Directing Axonal Growth: A Review on the Fabrication of Fibrous Scaffolds That Promotes the Orientation of Axons

You'll need this axonal sprouting so ask your doctor what this will do for your recovery.

Directing Axonal Growth: A Review on the Fabrication ofFibrous Scaffolds That Promotes the Orientation of Axons

Devindraan Sirkkunan , Belinda Pingguan-Murphy and Farina Muhamad * Department of Biomedical Engineering, Faculty of Engineering, Universiti Malaya, Kuala Lumpur 50603, Malaysia; ezvn2066@gmail.com (D.S.); bpingguan@gmail.com (B.P.-M.) * Correspondence: farinamuhamad@um.edu.my Abstract: Tissues are commonly defined as groups of cells that have similar structure and uniformly perform a specialized function. A lesser-known fact is that the placement of these cells within these tissues plays an important role in executing its functions, especially for neuronal cells. Hence, the design of a functional neural scaffold has to mirror these cell organizations, which are brought about by the configuration of natural extracellular matrix (ECM) structural proteins. In this review, we will briefly discuss the various characteristics considered when making neural scaffolds. We will then focus on the cellular orientation and axonal alignment of neural cells within their ECM and elaborate on the mechanisms involved in this process. A better understanding of these mechanisms could shed more light onto the rationale of fabricating the scaffolds for this specific functionality. Finally, we will discuss the scaffolds used in neural tissue engineering (NTE) and the methods used to fabricate these well-defined constructs. Keywords: cellular orientation; fiber alignment; neural tissue engineering 1. Introduction Over the past 25 years, neurological disorders (ND) have been the leading case of disability and death worldwide [1]. According to the Global Burden of Disease Study (2015), ND is the leading cause of disability-adjusted life-years (DALY) in 2015 (229.1 to 274.7 million or 10.2% of global DALYs) and the second leading group of deaths (9.1 to 9.7 million or 16.8% of global deaths) [1]. ND such as Alzheimer’s accounts for the second highest number of deaths, whereas other motor neuron diseases still account for a fairly large number of deaths globally [1]. These neurological diseases involve the loss of neurons and synapses in various parts of the brain, spinal cord, and other parts of the all-encompassing peripheral nervous system. Despite the substantial decrease in mortality rates from stroke and communicable ND, its burden has increased in the past 25 years due to gradual increment of the aged population [1]. Hence, there is a need to prepare more efficient methods to provide adequate treatment for the ever-growing number of patients with ND. The current treatment strategy for ND that involves neuronal loss, such as trauma to the spinal cord, is definitive surgical decompression and/or stabilization [2]. Autologous peripheral nerve graft has also been used as a treatment for Parkinson’s disease [2]. However, this procedure of stabilization requires the transference of nerve from another part of the nervous system, as seen in the excision of sural nerve containing Schwann cells and its delivery into the Parkinson’s disease affected substantia nigra [2]. The increment in morbidity to patients using surgical procedures drives the research for other avenues in ND treatment technologies. As an alternative to nerve transplant, stem cell therapy provides a renewable source of auxiliary cells and tissues for a variety of ND [3]. Bone marrow cell transplantation has been used to treat spinal cord injury, and it is shown to be a viable option for patients with complete spinal cord injury [4]. However, the study shows only small improvements in the treatment of acute and sub-acute groups, but not in chronic groups [4]. This is may be due to the fact that the stem cells require a scaffold and vector to improve its functionality. Biomimetic nanofibrous scaffolds has been developed for NTE in order to provide sustained growth factor/drug release or to support cell growth in situ [5,6]. The ability to fine tune the biochemical properties of the nanofibers enables researchers to produce biomaterials that could mimic the ECM of native tissues [7,8]. This potential coupled with a high surface area to volume ratio and superior biocompatibility provides a technique to reduce cell death or neuropathy due to nonphysiological local stress [8,9]. Many methods have been employed to fabricate these scaffolds according to the desired functionality and specifications. Electrospinning is frequently used to fabricate scaffolds due to its ability to manipulate the developmental parameters such as porosity, surface area, fiber diameter, and its alignment therein [10]. It is considered as a standard technique for producing nanofibers in the field of NTE [11,12]. Another immerging technique to fabricate neural scaffolds is microfluidics [13,14]. This method requires no application of high temperature or voltage [15,16]. There are also other novel methods being developed to produce neural scaffolds, such as isoelectric focusing [17], wet spinning [18], and thermal drawing process [19]; with each technique improving the scaffolds attributes that could enhance and direct cellular growth. Development of these fabrication techniques adheres to certain protocols when it comes to scaffold production for tissue engineering purposes. Regardless of tissue types, a sustainable scaffold needs to be biocompatible and biodegradable [20]. Mechanical properties of the scaffolds are also an important consideration because the various culturable cells requires ECM of different stiffness for efficient growth [20]. Scaffold architecture such as the level of porosity and its interconnectivity has to be taken into account for 3D cell culture [20]. Finally, the methods used to fabricate these scaffolds has to be cost effective and up-scalable [20]. The fabrication of neural scaffolds applies these criterion guidelines in a more cell specific manner. Neural cells respond to a distinct topological cue, where the neuronal outgrowth needs to be guided and the connection between neurons has to be established for efficient growth [21]. Bearing this in mind, the scaffolds should mimic the native tissue ECM’s topological, mechanical, biochemical, and electrical cues to promote better contact guidance, adhesion, and proliferation of neuronal cells [21]. The radical scavenging ability should also be incorporated in to the scaffolds to minimize secondary progression of injury [21]. These scaffolds that are shaped and solidified in vitro requires surgical insertion into the affected site. A more immediate method of therapy would include on-site treatment to reduce scarring or accumulation of inhibitory proteins. Hence, in recent years, researchers look to injectable hydrogels as a viable option for minimally invasive treatment of various injuries [22–24]. These hydrogels could be administered immediately after injury, and since it does not require surgery to apply the scaffold, the morbidity from trauma would be greatly reduced [25]. However, injectable hydrogels do not have defined microarchitecture that is found in in-vitro patterned scaffolds [26–28]. This topology on scaffolds plays an important role in orientating cellular growth and building appropriate micro-structures that could provide sustained tissue development [26–28]. Current advancements in this new class of hydrogels employs nanoparticles and nanotubes to direct polymer fibers therein and in effect, direct the alignment of cells [26–28]. In this review, the examination of currently used hydrogels for NTE is presented. The recent methods of fabricating scaffolds with aligned microarchitecture is also outlined; where it ranges from the most commonly used techniques hitherto such as electrospinning to more novel ones that are currently being developed. Overall, the processes that leads to cellular alignment within these scaffolds or during the fabrication procedure are reviewed.