Dan Kai

Towards lignin-based functional materials in a sustainable world

In light of the incessant consumption of raw materials in the world today, the search for sustainable resources is ever pressing. Lignin, the second most naturally abundant biomass, which makes up 15% to 35% of the cell walls of terrestrial plants, has always been treated as waste and used in low-value applications such as heat and electricity generation. However, its abundance in nature could potentially solve the problem of the rapidly depleting resources if it was successfully translated into a renewable resource or valorized to higher value materials. Advanced lignin modification chemistry has generated a number of functional lignin-based polymers, which integrate both the intrinsic features of lignin and additional properties of the grafted polymers. These modified lignin and its copolymers display better miscibility with other polymeric matrices, leading to improved performance for these lignin/polymer composites. This review summarizes the progress in using such biopolymers as reinforcement fillers, antioxidants, UV adsorbents, antimicrobial agents, carbon precursors and biomaterials for tissue engineering and gene therapy. Recent developments in lignin-based smart materials are discussed as well.

Biodegradable polymers for electrospinning: towards biomedical applications.

Electrospinning has received much attention recently due to the growing interest in nano-technologies and the unique material properties. This review focuses on recent progress in applying electrospinning technique in production of biodegradable nanofibers to the emerging field of biomedical. It first introduces the basic theory and parameters of nanofibers fabrication, with focus on factors affecting the morphology and fiber diameter of biodegradable nanofibers. Next, commonly electrospun biodegradable nanofibers are discussed, and the comparison of the degradation rate of nanoscale materials with macroscale materials are highlighted. The article also assesses the recent advancement of biodegradable nanofibers in different biomedical applications, including tissue engineering, drug delivery, biosensor and immunoassay. Future perspectives of biodegradable nanofibers are discussed in the last section, which emphasizes on the innovation and development in electrospinning of hydrogels nanofibers, pore size control and scale-up productions.

Long-Term Real-Time In Vivo Drug Release Monitoring with AIE Thermogelling Polymer

A new drug concentration meter is developed. In vivo drug release can be monitored precisely via a self-indicating drug delivery system consisting of a new aggregation-induced emission thermoresponsive hydrogel. By taking the advantage of a self-indicating system, one can easily detect the depletion of drugs, and reinject to maintain a dosage in the optimal therapeutic window.

Electrospinning of poly(glycerol sebacate)-based nanofibers for nerve tissue engineering

Nerve tissue engineering (TE) requires biomimetic scaffolds providing essential chemical and topographical cues for nerve regeneration. Poly(glycerol sebacate) (PGS) is a biodegradable and elastic polymer that has gained great interest as a TE scaffolding biomaterial. However, uncured PGS is difficult to be electrospun into nanofibers. PGS would, therefore, require the addition of electrospinning agents. In this study, we modified PGS by using atom transfer radical polymerization (ATRP) to synthesize PGS-based copolymers with methyl methacrylate (MMA). The synthesized PGS-PMMA copolymer showed a molecular weight of 82kDa and a glass transition temperature of 115°C. More importantly, the PGS-PMMA could be easily electrospun into nanofiber with a fiber diameter of 167±33nm. Blending gelatin into PGS-PMMA nanofibers was found to increase its hydrophilicity and biocompatibility. Rat PC12 cells were seeded onto the PGS-PMMA/gelatin nanofibers to investigate their potential for nerve regeneration. It was found that gelatin-containing PGS-based nanofibers promoted cell proliferation. The elongated cell morphology observed on such nanofibers indicated that the scaffolds could induce the neurite outgrowth of the nerve stem cells. Overall, our study suggested that the synthesis of PGS-based copolymers might be a promising approach to enhance their processability, and therefore advancing bioscaffold engineering for various TE applications.

