Salicylic acid-based poly(anhydride-ester) nerve guidance conduits: Impact of localized drug release on nerve regeneration

Polyanhydride Chemistry

Polyanhydrides (PAs) are a class of synthetic biodegradable polymers employed as controlled drug delivery vehicles. They can be synthesized and scaled up from low-cost starting materials. The structure of PAs can be manipulated synthetically to meet desirable characteristics. PAs are biocompatible, biodegradable, and generate nontoxic metabolites upon degradation, which are easily eliminated from the body. The rate of water penetrating into the polyanhydride (PA) matrix is slower than the anhydride bond cleavage. This phenomenon sets PAs as "surface-eroding drug delivery carriers." Consequently, a variety of PA-based drug delivery carriers in the form of solid implants, pasty injectable formulations, microspheres, nanoparticles, etc. have been developed for the sustained release of small molecule drugs, and vaccines, peptide drugs, and nucleic acid-based active agents. The rate of drug delivery is often controlled by the polymer erosion rate, which is influenced by the polymer structure and composition, crystallinity, hydrophobicity, pH of the release medium, device size, configuration, etc. Owing to the above-mentioned interesting physicochemical and mechanical properties of PAs, the present review focuses on the advancements made in the domain of synthetic biodegradable biomedical PAs for therapeutic delivery applications. Various classes of PAs, their structures, their unique characteristics, their physicochemical and mechanical properties, and factors influencing surface erosion are discussed in detail. The review also summarizes various methods involved in the synthesis of PAs and their utility in the biomedical domain as drug, vaccine, and peptide delivery carriers in different formulations are reviewed.

Biomaterials for Neural Tissue Engineering

The therapy of neural nerve injuries that involve the disruption of axonal pathways or axonal tracts has taken a new dimension with the development of tissue engineering techniques. When peripheral nerve injury (PNI), spinal cord injury (SCI), traumatic brain injury (TBI), or neurodegenerative disease occur, the intricate architecture undergoes alterations leading to growth inhibition and loss of guidance through large distance. To improve the limitations of purely cell-based therapies, the neural tissue engineering philosophy has emerged. Efforts are being made to produce an ideal scaffold based on synthetic and natural polymers that match the exact biological and mechanical properties of the tissue. Furthermore, through combining several components (biomaterials, cells, molecules), axonal regrowth is facilitated to obtain a functional recovery of the neural nerve diseases. The main objective of this review is to investigate the recent approaches and applications of neural tissue engineering approaches.

Methylcobalamin-Loaded PLCL Conduits Facilitate the Peripheral Nerve Regeneration

The feasible fabrication of nerve guidance conduits (NGCs) with good biological performance is important for translation in clinics. In this study, poly(d,l-lactide-co-caprolactone) (PLCL) films loaded with various amounts (wt; 5%, 15%, 25%) of methylcobalamin (MeCbl) are prepared, and are further rolled and sutured to obtain MeCbl-loaded NGCs. The MeCbl can be released in a sustainable manner up to 21 days. The proliferation and elongation of Schwann cells, and the proliferation of Neuro2a cells are enhanced on these MeCbl-loaded films. The MeCbl-loaded NGCs are implanted into rats to induce the regeneration of 10 mm amputated sciatic nerve defects, showing the ability to facilitate the recovery of motor and sensory function, and to promote myelination in peripheral nerve regeneration. In particular, the 15% MeCbl-loaded PLCL conduit exhibits the most satisfactory recovery of sciatic nerves in rats with the largest diameter and thickest myelinated fibers.

Tailoring synthetic polymeric biomaterials towards nerve tissue engineering: a review

The nervous system is known as a crucial part of the body and derangement in this system can cause potentially lethal consequences or serious side effects. Unfortunately, the nervous system is unable to rehabilitate damaged regions following seriously debilitating disorders such as stroke, spinal cord injury and brain trauma which, in turn, lead to the reduction of quality of life for the patient. Major challenges in restoring the damaged nervous system are low regenerative capacity and the complexity of physiology system. Synthetic polymeric biomaterials with outstanding properties such as excellent biocompatibility and non-immunogenicity find a wide range of applications in biomedical fields especially neural implants and nerve tissue engineering scaffolds. Despite these advancements, tailoring polymeric biomaterials for design of a desired scaffold is fundamental issue that needs tremendous attention to promote the therapeutic benefits and minimize adverse effects. This review aims to (i) describe the nervous system and related injuries. Then, (ii) nerve tissue engineering strategies are discussed and (iii) physiochemical properties of synthetic polymeric biomaterials systematically highlighted. Moreover, tailoring synthetic polymeric biomaterials for nerve tissue engineering is reviewed.

3D-printed nerve conduit with vascular networks to promote peripheral nerve regeneration

Peripheral nerve regeneration remains a challenge in tissue engineering and regenerative medicine. However, the existing approaches have limited regenerative capability. 3D-printed nerve conduits with well-defined properties are potent tools to facilitate peripheral nerve regeneration after injuries. Meanwhile, the vascular networks within the constructs can promote the exchange of oxygen, neurotrophic factors, and removal of waste products, thereby providing an advantageous microenvironment for tissue regeneration. It will be an interesting approach to integrate 3D-printed nerve conduit with vascular networks for the guidance of regenerated nerves. We hypothesize that 3D-printed vascularized nerve conduit will be an effective platform to promote nerve regeneration and functional restoration.

