Investigating the Effect of Aligned Microtube Density on the Mechanical Strength of Hydrogel Scaffolds
Authors: Halder Sampa , Qavi Imtiaz , Tan George 
Published: August 2024

Abstract
Tissue engineering is a pivotal research domain, central to advancing biomedical manufacturing processes with the aim of fabricating functional artificial organs and tissues. Addressing the pressing concern of organ shortages and myriad medical challenges necessitates innovative manufacturing techniques. Hydrogel scaffolds, due to their biocompatibility and extracellular matrix-mimicking porous structure, have emerged as prime candidates in this arena. Moreover, their hygroscopic properties and tunable mechanical characteristics render them suitable for various tissue engineering applications. Despite their promising attributes, a significant manufacturing challenge persists: the optimization of cellular growth within the confines of hydrogel scaffolds. Effective vascularization, essential for optimal cellular nutrient and oxygen supply, remains elusive. Our previous manufacturing research tackled this, introducing a novel hybrid Bio-Fabrication technique. This technique integrated coaxial electrospinning and extrusion-based bioprinting methodologies, yielding hydrogel scaffolds fortified with microtubes. These strategically embedded microtubes, modeled after capillary structures, function as microchannel diffusion conduits, enhancing cellular viability within the hydrogel matrix. A core aspect of scaffold manufacturing is ensuring the stability of its 3D architecture, especially post-swelling. Preliminary hypotheses suggest a gamut of factors — including microtube shape, size, orientation, alignment, and density — play determinant roles in shaping the scaffold’s mechanical attributes. This research rigorously examines the mechanical evolution of hydrogel scaffolds when supplemented with aligned electrospun microtubes across a spectrum of densities. A blend of sodium alginate (SA) and gelatin was selected for the hydrogel matrix due to their inherent biocompatibility and favorable mechanical properties. Different concentrations were prepared to assess the optimal mixture for mechanical stability. A co-axial electrospinning setup was employed where polycaprolactone (PCL) was used as the sheath material and polyethylene oxide (PEO) functioned as the core. This dual material approach was intended to leverage the structural rigidity of PCL with the biodegradability of PEO. The spinning parameters, including voltage, flow rate, and tip-to-collector distance, were meticulously adjusted to produce aligned microtubes of varied densities and diameters. Once the microtubes were synthesized, they were layered within the hydrogel constructs. The layering process involved depositing a hydrogel layer, positioning the microtubes, and then sealing with another hydrogel layer. The entire structure was then solidified using calcium chloride, resulting in a robust, multi-layered composite. Post-fabrication, the hydrogel scaffolds underwent mechanical evaluations. Compression tests were employed to measure the compressive modulus. Tensile tests were conducted to determine ultimate tensile strength. These tests were crucial to understanding the impact of microtube density on the overall mechanical properties of the hydrogel scaffolds. The high-density group, while showing improved mechanical properties over the control group, did not surpass the low-density group, suggesting a possible saturation point. In conclusion, our research methodically explored the influence of microtube density on the mechanical and structural attributes of hydrogel scaffolds. The manufacturing insights gleaned hold substantive implications, promising to propel the field of tissue engineering and drive transformative advancements in biomedical manufacturing.