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the material should be biocompatible and should not cause an unacceptable e ff ect on the human body. 123 However, the de - nition of biocompatibility has been debated in the literature. In addition to a modi ed de nition of “ biocompatibility ” , Ratner proposed “ biotolerability ” to describe biomaterials in medi- cine. 124 Biocompatibility was de nedby “ the ability of a material to locally trigger and guide non brotic wound healing, recon- struction and tissue integration ” , while biotolerability was proposed to be “ the ability of a material to reside in the body for long periods of time with only low degrees of in ammatory reactions ” . Novel biomaterials developed for biomedical appli- cations could be de ned by these terms with the target of limited brotic reactions, 125 and lignin may be within this group of biomaterials. Lignin is a material derived from biobased resources, with attractive properties for biomedical use, primarily with antioxidant and antibacterial characteristics. The antioxidant property of lignin is dependent on the phenolic hydroxy groups capable of free-radical scavenging. The antimi- crobial e ff ect is also caused by the phenolic compounds. 126 As expected, the antibacterial, antioxidant and cytotoxic properties may also depend on the type of lignin. 127,128 For example, kra lignin has been found to have less antibacterial properties compared to organosolv lignin due to the larger methoxyl content in organosolv lignins. 127 Several authors have attempted to draw on lignin's antibac- terial and antiviral properties, which could be useful in surfaces for biomaterials and biomedical applications. Antimicrobial coatings were, for example, prepared by Lintinen et al. via deprotonation and ion exchange with silver, 129 as shown in Fig. 12. Jankovic et al. also developed such surfaces by ash- freezing a dispersion of organosolv lignin and hydroxyapatite with or without incorporated silver. 130 A er freezing, the samples were dried by cryogenic multipulse laser irradiation, producing a non-cytotoxic composite, which were further tested on their inhibitory activity. A similar approach was taken by Erakovi´c et al. 131 The authors prepared silver doped hydroxy- apatite powder, which was then suspended in ethanol with organosolv lignin and coated via electrophoretic deposition onto titanium. 131 This composite showed su ffi cient release of silver to impose antimicrobial e ff ect, while posing non-toxic for healthy immunocompetent peripheral blood mononuclear cells at the applied concentrations. However, the use of silver has caused some environmental concerns that should be addressed. 132 As an alternative, copper has been reported with better antibacterial e ff ect than silver, which to our knowledge is currently unexplored in antimicrobial lignin complexes. 133 Lignin-titanium dioxide nanocomposites were prepared by precipitation from solution and tested for their antimicrobial and UV-blocking properties. 134 The authors concluded that the lignin could function as the sole capping and stabilization agent for the titanium dioxide nanocomposites. Better perfor- mance of the nanocomposites for antioxidant, UV-shielding, and antimicrobial properties was reported, as compared to the lignin or titanium dioxide alone. Kra lignin and oxidized Kra lignin were processed into colloidal lignin particles and coated with b -casein, which was further cross-linked. 29 This work aimed to produce biomaterials
Two approaches can generally be distinguished, based on either the use of neat or chemically modi ed lignin. Properties such as water-permeability and nitrogen or phosphor release can be positively a ff ected; however, chemical modi cationmay impair biodegradation. With that said, the work of Fertahi et al. should be noted, who coated triple superphosphate fertilizers with mixtures of carrageenan, PEG, and lignin. 118 The latter had been obtained from alkali pulping of olive pomace. The three mentioned coating-materials are in principle all biodegradable. Blending lignin with carrageenan or PEG improved the mechanical stability of the lms compared to lignin alone, while also increasing the swelling of the coatings. Similar blends were studied by Mulder et al. , who found that glycerol or polyols such as PEG 400 could improve the lm forming prop- erties. 120 The water resistance, on the other hand, was improved by using high molecular weight PEG or crosslinking agents such as Acronal or Styronal (commercial name). On the downside, the biodegradability will be negatively a ff ected by such cross- linking agents, especially acrylates or styrene-based chemistries. Chemical modi cation of lignin for coating of superphos- phate fertilizers was also conducted by Rotondo et al. , 119 where the technical lignin was either hydroxymethylated or acetylated. Apart from utilizing toxic chemicals in the synthesis, these modi cations alone do not pose as a detriment to biodegrad- ability. However, the Rotondo et al. also synthesized phenol- formaldehyde resin to coat the fertilizer cores, which could be troubling, as the authors basically suggested adding plastics to the soil. Zhang et al. furthermore modi ed lignin by gra ing quaternary ammonium groups onto it. 82 While the quaternary ammonium may conveniently bind anions and add nitrogen to the soil, some of its degradation products are highly toxic and hence concerning, unless the goal is to add biocides to the soil. A similar approach was done by Li et al. , 14 who synthesized multifunctional fertilizers. First, alkali lignin and NH 4 ZnPO 4 were mixed and dissolved to produce fertilizer cores, which were further coated with cellulose acetate butyrate and liquid para ffi n. A second coating was then applied as a superabsor- bent, which was based on alkali lignin gra ed with poly(acrylic acid) in a blend with attapulgite. Both the para ffi n and poly(- acrylic acid) gra should have been avoided due to environ- mental incompatibilities. At last, a di ff erent application was explored by Nguyen et al. , i.e. , the encapsulation of photo-liable compounds with a lignin coating layer. 122 In particular, the authors emulsi ed the insecticide deltamethrin in a corn oil nanoemulsion with polysorbate 80 and soybean lectin as emulsi er. The droplets were further coated with chitosan and lignosulfonate. The lignin contributed hereby to both the UV-protection of the emulsi ed insecticide, as well as to its controlled release. This approach is positive in several regards, as only biobased agents were used in the formulation, the lignosulfonates were not chemically modi ed, and the application drew on some of lignin's inherent properties. 3.3.2. Biomaterials and biomedical applications. A biomaterial, i.e. , a material intended for use in or on the human body, must comply with certain requirements. This implies that
RSCAdv. , 2023, 13 , 12529 – 12553 | 12541
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