Editorial Cite This: Biomacromolecules 2019, 20, 569−572
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Rational Design of Multifunctional Renewable-Resourced Materials
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relevant properties of cellulose nanomaterials such as improved specific surface area, hydrophilicity, and tailorability of the surface chemistry via surface grafting with cationic or anionic groups to make them accessible for use as wastewater treatment materials. Their utility as efficient adsorbent and flexible membranes or included in the preparation of hybrid materials for wastewater treatment was underlined. It has been recognized also that isolation of cellulose nanomaterials remains a challenge that need to address the affordable upscaling production and cost-effective surface modification routes. Lars Wågberg2 reported on chemically cross-linked highly porous nanocellulose aerogels with complex shapes prepared by freeze-linking procedures. The aerogel shapes ranged from simple geometrical three-dimensional bodies to swirls and solenoids. This was achieved by molding or extruding periodate oxidized cellulose nanofibril dispersions prior to chemical cross-linking or by reshaping already prepared aerogels followed by reshaping and locking the aerogels into its new shapes. The new shapes were retained by new cross-links formed between CNFs brought into contact by the deformation during reshaping. This self-healing ability to form new bonds after plasticization and redrying also contributed to the mechanical resilience of the aerogels, allowing them to be cyclically deformed in the dry state, reswollen with water, and redried with good retention of mechanical integrity. Akira Isogai3 highlighted work on CNFs with high aspect ratios and CNFs with low aspect ratios prepared by catalytic oxidation with 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO). Aqueous dispersions of TEMPO− CNFs and TEMPO−CNCs with different concentrations were prepared and the solid concentrations at the transition points from the dilute to semidilute regions and from the semidilute to dense gel regions were determined from dynamic light scattering (DLS) measurements. Orlando Rojas4 demonstrated the efficiency of a simple method based on emulsion gels for the synthesis of low-solid 3D scaffolds containing CNF and alginate. Polylactide (PLA) was added to the dispersed phase to build simple cubic scaffolds and complex geometric structures with high shape fidelity. The swelling behavior and robustness of dried scaffolds suggested their potential relevance to 3D-printable ink applications. Ulrica Edlund5 revealed new engineered polysaccharide materials by applying a water-based admicellar polymerization strategy for the production of hydrophobic CNFs. Hexyl acrylate was used as the adsolubilized monomer generating CNFs coated with fatty acrylate polymers. This approach proved beneficial for altering the interfibrillar interactions and improving the CNFs compatibility with a degradable composite matrix, poly(butylene adipate-co-terephthalate) (PBAT) leading to significant mechanical property improvements of the resulting composites.
urrently, our society has to respond to an increasing demand for polymer materials that are more environmentally friendly. Therefore, Renewable-Resourced Materials (RRMs) have recently gained considerable interest due to their potential to address this challenge and to expedite application efforts for a wide range of targets. Advances in RRMs that could be made available in large quantities at low cost, while satisfying various functional requirements and being safe and durable, are extremely important in achieving this ultimate goal. The development of new concepts, novel and efficient synthetic methodologies and processes constitutes a key factor to overcome the limitations of the classical boundaries between different types of materials. Breakthroughs are needed in the way the renewables are exploited and for finding more efficient strategies to manage the valorization and reuse of natural resources. Improving the performance by providing multifunctionality character through rational design should be a major focus in the RRM field. The Symposium Rational Design of Multifunctional Renewable-Resourced Materials held during the ACS National Meeting August 19−23, 2018 in Boston was the first Symposium organized on this topic. It covered the area of rationally designed polymeric materials from ground up to perform specific functions with degradability being part of them. The main topics included the principles of biodegradability such as enzymatically catalyzed processes, acid or base catalyzed hydrolysis, oxidation and other processes, correlations of differences in polymer structure with differences in polymer function, thereby facilitating the rational design structure−property relationships, new synthetic methodologies for the bottom-up construction of multifunctional materials with degradation on demand characteristics, use of renewable materials and biomass to create such materials. Challenging questions about the significance of cellulose nanocrystals (CNCs) and cellulose nanofibrils (CNFs) and on how far are we from being able to create polymeric materials that are rationally designed to perform the biodegradation task with the same specificity that is found in biomolecules were vigurously debated during the meeting sessions. “Convergence” of the scientific fields of macromolecular and bio sciences is already the mission of Biomacromolecules journal. The message on the need of “designed to degrade” materials was reflected during the symposium in particular for polymers used in medicine and pharmaceutics but also expanding to incorporate these features for materials used in other fields such as electronics, computers and the like. The symposium was organized in four subsections on the following main topics: (1) CNC/CNF Nanocellulose Composites, (2) Synthesis of Renewable Materials, (3) Nanoparticle Structures and Properties and (4) New Applications. The section on CNC/CNF Nanocellulose Composites was opened by Wim Thielemans who delivered a comprehensive view of understanding the mechanism and modulating surface interactions at cellulose nanocrustal surfaces. In the same section, Alain Dufresne1 discussed the © 2019 American Chemical Society
Special Issue: The Rational Design of Multifunctional RenewableResourced Materials Published: February 11, 2019 569
DOI: 10.1021/acs.biomac.9b00060 Biomacromolecules 2019, 20, 569−572
Biomacromolecules
Editorial
The macrocyclic radiometal chelator, DOTA (1,4,7,10tetraazacyclododecane-1,4,7,10-tetraacetic acid), was covalently installed on the surface of CNCs by two different conjugation methods. First, DOTA was conjugated to the aldehyde group (−CHO) at the reducing end of CNC as a hydrazide. Second, the hydroxyl groups (−OH) on the CNCs surface were activated with 1,1′-carbonyldiimidazole (CDI) and further conjugated with a DOTA-amine. The in vitro cytotoxicity of the two types of DOTA-installed CNCs was investigated in murine RAW 264.7 macrophages and murine 4T1 mammary adenocarcinoma cell line. The in vivo behavior of DOTA-CNC prepared using the two synthetic strategies was determined in healthy and 4T1-tumor-bearing mice by in vivo small-animal SPECT/CT imaging and ex vivo gamma counting of the excised tissues. It appeared that the terminal aldehyde modification could prevail as a strategy as it renders the OH-groups on the CNC surface free for subsequent functionalization and payload incorporation. These data could prove useful for investigations of CNC drug delivery systems in vivo and for guiding structural optimization in biomedical applications and clinical translation. The section on Nanoparticle Structures and Properties was opened by Virgil Percec12 who used Nature as a model to generate constitutional isomeric libraries of self-assembling dendrons and dendrimers, based on natural phenolic acids, that are the simplest examples of programmed synthetic macromolecules. Demonstrations were made on the synthesis and structural analysis of a library containing 13 amphiphilic Janus dendrimers containing linear and branched alkyl chains on their hydrophobic part. These were prepared by an optimized iterative modular synthesis starting from natural phenolic acids. Monodisperse dendrimersomes were produced by injection and giant polydisperse by hydration. Both were structurally characterized to select the molecular design principles that provide unilamellar dendrimersomes in higher yields and shorter reaction times than under previously used reaction conditions. A systematic study on the size, structure, stability, and polydispersity of the self-assembled DSs was conducted at different concentrations by injection and monitored by dynamic light scattering, cryo-TEM, and confocal microscopy. These dendrimersomes are expected to provide important tools for synthetic cell biology, encapsulation, and drug delivery. Marc Hillmyer covered the interesting topic of renewable, degradable aliphatic polyester elastomers and described the high performance achieved using different preparation approaches. Karin Odelius reported on using ring-opening reaction strategies for the design of renewable polymeric materials with tunable characteristics. Michael Monteiro13 presented different shapes of nanostructures (spheres, worms, rods, nanorattles, and toroids) prepared with a poly(N-isopropylacrylamide) (PNIPAM) coating using their temperature-directed morphology transformation (TDMT) method. The PNIPAM coating was designed to have a fraction of the PNIPAM in its globular conformation shown to lead not only to greater cell uptake, but also enhanced enzyme activity. The incorporation of a globular component on the nanostructure surface, studied by in vivo biodistribution in a mouse model as a function of time indicated that it was influenced by the structure and PNIPAM surface composition. The data obtained suggested that the PNIPAM coating not only directed the nanostructures to specific organs (e.g., spleen) but were irreversibly captured there. The rods, in particular, accumulated at the red pulp
