin (Femenia, 2007). Even at low concentrations, these products can improve the moisturizing effect of the skin and reduce water loss (Damasceno et al., 2016).

Among plants, Aloe vera extract is widely used as a natural raw material in the cosmetic field, where polysaccharides as the main active ingredients are mainly composed of monosaccharides, such as glucose and mannose polymers (Nema, 2012), and are representative of plant-derived moisturizing polysaccharides with excellent moisturizing activity and some whitening and antioxidant properties. Dal'Belo et al. (2010) applied freeze-dried aloe extracts containing polysaccharides at different concentrations in cosmetic formulations to explore their effects on skin hydration. Aloe extract containing polysaccharides is a natural effective ingredient that improves skin hydration, and can be used in moisturizing cosmetic formulations and as a supplement for the treatment of dry skin. Active polysaccharide from Aloe vera has a significant moisturizing effect and strong skin penetration and high security on the human skin (Ren et al., 2012). In addition, the moisturizing effect of aloe polysaccharides is endogenous to certain extent. Aloe polysaccharides have the effect of moisture absorption and moisturizing on the surface of the skin, and make human fibroblasts proliferate actively, promoting the expression and synthesis of type I and type III collagens, hyaluronic acid and hydroxyproline. The secretion of hyaluronic acid and hydroxyproline can nourish and protect the skin, and improve the dry skin from the inside (Liu et al., 2010).

Some other polysaccharides from different parts of other plants have been gradually discovered in recent years to explore the feasibility and utilization as moisturizing components, and some of them have better moisturizing effects. Polysaccharide fractions obtained from Eragrostis Teff show a moisture retention rate of 103.29 % (20 °C, 81 %RH, 24 h) (Table 2), which is significantly higher than those of glycerol, polyethylene glycol 400, and chitosan (Zhu & Sun, 2013). The four polysaccharide fractions obtained from onion show less variation in moisturization rate, which are all-around 55 % (20 °C, 48 h) (Zhu et al., 2017). Some polysaccharides from plant petals, such as RRPS-2 isolated from Rosa rugosa petal (Zhang et al., 2019), and NHP-2 extracted and purified from Nymphaea hybrid (Cong, 2018), show both outstanding moisturizing properties and excellent antioxidant activity. In addition, polysaccharides extracted from the fruits of Litchi chinensis (Jing et al., 2016), Blackcurrant (Xu et al., 2021), although not outstanding in moisturizing properties, have great potential for application because of their antioxidant and hypoglycemic effects.

The water extracts of polysaccharides from Dalbergia sissoo, Tectona grandis and Mimosa diplotricha show effective moisture retention properties in comparison with hyaluronic acid and glycerol. The polysaccharides from Dalbergia sissoo in all antioxidant assays are more active than the other two polysaccharides (Rana et al., 2014). The polysaccharides from Mentha haplocalyx (MHP) by a high-pressure ultrasound-assisted extraction method exhibits substantial hygroscopicity and a relatively strong moisture retention capacity and may be used as a supplement in cosmetics, although the hygroscopicity and moisture retention of MHP (28.11 % and 35.14 %) are lower than glycerin (57.96 % and 41.97 %) (Chen et al., 2019). The protective effects of green tea polysaccharides (TPS) and polyphenols (TPP) on skin were investigated (Wei et al., 2009). TPS and TPP have good moisture absorption and water retention properties. TPS can hardly protect the skin from solar ultraviolet (UV), but TPP can. TPP and TPS have certain complementary advantages, and can be used in combination as cosmetic active ingredients. Besides, the properties of the moisture retention and moisture absorption of polysaccharides from Acanthopanax gracilistylus were studied and compared with common humectants in relative humidity 43 %and 75 %. Results show the polysaccharides of AGSL-P-2 are excellent moisturizers (Liu et al., 2015).