3D melatonin nerve scaffold reduces oxidative stress and inflammation and increases autophagy in peripheral nerve regeneration

Peripheral nerve defect is a common and severe kind of injury in traumatic accidents. Melatonin can improve peripheral nerve recovery by inhibiting oxidative stress and inflammation after traumatic insults. In addition, it triggers autophagy pathways to increase regenerated nerve proliferation and to reduce apoptosis. In this study, we fabricated a melatonin-controlled-release scaffold to cure long-range nerve defects for the first time. 3D manufacture of melatonin/polycaprolactone nerve guide conduit increased Schwann cell proliferation and neural expression in vitro and promoted functional, electrophysiological and morphological nerve regeneration in vivo. Melatonin nerve guide conduit ameliorated immune milieu by reducing oxidative stress, inflammation and mitochondrial dysfunction. In addition, it activated autophagy to restore ideal microenvironment, to provide energy for nerves and to reduce nerve cell apoptosis, thus facilitating nerve debris clearance and neural proliferation. This innovative scaffold will have huge significance in the nerve engineering.

The effect of four types of artificial nerve graft structures on the repair of 10-mm rat sciatic nerve gap

Investigating the effect of four types of artificial nerve graft (ANG) structures on rat sciatic nerve defect repair will aid future ANG designs. In this study, fibroin fibers and polylactic acid were used to prepare four ANGs with differing structures: nerve conduit with micron-sized pores (Conduit with pore group), nerve conduit without micron-sized pores (Conduit group), nerve scaffold comprising Conduit with pore group material plus silk fibers (Scaffold with pore group), and nerve scaffold comprising Conduit group material plus silk fibers (Scaffold group). ANGs or autologous nerves (Autologous group) were implanted into 10 mm rat sciatic nerve defects (n = 50 per group). Twenty weeks after nerve grafting, the time required to retract the surgical limb from the hot water was ranked as follows: Conduit with pore group > Scaffold with pore group > Conduit group > Scaffold group > Autologous group. The static sciatic index was ranked in descending order: Autologous group > Scaffold group > Conduit group > Scaffold with pore group > Conduit with pore group. Immunofluorescence staining identified significant differences in the distribution and number of axons, Schwann cells, and fibroblasts. These findings indicate that ANGs with micron-sized pores had a negative impact on the repair of peripheral nerve defects, while internal microchannels were beneficial. © 2017 The Authors. Journal of Biomedical Materials Research Part A Published by Wiley Periodicals, Inc. J Biomed Mater Res Part A: 105A: 3077-3085, 2017.

Polymeric prodrug-functionalized polypropylene films for sustained release of salicylic acid

Medical devices decorated with salicylic acid-based polymer chains (polymeric prodrug) that slowly release this anti-inflammatory and anti-biofilm drug at the implantation site were designed. A "grafting from" method was implemented to directly grow chains of a polymerizable derivative of salicylic acid (2-methacryloyloxy-benzoic acid, 2MBA) onto polypropylene (PP). PP was modified both at bulk and on the surface with poly(2MBA) by means of an oxidative pre-irradiation method ((60)Co source), in order to obtain a grafted polymer in which salicylic acid units were linked by means of labile ester bonds. The grafting percent depended on absorbed dose, reaction time, temperature and monomer concentration. The functionalized films were analyzed regarding structure (FTIR-ATR, SEM-EDX, fluorescence microscopy), temperature stability (TGA), interaction with aqueous medium (water contact angle and swelling), pH-responsive release and cytocompatibility (fibroblasts). In the obtained poly(2MBA)-grafted biomaterial, poly(2MBA) behaved as a polymeric prodrug that regulates salicylic acid release once in contact with aqueous medium, showing pH-dependent release rate.

Polyanhydrides: Synthesis, Properties, and Applications

This review focusses on polyanhydrides, a fascinating class of degradable polymers that have been used in and investigated for many bio-related applications because of their degradability and capacity to undergo surface erosion. This latter phenomenon is driven by hydrolysis of the anhydride moieties at the surface and high hydrophobicity of the polymer such that degradation and mass loss (erosion) occur before water can penetrate deep within the bulk of the polymer. As such, when surface-eroding polymers are used as therapeutic delivery vehicles, the rate of delivery is often controlled by the rate of polymer erosion, providing predictable and controlled release rates that are often zero-order. These desirable attributes are heavily influenced by polymer composition and morphology, and therefore also monomer structure and polymerization method. This review examines approaches for polyanhydride synthesis, discusses their general thermomechanical properties, surveys their hydrolysis and degradation processes along with their biocompatibility, and looks at recent developments and uses of polyanhydrides in drug delivery, stimuli-responsive materials, and novel nanotechnologies.