Engineering highly stretchable lignin-based electrospun nanofibers for potential biomedical applications

Lignin, one of the most abundant biopolymers on Earth, has been recognized as a renewable alternative to traditional petroleum-based plastics. The integration of lignin with synthetic and engineering plastics is an important approach to develop sustainable polymers. However, it is challenging to blend lignin with other polymers due to its brittle nature and poor dispersion in many composites. In order to improve the miscibility and compatibility of lignin with other plastics, a series of poly(methyl methacrylate) (PMMA) grafted lignin copolymers were prepared from atom transfer radical polymerization. The chain length of PMMA oligomers and glass transition temperature of the lignin copolymers was controlled by varying the lignin: methyl methacrylate ratio. The lignin mass fractions in the copolymers varied from 5.6% to 46.1%. These lignin-PMMA copolymers were further blended with poly(ε-caprolactone) (PCL) and engineered into nanofibrous composites by electrospinning. Tensile test and dynamical mechanical analysis showed that the incorporation of lignin-PMMA copolymers significantly improved the tensile strength, Youngs modulus, and storage modulus of the resulting nanofibrous composites. The length of the PMMA chain played a crucial role in the miscibility of lignin in PCL, and therefore enhanced the stiffness and ultimate elongation of the resulting nanofibers. Cell culture studies suggested that these PCL/lignin-PMMA nanofibers were biocompatible and promoted the proliferation, attachment and interactions of human dermal fibroblasts. With reinforced mechanical properties and good biocompatibility, these green and stretchable electrospun nanofibers are potentially useful as biomaterial substrates for biomedical applications.

Elastic poly(ε-caprolactone)-polydimethylsiloxane copolymer fibers with shape memory effect for bone tissue engineering.

A porous shape memory scaffold with biomimetic architecture is highly promising for bone tissue engineering applications. In this study, a series of new shape memory polyurethanes consisting of organic poly(ε-caprolactone) (PCL) segments and inorganic polydimethylsiloxane (PDMS) segments in different ratios (9 : 1, 8 : 2 and 7 : 3) was synthesised. These PCL-PDMS copolymers were further engineered into porous fibrous scaffolds by electrospinning. With different ratios of PCL: PDMS, the fibers showed various fiber diameters, thermal behaviour and mechanical properties. Even after being processed into fibrous structures, these PCL-PDMS copolymers maintained their shape memory properties, and all the fibers exhibited excellent shape recovery ratios of  >90% and shape fixity ratios of  >92% after 7 thermo-mechanical cycles. Biological assay results corroborated that the fibrous PCL-PDMS scaffolds were biocompatible by promoting osteoblast proliferation, functionally enhanced biomineralization-relevant alkaline phosphatase expression and mineral deposition. Our study demonstrated that the PCL-PDMS fibers with excellent shape memory properties are promising substrates as bioengineered grafts for bone regeneration.

Biocompatible electrically conductive nanofibers from inorganic-organic shape memory polymers

A porous shape memory scaffold with both biomimetic structures and electrical conductivity properties is highly promising for nerve tissue engineering applications. In this study, a new shape memory polyurethane polymer which consists of inorganic polydimethylsiloxane (PDMS) segments with organic poly(ε-caprolactone) (PCL) segments was synthesized. Based on this poly(PCL/PDMS urethane), a series of electrically conductive nanofibers were electrospun by incorporating different amounts of carbon-black. Our results showed that after adding carbon black into nanofibers, the fiber diameters increased from 399±76 to 619±138nm, the crystallinity decreased from 33 to 25% and the resistivity reduced from 3.6 GΩ/mm to 1.8 kΩ/mm. Carbon black did not significantly influence the shape memory properties of the resulting nanofibers, and all the composite nanofibers exhibited decent shape recovery ratios of >90% and shape fixity ratios of >82% even after 5 thermo-mechanical cycles. PC12 cells were cultured on the shape memory nanofibers and the composite scaffolds showed good biocompatibility by promoting cell-cell interactions. Our study demonstrated that the poly(PCL/PDMS urethane)/carbon-black nanofibers with shape memory properties could be potentially used as smart 4-dimensional (4D) scaffolds for nerve tissue regeneration.

