Properties and chemical modifications of lignin: Towards lignin-based nanomaterials for biomedical applications

Abstract

Biorenewable polymers have emerged as an attractive alternative to conventional metallic and organic materials for a variety of different applications. This is mainly because of their biocompatibility, biodegradability and low cost of production. Lignocellulosic biomass is the most promising renewable carbon-containing source on Earth. Depending on the origin and species of the biomass, lignin consists of 20-35% of the lignocellulosic biomass. After it has been extracted, lignin can be modified through diverse chemical reactions. There are different categories of chemical modifications, such as lignin depolymerization or fragmentation, modification by synthesizing new chemically active sites, chemical modification of the hydroxyl groups, and the production of lignin graft copolymers. Lignin can be used for different industrial and biomedical applications, including biofuels, chemicals and polymers, and the development of nanomaterials for drug delivery but these uses depend on the source, chemical modifications and physicochemical properties. We provide an overview on the composition and properties, extraction methods and chemical modifications of lignin in this review. Furthermore, we describe different preparation methods for lignin-based nanomaterials with antioxidant UV-absorbing and antimicrobial properties that can be used as reinforcing agents in nanocomposites, in drug delivery and gene delivery vehicles for biomedical applications. Response to Reviewers: RESPONSE TO THE REVIEWERS' COMMENTS REVIEWER #1In this review, the authors summarized the current state of art for the preparation of lignin-based nanomaterials. Recent advances in the application of lignin-based nanomaterials in drug delivery and gene delivery vehicles were also discussed in the manuscript. However, this review is premature for publication in Progress in Materials Science. Some specific comments are listed as follows.2. "4.1 Lignin Depolymerization": There are too many descriptive statements. In order to im-prove the understanding of different lignin depolymerization processes, main reaction equa-tions (similar to fig. 4) should be added in this section. R.: As each depolymerization process can originate different products, with different chemical structures, we decided to added a figure (Fig. 4) that summarize the different conditions used for each lignin depolymerization process, with the different products that can obtained from each pro-cess. "4.4 Production of Lignin GraftCopolymers": This section is too long to read. The authors should divide it into several parts and provide some subtitles here. In this way, the readers can easily follow. R.: In order to facilitate the reading, the following subsections were created in the "4.4. Production of Lignin Graft Copolymers" section, according to the processes described in the Figures 7 and 8: 4.4.1. "Grafting from" Technique 4.4.1.1. Ring Opening Polymerization 4.4.1.2. Radical Polymerization 4.4.1.3. Atom Transfer Radical Polymerization 4.4.2. "Grafting to" Technique R. : We thank the...

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