In the section on Synthesis of Renewable Materials of the symposium, Kevin Edgar6 described the synthesis of glycosoaminoglycan (GAG) analogs bearing amine and carboxyl groups, or a combination of amines/amides and carboxyl groups by chemical modification of cellulose acetate. The synthetic strategy involved regiospecific C6 bromination and azide displacement, placing the amine/amide precursor at C6. Saponification of the acetyl moieties exposed the remaining C6 oxygen moieties as primary hydroxyls susceptible to TEMPO oxidation, carried out with high chemoselectivity to afford a water-soluble polymer. The azide reduction by dithiothreitol (DTT) generated GAG analogs that mimicks the heparin backbone. Similarly, thioacetic acid reduction produced a new GAG analog containing both, amine and amide groups. Significantly, each reaction step from the azide onward was carried out in aqueous media, providing a relatively green, selective, and efficient pathway from a cellulose ester to these analogs. Reporting on green chemistry available for the modification of polysaccharides, Tadahisa Iwata7 presented results on the regioselective modification of dextrin using lipase enzymes as biocatalysts. Dextrin esters with varying side-chain lengths (C2−C12) were obtained under optimum conditions using Lipozyme TL IM and increased reaction times. The initial reaction rates and final DS values of the dextrin esters were proven to be affected by the chain length of the vinyl acyl donor. Furthermore, the substitution reaction was shown to occur preferentially at the C6 position of dextrin. The resulting monosubstituted dextrin esters exhibited thermal stabilities between those of unmodified dextrin and the fully substituted dextrin esters. Monika Ö sterberg8 disclosed results on producing cellulose nanofibril nanocomposite films utilizing a variety of different lignin morphologies using a pressurized filtration method. Tensile measurements of nanocomposite films that contained colloidal lignin particles displayed a significant improvement in their mechanical properties, exhibiting nearly 2-fold increase in toughness as compared to CNT films. In addition, these films were proven to block efficiently the UV light, while maintaining a higher visible light transmittance in comparison to the films that contained irregular lignin aggregates instead of nanoparticles. The long lasting radical scavenging activity delivered by lignin particles was thought also to be useful in packaging and healthcare applications. Andrea Kasko9 reported on new structures of aromatic poly(ester-amides) and poly(ether-amides) synthesized from monolignol-based precursors and their thermal properties and hydrolytic stability. Lars Berglund10 introduced different routes for the preparation of thermoplastic cellulose nanocomposites with CNFs oriented random-in-plane. These included physical blends of CNFs and poly(methyl methacrylate) (PMMA), with native CNF or surface-modified with allyl glycidyl ether (AGE)). The latter was used also during PMMA in situ polymerization to form grafted CNF-PMMA links at the interface. The grafted CNF-PMMA showed the highest optical transmittance followed by the AGE-modified CNF-PMMA blend and native CNF-PMMA blend. Of the three materials, the grafted nanocomposites also showed the best thermal stability and mechanical properties (modulus and strength) in moist state. The results are postulated to have implications for nanocomposite design at the nano- and molecular scale and for processing strategies of thermoplastic nanocomposites with high cellulose content. Mauri Kostiainen11 described new molecular imaging probes based on nanocrystalline cellulose. 570
DOI: 10.1021/acs.biomac.9b00060 Biomacromolecules 2019, 20, 569−572
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and poly(L-lysine) (MAL-TEG-PLL). Through electrostatic interactions with plasmid DNA (pDNA), MAL-TEG-PLL formed micelles with maleimide groups on the surface allowing postmodifications with cysteine-containing functional peptides and nuclear localization signal via thiol−maleimide conjugation. This strategy is thought to have potential to design highly efficient plant gene delivery systems. In his presentation, Taizo Kabe described the ordered structure of Curdlan propionate materials and presented results on mechanical properties of their melt spun fibers. There is no doubt that advances in new materials rationally designed and equipped with highly desirable functions, and that could be made available in large quantities at low cost, while being safe and durable, are critically important in different types of applications. Design strategies that led to significant improvements in the performance and stand out for a continuing progress are well under way and moving forward in different fields. The first symposium on Rational Design of Multifunctional Renewable-Resourced Materials represented a clear illustration of multiple efforts undertaken in this domain in different laboratories and academic institutions and provided an excellent platform for the scientific communities engaged in advancing this type of materials. In the following special issue, we feature 18 of the papers presented at the symposium. These papers further illustrate the ongoing research worldwide and the challenges with rational design of multifunctional renewable-resourced materials.