3.3 Moisturizing activity of polysaccharides from bacteria and fungus

Extracellular polysaccharides gradually become effective alternatives to polysaccharide products from plants and animals owing to their non-toxicity, safety, short production cycle, easy isolation and purification, excellent physical and chemical properties and bioactivity, and the demand for EPSs is increasing (Liu et al., 2016; Reitas et al., 2011). For some fungal polysaccharides, research on moisturizing is less, and polysaccharides from Tremella fuciformis are the most studied.

Microbial polysaccharides have great potential in cosmetics, pharmaceuticals, and biomedicine because they meet the required degree of purity and certain specific functional properties compared to traditional polymers (Liu et al., 2016). Microorganisms are more suitable for polysaccharide production than plants or algae because of their faster growth rate and easier technical manipulation (Raza et al., 2012). Microbial polysaccharides are divided into extracellular polysaccharides, cell wall polysaccharides, and intracellular polysaccharides (Zhu & Tong, 2012). Common microbial polysaccharides such as Xanthan gum, Gellan gum, and Levan are often used as gelling agents, flocculants, and film-forming agents in many fields, such as food and pharmaceuticals because of their water solubility, low viscosity, and skin protection and moisturizing properties (Freitas et al., 2015).

The polysaccharides with good moisturizing effect obtained from microorganisms are all extracellular polysaccharides, which are easily separated from bacteria and secreted outside of cells during the growth and metabolism of microorganisms (Table 3). Microbial extracellular polysaccharides are essential for biofilm formation and are involved in bacteria protection from harmful environments and adhesion. High viscosity is formed by the accumulation of various polymeric substances in EPS (exopolysaccharides) layers. They tend to be hygroscopic and aerobic bacteria and usually contain more water than the surrounding environment (Helm et al., 2000). The moisturizing retention of EPS plays an important role in cosmetics and clinical medicine. In terms of the in vitro moisturizing effect, all these reported polysaccharides have excellent moisturizing properties. The moisturizing rate of Lachnum YM262 is up to 79.6 % (25 °C, RH43 %, 72 h) (Chen et al., 2017). The extracellular polysaccharide obtained from Zunongwangia profunda SM-A87 outperforms sodium alginate (25 °C, RH81 %,70 h) (Sun et al., 2014), P. fluorescens PGM37 extracellular polysaccharides are close to hyaluronic acid (Zhao et al., 2013). Interestingly, some bacterial strains produce extracellular polysaccharides with moisturizing effects even better than hyaluronic acid. The moisturizing rates of extracellular polysaccharides produced by Phyllobacterium sp.921f (Liet al., 2017a) and Polaribacter sp.sm1127 (Sun et al., 2015) are all about 93.4 % (25 °C, 43 % relative humidity), which is higher than that of hyaluronic acid. Similarly, the extracellular polysaccharide from Rhodococcus SM-1strain (Urai et al., 2002) is more hygroscopic than glycerol (a strong hygroscopic polyol), and is better than hyaluronic acid in moisturizing performance.

Some fungal polysaccharides also have good moisturizing activity. The moisturizing activity of fungal polysaccharides is mainly concentrated in Tremella (Chen, 2017), Tremella fuciformis (Wang et al., 2019; Xu et al., 2020), Pholiota nameko (Chou et al., 2019), and Cordyceps sinensis (Chen et al., 2014). The polysaccharides from Tremella fuciformis and its fruiting bodies show good moisturizing activity.

4.1 Effect of molecular weight of polysaccharides on moisturizing activity

Based on Tables 1 to 3, the molecular weights of polysaccharides obtained from algae, plants, and microorganisms are 5.54 × 10³ –3.64 × 10⁶ Da, 6.366 × 10³ –7.702 × 10⁶ Da, and 4.3 × 10³ –6.35 × 10⁶ (mostly 10⁵ –3 × 10⁵) Da respectively. This is in agreement with another report (Sandford & Laskin, 1977). As for the separation and purification of the above polysaccharides, crude polysaccharides extracted from algae or plants often yield multiple fractions after pure characterization, and the molecular weights of the polysaccharides were further reduced by the impurity removal after purification, whereas microbial polysaccharides often exhibit single components after purification.

