75
to its Mw (Ugartondo et al. 2008). The carbohydrates in lignins would also affect
their cytotoxicity: lignin with low carbohydrate content was reported as the most
cytotoxic (Ugartondo et al. 2008).
Lignin is heterogeneous and has an amorphous structure. Its chemical and biological properties vary with different extraction methods and different plant sources.
It is thus necessary to evaluate the cytotoxic properties of lignin before application
as functional polymers used in cosmetics, food, healthcare products and
pharmaceuticals.
5.5 Applications of Lignin as an Antioxidant
5.5.1 Applications in Anti-UV Agents
and Photosensitive Materials
Structurally, lignin has several UV chromophoric groups, such as carbonyl, cinnamaldehyde and hydroxyl groups, and double bonds. These groups give lignin the
ability to effectively absorb UV-A and UV-B light. As one of the main components
of wood, lignin absorbs 80–95% of the total amount of UV light absorbed.
Prolonged exposure to UV rays can lead to the formation of different free radicals, such as hydroxyl radicals, reactive oxygen species (ROSs) and superoxide
anion radicals, in the skin. This causes adverse effects on the skin, such as aging,
changes in pigmentation and even skin cancer (Gonzaga 2009). Sunscreen agents
offer protection against UV-induced damage. Adding lignin to the sunscreen
improves its protection against UV rays, while providing antioxidant properties.
Kai et al. (2016a) prepared lignin-poly(ethylene glycol) methacrylate (PEGMA)
copolymers through atom transfer radical polymerization (ATRP) to increase the
dispersion efficiency of lignin in commercial creams (Fig. 5.7). These ligninPEGMA copolymers have been incorporated into commercial creams, thus significantly increasing sun protection. For example, the sun protection factor of Shiseido
White Lucent brightening protective cream SPF15 was increased from 15 to 38+ by
the addition of lignin-PEGMA copolymers. Through further research, such lignin
copolymers can be used more widely in other cosmetics. Meanwhile, the reason and
mechanism by which lignin and its copolymers improve the sun protection factor of
cosmetics is currently under investigation. In addition to lignin, the sunscreen active
ingredients in cosmetics are ensulizole, octinoxate and titanium dioxide (TiO 2 ).
Qian et al. (2016) attributed the synergistic influence between octinoxate and lignin
to the formation of J-aggregation (π-π∗ stacking) between them, so that the
J-aggregation improved UV absorbance of the copolymers. As a sunscreen ingredient, TiO 2 can absorb UV rays from sunlight. However, under UV radiation, the
photocatalytic activity of TiO 2 will produce superoxide and hydroxyl radicals.
These free radicals can cause damage to human skin. The lignin in lignin/TiO 2 composites quenches the hydroxyl radicals produced by TiO 2 (Ibrahim et al. 2019).
5 Lignin as a Natural Antioxidant: Property-Structure Relationship and Potential…
to its Mw (Ugartondo et al. 2008). The carbohydrates in lignins would also affect
their cytotoxicity: lignin with low carbohydrate content was reported as the most
cytotoxic (Ugartondo et al. 2008).
Lignin is heterogeneous and has an amorphous structure. Its chemical and biological properties vary with different extraction methods and different plant sources.
It is thus necessary to evaluate the cytotoxic properties of lignin before application
as functional polymers used in cosmetics, food, healthcare products and
pharmaceuticals.
5.5 Applications of Lignin as an Antioxidant
5.5.1 Applications in Anti-UV Agents
and Photosensitive Materials
Structurally, lignin has several UV chromophoric groups, such as carbonyl, cinnamaldehyde and hydroxyl groups, and double bonds. These groups give lignin the
ability to effectively absorb UV-A and UV-B light. As one of the main components
of wood, lignin absorbs 80–95% of the total amount of UV light absorbed.
Prolonged exposure to UV rays can lead to the formation of different free radicals, such as hydroxyl radicals, reactive oxygen species (ROSs) and superoxide
anion radicals, in the skin. This causes adverse effects on the skin, such as aging,
changes in pigmentation and even skin cancer (Gonzaga 2009). Sunscreen agents
offer protection against UV-induced damage. Adding lignin to the sunscreen
improves its protection against UV rays, while providing antioxidant properties.
Kai et al. (2016a) prepared lignin-poly(ethylene glycol) methacrylate (PEGMA)
copolymers through atom transfer radical polymerization (ATRP) to increase the
dispersion efficiency of lignin in commercial creams (Fig. 5.7). These ligninPEGMA copolymers have been incorporated into commercial creams, thus significantly increasing sun protection. For example, the sun protection factor of Shiseido
White Lucent brightening protective cream SPF15 was increased from 15 to 38+ by
the addition of lignin-PEGMA copolymers. Through further research, such lignin
copolymers can be used more widely in other cosmetics. Meanwhile, the reason and
mechanism by which lignin and its copolymers improve the sun protection factor of
cosmetics is currently under investigation. In addition to lignin, the sunscreen active
ingredients in cosmetics are ensulizole, octinoxate and titanium dioxide (TiO 2 ).
Qian et al. (2016) attributed the synergistic influence between octinoxate and lignin
to the formation of J-aggregation (π-π∗ stacking) between them, so that the
J-aggregation improved UV absorbance of the copolymers. As a sunscreen ingredient, TiO 2 can absorb UV rays from sunlight. However, under UV radiation, the
photocatalytic activity of TiO 2 will produce superoxide and hydroxyl radicals.
These free radicals can cause damage to human skin. The lignin in lignin/TiO 2 composites quenches the hydroxyl radicals produced by TiO 2 (Ibrahim et al. 2019).
5 Lignin as a Natural Antioxidant: Property-Structure Relationship and Potential…