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Using diverse technological approaches, many types of delivery devices have been used to supply plant nutrients at a controlled rate in the soil. One new approach is the use of hydrophilic polymers as carriers of plant nutrients. These polymers may be generally classified as 1) natural polymers derived from polysaccharides, 2) semi-synthetic polymers (primarily cellulose derivatives), and 3) synthetic polymers. By controlling the reaction conditions when forming the polymers, various degrees of cross-linking, anionic charge, and cationic charge can be added, thereby changing their effectiveness as fertilizer carriers.
Natural polymers, such as cellulose and latex, were first chemically modified in the 19th century to form celluloid and vulcanized rubber. The first totally synthetic polymer, Bakelite, was produced in 1907. The first semisynthetic fiber, rayon, was developed from cellulose in 1911. However, it was not until the global disruption caused by World War II, when natural sources of latex, wool, silk, and other materials became difficult to obtain, that synthetics were mass produced. Synthetic rubber was needed for tires, and nylon was needed as a replacement for silk for parachutes. Today synthetic polymers in the form of plastics are in wide use, and the plastics industry is one of the fastest growing in the United States and around the world. The industry produces approximately 150 kilograms of polymers per person annually in the United States.
Since the last decade, the methods of polymer synthesis, processing, and characterization are developing rapidly, which bring both challenges and opportunities to design novel polymeric biomaterials as well as to understand the biological behaviors between biological systems and polymeric materials. Therefore, we launch this special issue, including two review articles and four research articles, to summarize the application of synthetic polymers in biomedical engineering and to illustrate the new development of polymeric biomaterials.
In summary, this special issue connects the synthetic polymers to biomaterials science and engineering. We sincerely hope that the readers enjoy reading the presented original research work in this special issue and get inspired for their future studies.
As outlined earlier, a small number of plastic-degrading microbes have been isolated from plant- and animal-associated microbiomes [149, 151, 152]. However, most isolates reported in the literature were derived from soil [153, 154] or from waste processing sites such as composting facilities  and landfills . An additional source comprises bacteria and fungi already deposited in culture collections . All major synthetic polymers have species reported to degrade them, for instance PE [158, 159], PET [160, 161], PP , PS , PU  and PVC . However, the strength of evidence for degradation varies by plastic type. To date, PET biodegradation has been studied the most comprehensively. A notable example includes the PET-degrading bacterium, Ideonella sakaiensis, isolated from sediment in the vicinity of a Japanese bottle recycling plant . I. sakaiensis is the first organism for which the degradation of PET was well-described and the enzymatic degradation of PET elucidated, characterised  and enhanced . Conversely, there is only weak evidence for the biodegradation of synthetic polymers such as nylon, PP, PS and PVC. For instance, nylon-oligomer biodegradation by the bacterium Agromyces sp. KY5R has been shown by Yasuhira, et al.  and the genes and corresponding enzymes responsible for the biodegradation activity have been identified; however, biodegradation of the plastic polymer (i.e. not just monomers and oligomers) is yet to be confirmed.
Polymers, either synthetic or natural in origin, have been extensively evaluated as a solution for restoring functions in damaged neural tissues. Polymers offer a wide range of versatility, in particular regarding shape and mechanical characteristics, and their biocompatibility is unmatched by other biomaterials, such as metals and ceramics. Several studies have shown that polymers can be shaped into suitable support structures, including nerve conduits, scaffolds, and electrospun matrices, capable of improving the regeneration of damaged neural tissues. In general, natural polymers offer the advantage of better biocompatibility and bioactivity, while synthetic or non-natural polymers have better mechanical properties and structural stability. Often, combinations of the two allow for the development of polymeric conduits able to mimic the native physiological environment of healthy neural tissues and, consequently, regulate cell behaviour and support the regeneration of injured nervous tissues.