However, the moisture retention of polysaccharides extracted by different methods from Codium fragil (Song, 2010), onion (Zhu et al., 2017), Bryopsis Plumosa (Song, 2010), and Tremella fuciformisonion (Wang et al., 2019) is not significantly related to the molecular weight. This may be because the influence of the extraction method on the structure of polysaccharide results in great differences in the chemical composition and physical properties.

On the one hand, studies confirm that polysaccharides with larger molecular weights have better moisturizing effect. For the polysaccharide fractions obtained from the isolated and purified from papaya (Zhang et al., 2012) and Pholiota nameko (Chou et al., 2019) by graded alcoholic precipitation, their moisturizing and hygroscopic properties were further enhanced with the increase of molecular weight. On the other hand, other studies confirm that the degradation of large-molecular-weight polysaccharides into small-molecular-weight polysaccharides helps improve the moisturizing activity. Noticeably, the bioactivity of Ganoderma lucidum polysaccharides can usually be improved by reducing the molecular weight of polysaccharides through degradation (Xuet al., 2019). The crude polysaccharides extracted from Sargassum horneri (Shao et al., 2015), Rosa rugosa petals (Zhang et al., 2019), or Blackcurrant (Xu et al., 2021) were purified and degraded to small-molecular-weight polysaccharides, which significantly increased moisture retention and absorption.

The method of polysaccharide degradation may also affect moisturizing activity. Unlike acid digestion, H₂O₂ oxidative degradation, ultrasonic degradation, and enzymatic digestion have high efficiency and mild reaction conditions. Especially, ultrasound-assisted oxidative degradation can reduce the molecular weight of polysaccharides without changing their primary structure (Huang et al., 2020). The crude polysaccharides extracted from Sarcinochrysis marina Geitle (Wang et al., 2012), or Pavlova viridi (Wang & Sun, 2012) were degraded using an ultrasound-assisted H₂O₂-VC system. The moisture retention effect of the polysaccharide fractions of Ulva pertusa after H₂O₂ degradation was improved with an increase in the molecular weight. However, after degradation, the hygroscopicity of Tremella polysaccharides (Chen, 2017) increases with the decrease of molecular weight. Moreover, Saccharina japonica (Wang et al., 2013), Porphyra haitanensis (Wang et al., 2013), and E. prolifera (Li et al., 2017) all have better moisture absorption and moisturizing properties of polysaccharides after degradation with decreasing molecular weight. Huang et al. (2014) separated polysaccharides from Gracilaria verrucosa into three fractions: less than 10⁴ Da, 10⁴ –10⁵ Da, and more than 10⁵ Da by using radial flow coupled ultrafiltration. The polysaccharide fractions below 10⁴ Da were improved in terms of preventing desorption and moisture absorption. Qin et al. (2002) found the hygroscopic activity of modified chitosan increased with decreasing molecular weight and increased rapidly when the molecular weight varied from 1.0 × 10⁴ to 0.4 × 10⁴ Da.

When the polysaccharides are in a certain molecular weight range, the moisturizing activity can be significantly displayed. As the polysaccharide molecules are a linear long-chain structure, the larger relative molecular weight of polysaccharides within a certain range indicates the presence of more polar groups. The formation of more hydrogen bonds makes it easier to form a network structure and avoids water loss, so the moisture retention is better. Research on the antioxidant activity of related polysaccharides confirms that the degraded polysaccharides have better water solubility. The smaller molecular weight makes it easier to expose the active groups and trap free radicals. Hence, the contact area between active sites and free radicals is larger(Ma et al., 2005; Zhao et al., 2006).