regions in the spleen and had a high cell uptake due to the PNIPAM coating. The findings on the accumulation of PNIPAM rods selectively into the spleen could represent a new targeted design strategy for diagnostic or therapeutic applications. Jukka Seppälä14 reported on a combined approach using electrospinning and cryogelation for the synthesis of the guidance channel that mimics the native architecture of the physiologically present peripheral nerve. The electrospun polyurethane external guide was chosen due to its pore size, porosity, wall thickness for nutrient exchange, and mechanical stability. In turn, the biodegradable aligned chitosan-gelatin cryogel filler supplied topographical cues through interconnected porous channels for directional regeneration of axons and also provided haptotactic cues for SC adhesion and migration. Future studies could be performed to show the potential of advanced nerve guidance channels for in vivo peripheral nerve regeneration. Matthew Becker described the synthesis of polylactone and poly(propylene fumarate) (PPF) block copolymers with well-defined molecular masses and molecular mass distributions using sequential, ring-opening polymerization and ring-opening copolymerization methods. A “click” functionalization using a coppermediated azide−alkyne cycloaddition brings the resulting structures closer for potential use in regenerative medical applications. The section on New Applications was opened with a presentation by Minna Hakkarainen15 about multifunctional three-dimensional (3D) scaffolds obtained by surface grafting cellulose-derived nanographene oxide (nGO) on the surface of porous poly(ε-caprolactone) (PCL) scaffolds. Grafting of nGO was shown to induce a significant increase in compressive strength and to promote mineralization with the formation of calcium phosphate precipitates on the surface of the scaffolds. The potential of surface-grafted nGO scaffolds as nanocarriers of antibiotics (e.g., ciprofloxacin) was also established in terms of drug loading and delivery capability. George Guo-Qiang Chen16,17 describes in his papers the fabrication and properties of a biodegradable and biocompatible material poly(R-3-hydroxybutyrate-co-R-3-hydroxyhexanoate) (PHBHHx), a family member of microbial polyhydroxyalkanoates (PHA). This material was shown to be superhydrophobic with potential for applications involving the cleaning of environmental oil or solvent pollutions along with advantage in disposal after the usage due to its biodegradability. In addition, films of PHBHHx evaluated for antibioadhesion properties exhibited up to 100% reductions for platelet adhesion on their surfaces in comparison with control material surfaces, which could be beneficial for implant applications. Rachel Auzely-Velty gave a presentation on injectable and self-healing polysaccharide hydrogels by the route of boronate ester bonds and defined the relationships between the binding mode of boronic acids to saccharide moieties and the mechanical properties. Keiji Numata18 reported on use of chemoenzymatic polymerization as an advantageous approach to synthesize artificial polypeptide materials. Of particular interest in terms of functionality and properties were those inspired by natural functional and structural proteins present in living systems. The outstanding mechanical properties of spider dragline silk were shown to be highly dependent on the repetitive sequences of the component proteins. Additional research from this laboratory included in this special issue describes the development of the block copolymer maleimide-conjugated tetra(ethylene glycol)
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Simona Percec Ann-Christine Albertsson
AUTHOR INFORMATION
ORCID
Ann-Christine Albertsson: 0000-0001-8696-9143 Notes
Views expressed in this editorial are those of the authors and not necessarily the views of the ACS.
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REFERENCES
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DOI: 10.1021/acs.biomac.9b00060 Biomacromolecules 2019, 20, 569−572