In neural tissue engineering, the use of natural polymers is highly beneficial due to their high biocompatibility and natural biodegradation kinetics combined with chemically tuneable properties. Often, natural polymers are analogues, if not identical like in the case of collagen, to substances already present in the human body, minimising the risks of cytotoxicity and immunogenic reaction upon implantation in the body . In neural tissue engineering, natural polymers can fulfil different roles, including matrix formers, gelling agents, or drug release modifiers, and they can be easily adjusted to fit a defect in a difficult physiological geometry, such as the spinal cord [48, 49]. Natural polymers applied in neural tissue engineering have different origins, such as extracellular matrix components (ECM), like collagen, polymers derived from marine life, like alginate, polymers derived from crustaceans, like chitosan, and polymers derived from insects, like silk. Natural polymers are the most researched type of polymer in neural tissue engineering and they have been preclinically studied at length in numerous animal models, including primates. In addition, collagen is the only biopolymer currently approved for clinical studies aimed at peripheral nerve regeneration. However, weak mechanical characteristics due to complex chemical structures, thermal sensitivity, and processing difficulties that frequently require use of solvents, hinder the efficacy of natural polymers, prompting researchers to combine them with synthetic or electroconductive polymers. Table 1 summarises the main natural polymers used in neural tissue engineering and their applications.
Primarily, gelatin has found applications in neural tissue engineering as electrospun combinations with other polymers, synthetic or natural in origin. The use of electrospinning as a fabrication technique for gelatin-based nerve conduits is particularly advantageous because it allows the optimisation and manipulation of mechanical, biological, and kinetic properties. In particular, electrospinning allows control over the orientation of the nanofibres, which is a key component in the creation of a functional scaffold . Fig. 3 offers a simple overview of the structure of as nerve conduit focusing in its three crucial parts: oriented substratum achieved through electrospinning, seeded support cells, and controlled release of NGF. 3D electrospun nanofibrous gelatin conduits allowed differentiation of motor neuron-like cells, showing great potential for applications in the CNS [66, 67]. The most common combination of hybrid polymer conduits is gelatin/PCL. Gelatin combined with PCL acted as a positive cue to support neurite outgrowth and allowed culture and proliferation of Schwann cells in vitro [68,69,70] and, more recently, a PCL/collagen blend incorporated into a gelatin matrix was used to bridge a 15mm gap in sciatic nerve in rats . Gelatin has also been successfully blended and electrospun with PLA, increasing differentiation into motor neurons lineages and promoting neurite outgrowth .
HA can be combined with other natural biopolymers, especially collagen due to the similar nature of the two biomaterials. For instance, Zhang et al. used neural stem cells embedded in a HA/collagen conduit to promote the regeneration of a 5mm facial nerve gap in rabbits . Combinations of HA and chitosan have also been successful in peripheral nerve regeneration. Li et al treated peripheral nerve crush injury in a rat model using chitosan conduits combined with HA , and Xu et al. used an injectable chitosan/HA biodegradable hydrogel for the regeneration of peripheral nerve injury . Further, blends of HA and biodegradable synthetic polymers, such as PLGA and poly-L-lysine, showed great potential for controlled delivery of drugs aimed at axonal regrowth after spinal cord injury in vitro  and in vivo .
Alginate has been recently used to create scaffolds for neural applications. Usually, hybrid scaffolds combine the biological characteristics of alginate with the mechanical properties of other biopolymers, both natural and synthetic in origin such as HA or PVA, showing great potential for peripheral nerve regeneration [105,106,107]. The leading application of alginate in neural tissue engineering is the treatment of spinal cord injury in rats, where it has been continuously successful in regenerating small nerve gaps, ranging from 2 to 4 mm [108,109,110,111].
Often, chitosan is used to enhance the biocompatibility of synthetic polymers with better mechanical characteristics. For example, chitosan was used to increase the biocompatibility of PVA in nanofibrous scaffolds, enhancing viability and proliferation of PC12 cells in vitro , and in PVA/SWCNTs structures, increasing the in vitro proliferation rate and integration of human derived brain cells U373 . Aligned PCL/chitosan fibres supported PC12 cells adhesion and growth, enhancing neurite extension along the fibre orientation . PLGA/chitosan scaffolds guided neuronal differentiation for peripheral nerve regeneration both in vitro and in vivo [121, 122]. 2b1af7f3a8