Therefore, the above results indicate that the molecular weight of polysaccharides does not regularly change with the moisturizing activity. However, it can also be inferred that the reduction of polysaccharide molecular weight may lead to a loose structure, enlarge the contact area between water molecules and polysaccharides, and increase the exposure opportunity of hydrophilic groups (such as carboxyl and hydroxyl). The hydrophilic groups on these newly-added sites easily associate with water molecules in the surrounding environment to form hydrogen bonds, which strengthen the binding of water molecules and improve their moisture retention. Meanwhile, the molecular chain becomes shorter and a more complex three-dimensional network structure is gradually formed. When the water molecules enter, the water molecules become less movable due to the hindrance of the grid structure, and can hardly escape. As a result, the moisturizing effect is further improved. The above research indicates that most polysaccharides with a molecular weight of about 10⁴ Da show good moisture retention. Nevertheless, if the molecular weight of the polysaccharides is too large, the water solubility is poor, which often limits their biological effects. If the molecular weight of the polysaccharides is too small, the network structure may be destroyed, and no active polymerization structure space can be formed, weakening the inhibitory effect on water and significantly reducing the moisture retention.

4.2 Effect of monosaccharide composition on moisturizing activity

The different structures of polysaccharides may be caused by different monosaccharide compositions, and are important reasons for the differences in the biological activity of polysaccharides (Yi et al., 2017). Among the polysaccharides from different sources, the monosaccharide composition of galactose, mannose, glucose appears more frequently (Table 1-3). Algae and plant polysaccharides contain many types of monosaccharides. On the contrary, the types of monosaccharides contained in bacteria and fungus are relatively single, mainly glucose, galactose, mannose, and some neutral monosaccharides such as rhamnose. This result is consistent with related literature (Freitas et al., 2021). For algae polysaccharides, studies on the monosaccharide composition confirm that the frequency of rhamnose is higher. For brown algae, there is more fucose in monosaccharide composition (Table 1). In addition to a certain moisturizing effect, fucose also has anti-aging effects, such as increasing skin thickness and accelerating skin tissue regeneration. Polysaccharides are affected not only by the composition of monosaccharides but also by various other factors. Only certain speculations are made about the relationship between the moisturizing activity of polysaccharides and the composition of monosaccharides. Some microbial polysaccharides are even mainly composed of two monosaccharides. In particular, glucose occurs most frequently, which may be the underlying reason for its moisturizing effect.

4.3 Effect of functional groups of polysaccharides on moisturizing activity

The activity of polysaccharides is not only affected by molecular weight and monosaccharide composition, but also is closely related to functional groups such as carboxyl, hydroxyl, amino, ethyl, acetyl, and sulfate groups (Ramesh & Tharanathan, 2003). Similarly, the reason why some polysaccharides have strong moisturizing ability may be due to the strong interaction between water molecules and the hydrophilic groups of polysaccharides (Chen et al., 2003, 2017).That is, polysaccharides contain polar groups such as polyhydroxy and carboxyl groups, which endow them strong electronegativity (Chou et al., 2019), allowing the polysaccharides to form hydrogen bonds with water. For example, HA is widely used as a humectant component in biochemical products and cosmetics, because the carboxyl and acetamide groups on the HA polysaccharide chain may attract water molecules through intermolecular hydrogen bonds (Zhang, et al., 2013). Another reason is that its relatively high viscosity (Li et al., 2018) is conducive to the moisturizing of polysaccharides (Chen et al., 2017; Helm et al., 2000).