Multi-arm carriers composed of an antioxidant lignin core and poly(glycidyl methacrylate-co-poly(ethylene glycol)methacrylate) derivative arms for highly efficient gene delivery

A lignin-based copolymer with good biocompability was successfully prepared via atom transfer radical polymerization (ATRP) for efficient gene delivery. Kraft lignin was modified into lignin-based macroinitiators and then poly(glycidyl methacrylate)-co-poly(ethylene glycol)methacrylate (PGMA-PEGMA) side chains were prepared via ATRP grafting onto lignin. Ethanolamine was sequentially functionalized onto lignin-PGMA-PEGMA and a cationic lignin-PGEA-PEGMA copolymer consisting of a lignin core and different-length PGEA-PEGMA side chains was produced. Lignin-PGEA-PEGMA copolymers could efficiently compact pDNA into nanoparticles with sizes ranging from 150 to 250 nm at N/P ratios of 10 or higher. The gene transfection efficiency depends greatly on the mass percentage of PGEA units and the N/P ratio. The lignin-PGEA-PEGMA with 46.9% (mass%) of PGEA units (i.e. LG100) has highest transfection efficiency in comparison with the copolymers with a lower amount of PGEA units. In addition, LG100 has high transfection efficiency under serum conditions, which is comparable to or much higher than PEI control in HEK 293T and Hep G2 cell lines. More importantly, lignin-PGEA-PEGMA copolymers have excellent antioxidant activity. The novel cationic lignin-PGEA-PEGMA copolymers can be promising gene carriers for gene delivery.

The role of hydrogen bonding in alginate/poly(acrylamide-co-dimethylacrylamide) and alginate/poly(ethylene glycol) methyl ether methacrylate-based tough hybrid hydrogels

The interpenetrating alginate-based hybrid hydrogel network is a tough yet recoverable material. This is believed to be caused by the combination of the strength of a covalent network, and the reversibility of an ionic network. However, hydrogen bonds are believed to also be responsible for the exceptional properties of these hydrogels. In this paper, the effect of varying the reactant concentrations on the mechanical properties of the hydrogels was first studied. By changing the monomer used (from polyacrylamide to polydimethylacrylamide) in the fabrication of the hydrogel, the effect of hydrogen bonding was studied. Compression test results showed that the presence of hydrogen bonds is critical for the high toughness of the hybrid hydrogel. Additionally, co-polymeric hybrid hydrogels were synthesized and shown to have improved mechanical properties over the original hybrid hydrogel, with an elastic modulus and compressive toughness of 350 kPa and 70 J m−3, respectively. The results of this experiment can be used to optimise the mechanical properties of future hybrid hydrogels.

Biodegradable electronics: cornerstone for sustainable electronics and transient applications

Electronic devices have become ubiquitous in modern society and are prevalent in every facet of human activities. Although electronic devices have brought much convenience and value, the insatiable appetite for newer and more attractive devices has also created a growing ecological problem: managing electronic waste or e-waste. As the lifetime of electronic devices gets shorter and shorter, the pressure on e-waste management systems is mounting with no abate in sight. Therefore, an alternative to traditional electronics must be sought. Bio-degradable electronics have thus emerged as the most viable and ideal replacement to address the issue of uncontrollable e-waste. Bio-degradability will ensure that the waste generated will be at least non-toxic and even environmentally friendly. Furthermore, bio-degradable organic materials have also been shown to be biocompatible and human-friendly, being able to be metabolized safely in the body without causing adverse physiological reactions. As such, this developing class of “green” electronics is not only able to alleviate the growing e-waste problem, but also fulfils niche applications interfacing with the human body. This Review will introduce various bio-degradable organic materials that can serve as substitutes for the different components of an electronic device, highlight recent research achievements and applications in implementing such bio-degradable devices as well as present an overview of the printing technologies available that provide the low-cost and high throughput advantages of solution-processable organic materials over the traditional inorganic materials.