(668h) Using Magnetic Fields to Control Fiber Alignment within Fiber-Hydrogel Composites

INTRODUCTION: Fibrous connective tissues, such as ligaments and tendons, are commonly injured. Current treatment options fail to regenerate the highly organized fibrous structure present in these tissues, often resulting in poor clinical outcomes. Tissue engineering is a promising approach that uses a combination of cells, signaling cues, and a scaffold to regenerate functional tissue. Further research is required to understand the role of physical cues (e.g. fiber alignment) in directing new tissue organization. To address this need, we designed a fiber-hydrogel composite with in-situ control over fiber alignment. Magnetic nanoparticles were incorporated into short, electrospun, fibers to enable alignment in the presence of a magnetic field. Magnetically-responsive fibers were embedded within static, covalently crosslinked, hydrogels and dynamic, non-covalently crosslinked, hydrogels. Fiber alignment is locked in place once crosslinking occurs in static hydrogels, while fiber alignment can be continuously modified at user-controlled timepoints in dynamic hydrogels. In this study, we characterized fiber alignment as a function of magnetic field strength and exposure time within both hydrogel systems. Additionally, preliminary studies evaluating cell viability and morphology within these materials is ongoing.

METHODS: Fibrous mats were formed by electrospinning polycaprolactone dissolved in 90:10 chloroform: acetic acid which contained superparamagnetic iron oxide nanoparticles (SPIONs) and rhodamine for visualization. The fibrous mats were cut using a cryotome to form short, 40 µm length fibers. Static hydrogels were synthesized using norbornene-modified hyaluronic acid (HA) and crosslinked using UV light. Dynamic hydrogels were synthesized using guest-host chemistry by combining adamantane-modified HA (guest) with β-cyclodextrin-modified HA (host). Short fibers were mixed within the hydrogel precursor solutions to create the fiber-hydrogel composites. Rheology was performed to determine the composite’s mechanical properties. Fiber alignment within the hydrogels was visualized using microscopy and quantified as a function of magnetic field strength (100-300 mT), SPION concentration (0, 3, 5, and 7 wt%), and exposure time (0.5-20 mins). Cell compatibility and morphology will be assessed using a colorimetric cell viability assay, as well as DAPI (cell nucleus) and phalloidin (F-actin) staining.

RESULTS AND DISCUSSION: To create magneto-responsive fibers, SPIONs were encapsulated in electrospun polycaprolactone fibers and chopped to 40 µm in length to allow for embedding within the hydrogels. When a magnetic field was applied to the fiber-hydrogel composites, the fibers aligned in the same direction as the magnetic field. Fiber alignment increased with exposure time and plateaued around 10 minutes of exposure. After removing the magnetic field, the fibers did not magnetically attract to nearby fibers and remained aligned. This demonstrates the superparamagnetic properties of the SPIONs. Fibers without SPIONs served as controls and did not change alignment with magnetic field exposure, thus remaining in the unaligned state. We anticipate that fibers with more SPIONs will align more quickly than fibers with less SPIONs. The inclusion of fibers did not affect the shear-thinning or self-healing properties of the dynamic hydrogel. This work demonstrates that using magneto-responsive fibers allows for in situ control over fiber alignment at user-defined timepoints within dynamic hydrogels. Ongoing work is evaluating the in vitro cell viability and morphology in response to alignment within these fiber-hydrogel composite materials.

Figure 1. Brightfield microscopy images of short fibers in HA solution a) before and b) after exposure to a 300 mT magnetic field for 10 minutes. Images were modified using ImageJ to improve fiber visualization and c) calculate fiber alignment.