The existence of some special peripheral groups may also help adjust the rheological properties of polysaccharides, thereby affecting their water retention capacity and application potential. Sulfate content is very positively correlated with the antioxidant activity of sulfated polysaccharides(Cui, Liu, Li, Hao, & Lu, 2018). The moisturizing activity of polysaccharides is also closely related to sulfate content of polysaccharides. Besides sulfated polysaccharides from plants with good moisture retention, sulfate components are also found in microbial polysaccharides, especially algae polysaccharides that generally contain sulfate groups(Percival, 1979). The sulfated polysaccharide extracted in Ulva fasciata has higher viscosity and hydroxyl content, and sulfation can improve water solubility, which positively affects its hygroscopicity and moisture retention (Shao et al., 2015). Research on the polysaccharide components extracted from E.prolifera (Li et al., 2017), Sargassum vachellianum (Jesumani et al., 2020), and Sargassum (Jesumani et al., 2019) confirms that the moisture retention of polysaccharides slightly rises with the increase of sulfate content. When investigating the relationship of water absorption and retention capacity with the content of sulfate groups, Wang et al. (2013) found that the sulfate contents of the five algae polysaccharides had inconsistent effects on the moisture absorption capacity. The sample with the highest sulfuric acid content exhibited the highest moisture absorption capacity, while the sample with the second-highest sulfuric acid content exhibited extremely low moisture absorption capacity. As the sulfate content decreased, the moisturizing retention of polysaccharides was weakened. This result may be affected by factors such as the molecular weight of polysaccharides and the complexity of monosaccharide structure. Because the chemical composition of the sulfated polysaccharides of the seaweed family is specific, and the degree and distribution of sulfate groups are different, it may also have a certain relationship with the algae species.

The glyoxylate group significantly influences the physiological, biological, and pharmacological properties of polysaccharides. This is because changes in the basic structure of biopolymers alter their physical and chemical properties, resulting in characteristic bioactive and solubility behavior (Chen et al., 2004). Reportedly, polysaccharides with a larger proportion of glyoxalate are mostly negatively charged, leading to a smaller spatial site resistance that allows a simple way to react with free radicals. Therefore, polysaccharides with a higher content of aldehyde acids have better free radical scavenging activity (Ye et al., 2012). Some recent reports also suggest a link between the glyoxalic acid contained in polysaccharides and the moisturizing activity of polysaccharides. Studies on polysaccharides extracted from Onion (Zhu et al., 2017), Tremella fuciform (Wang et al., 2015), Rosa rugosa petals (Zhang et al., 2019), and Blackcurrant (Xu et al., 2021) found an increase in water retention with increasing glyoxylate content, which may be because the presence of glyoxylate can alter the rheological properties of polysaccharides by imparting charge and interfering with the conformation. Among polysaccharide molecules, uronic acid can also form intermolecular hydrogen bonds with other polar groups on the polysaccharide chain, which further strengthens the network structure and facilitates water preservation (Chou et al., 2019; Xu et al., 2020).

In addition to the above sulfuric acid and uronic acid groups, some microbial polysaccharides also contain certain amounts of pyruvate groups (Freitas et al., 2021). The presence of pyruvate substituents in some bacterial extracellular polysaccharides can greatly change their properties in solutions by imparting charges and interfering with confirmation(Cesàro et al., 1992). Li et al.(2017a) analyzed the structure and moisturizing activity of the extracellular polysaccharide produced by Phyllobacterium sp. 921F, and speculated that the pyruvate substituent can enhance its moisturizing properties.

4.4 Effect of modificationof polysaccharides on moisturizing activity

Although natural polysaccharides exhibit strong bioactivity, the existence of various functional groups provides a broad space for chemical structure modification. In addition, modification is of great significance for studies on the relationship between polysaccharide structure and activity. Therefore, in-depth study of structural modification methods and mechanisms can promote the application of polysaccharides (Lee et al., 2010; Liu et al., 2012; Silva et al., 2004). In fact, there are many studies on sulfated, acetylated, glyoxylated, and carboxymethylated derivatives of polysaccharides. Most studies demosntrate that the moisturizing effect of polysaccharides plays a significant role through the modification of polysaccharides.

The moisture absorption and moisturizing retention of polysaccharides are generally enhanced after modification, and are generally superior over the traditional moisturizing component HA, achieving significant effects (Table 4). For example, extracellular polysaccharides of Phomopsisliquidambari NJUSTb1(Liu et al., 2021) outperform unmodified polysaccharides in terms of moisture absorption and moisturizing ability after sulfation modification. When the polysaccharides obtained from Actinidia chinensis roots were subjected to a SO₃-pyridine procedure to prepare sulfated polysaccharides with different degrees of substitution, the moisturizing retention increased with the rise of substitution (Lin et al.,2014). This may be because sulfation of polysaccharides can increase their solubility (Shao et al., 2015) and change the conformation of the chains, which leads to changes in their bioactivity. Yang et al. (2009) prepared hyaluronic acid quaternary ammonium salt (QHA), starch quaternary ammonium salt (QS) and chitosan quaternary ammonium salt (QCTS), and measured their moisturizing activities. The moisture absorption of QHA was superior over HA at both 43 % and 81 % RH. The same trend was observed for QS and QCTS. In addition, the moisturizing retention of all three quaternate polysaccharides was better than the corresponding polysaccharides at 81 % RH, 43 % RH and dry conditions. The N⁺(CH₃)₃ critically affects the moisture absorption and moisturizing retention of quaternate polysaccharides. QCTS and QHA have comparable to or better moisture absorption and moisturization than HA and thus can be used as moisturizing ingredients in cosmetics. There are also modifications of polysaccharides such as carboxymethylation and acetylation (Chen et al., 2021). The transverse order and orientation of polysaccharide molecules are changed by introducing acetyl groups, exposing polar groups such as hydroxyl and carboxyl groups, which improve the water solubility of polysaccharides. The introduction of carboxymethyl improve the electronegativity and water solubility of polysaccharides, which in turn enhances the polysaccharide activity or generates new activity. The carboxymethyl substitution at the OH-3 site on chitosan is beneficial to increasing moisturizing property, and a larger molecular weight of polysaccharides makes it easier to form a net-like structure to prevent water loss, and leads to the better moisturizing retention (Chen et al., 2003).

In fact, the moisture retention of polysaccharides and the substitution degree of polysaccharide modification may only show a direct positive correlation in a certain range. The moisturizing retention may increase with the increment of carboxymethyl substitution, but when the substitution degree increases further, the moisturizing property will decrease instead. Guo et al. (2007) studied the relationship between the molecular structure of succinyl chitosan and moisturizing retention, and found 50 % deacetylation showed the greatest moisture absorption under high humidity environments. When the degree of substitution is between 0.65 and 0.79, the moisture retention of succinyl chitosan is better than that of HA. Skin hydration in the moisturizing assay coated area increased first and then decreased with increasing degree of substitution, and the same trend was observed for transcutaneous water loss, with a higher moisturizing property at a degree of substitution of 0.45 (Zheng et al., 2017). Based on the above polysaccharide modification effects, we further know that most microbial polysaccharides have varying degrees of acetyl substitution. For example, the naturally occurring gellan gum has acetyl and glyceryl groups substituted on its glucose monomer (Zhu & Tong, 2012), which may be another reason why microbial polysaccharides show better moisture retention.

4.5 Relationship between apparent structure of polysaccharides and moisturizing activity

The surface morphology of the substance is closely related to its physicochemical properties (Fig. 2), which may affect its biological function (Emanuele et al., 2010). Some natural polysaccharides can form helical structures such as β-(1 → 3)-d-glucan, which has high viscosity and gel formation ability (Laroche & Michaud, 2007). In addition, the straight chain and branched chain structures of polysaccharides are related to the moisturizing of polysaccharides (Chen et al., 2017). In contrast to modern instruments for detecting the structure of polysaccharides, such as infrared spectroscopy and nuclear magnetic resonance, scanning electron microscopy (SEM) allows to understand polysaccharide wholeness, obtain relatively intuitive information and reveal the apparent structure of polysaccharides. Atomic force microscopy (AFM) is a powerful tool for imaging single-molecule chains and polysaccharide aggregates under “near-native” conditions, providing information on the size and morphology of individual polysaccharide chains.

Close

Fig. 2

(A) Scanning electron micrographs of Nymphaea hybrid polysaccharide (at manification of 1000, 5000, 20,000 and 50000)(Cong, 2018); (B) Surface morphology of nostoglycan by scanning electron microscopy(at manification of 200, 2000)(Li et al., 2011); (C) SEM images of three Laminaria japonica polysaccharide samples (at manification of 5000)(Cai et al., 2019); (D) Tapping mode AFM images of the EPS from Phyllobacterium sp. 921F solution (5 mg/L;10 mg/L)(Chi et al., 2019); (E) The molecular morphology of TFP at the concentration of 10 μg/mL via AFM(Xu et al., 2020).

Export to PPT

In the SEM of extracted crude polysaccharide samples of Lotus fragrance (Fig. 2A-B), Cong (2018) and Li (2011) found a flaky or crumbly accumulation with cotton wool-like fibers and pore channels of different sizes on the surface, with good adsorption to water. Cai et al. (2019) found the surface of acetylated degraded polysaccharides from Laminaria japonica was rough and showed a certain pore-like structure at a magnification of 5000 times (Fig. 2C), which resulted in its better water solubility and larger specific surface area. The unmodified polysaccharide of Laminaria japonica had a smoother surface and no porous structure, which are consistent with its lower water solubility and higher relative viscosity. The microalgae polysaccharide with better moisture retention (Li et al., 2018) had a rough surface under 200 × magnification, indicating the polysaccharide is amorphous. The 2000 × magnification showed its multilayered and lamellar morphology, similar to its reported porous polysaccharide of a drought-tolerant plant. Therefore, we also speculate that polysaccharides with better moisture-absorbing ability at the polysaccharide phenotype level may usually contain more pore-like structures.

Chi et al. (2019) similarly used AFM to obtain single-molecule chain images of exopolysaccharides (EPS) at a concentration of 5 mg/L (Fig. 2D). The characteristics of EPS are consistent with a hard elongated molecule without branches. The average length of polysaccharides in an aqueous solution was about 600 nm. As the concentration increased to 10 mg/L, EPS clustered and the chain height observed in the AFM image increased. This also shows the high molecular weight, linear chain, and polyhydroxylation, which explain the strong hydrogen bonds between molecules and subunits. Xu et al. (2020) used AMF to observe the chain characteristics and thickness of TFP molecules dispersed on the surface of mica matrix, and revealed the existence of TFP molecules as flexible chains from morphological analysis, with an average thickness of 1.1 nm (Fig. 2E). The TFP is higher than that of a single polysaccharide chain, and the average length is 0.1–1 nm, indicating the branches in the TFP structure are intertwined. Some aggregation between TFP chains may be due to strong intramolecular and intermolecular interactions.

5.1 Extrinsic moisurizing retention

Hydroxyl and carboxyl groups and some other polar groups hydrate with water molecules through hydrogen bonds. After a certain amount of water is absorbed by hygroscopicity, the polysaccharide molecular chain network expands, allowing more polar groups to contact water molecules and combine. Under the action of intermolecular hydrogen bonds and electric charges, polysaccharide molecular chains are cross-linked to form a three-dimensional grid structure, so that water molecules are bound in the grid, limiting the escape of water molecules, and further forming a hydration film on the skin surface to prevent water evaporation. In this process, hydrogen bonding, network structure, and film formation are important links, which will be described in detail below.

5.2 Intrinsic moisturizing retention

In addition to in vitro moisturizing effects, polysaccharides also have endogenous moisturizing effects. The epidermis, dermis, and subcutaneous tissue in the skin are the three-layer structure of human skin, and each layer influences the skin nourishing and moisturizing effects to varying degrees. Keratin fibers are concentrated in the epidermal layer. At this time, the skin can prevent both the penetration of chemical substances and the extravasation of body fluids. The moisture content of the stratum corneum is certainly the key point to skin moisturizing, but in fact, the moisture content of the skin depends on the moisture content of each layer. Moisturizing is the process of moisture transportation from the dermis to the stratum corneum for dynamic distribution. From the inside to outside of the skin, the moisturizing functions can be summarized as water capturing, water locking, water activating, and water storing (Yun et al., 2017) (Table 6). Polysaccharides can promote the skin's moisturizing system.