(10c) Fabricating Scaffolds with Gradients in Fiber Alignment and Chemistry for Interfacial Tissue Engineering

INTRODUCTION: Although there have been significant advances within tissue engineering, translation has proven difficult with ongoing challenges regenerating complex tissues with a heterogeneous structure and multiple cell types. For example, the interfacial tissue between adjacent tissues has complex gradients of structure, cell type, and chemical composition. In particular, the tendon to bone junction transitions from highly aligned tendon tissue to calcified bone [1]. For musculoskeletal interfacial tissues (e.g., ligament or tendon to bone, muscle to tendon, cartilage to bone, etc.), the interfacial region is critical for dispersing mechanical loads from one tissue type to another. Designing innovative biomaterial scaffolds that mimic the heterogeneous properties of these interfacial tissues, as well as other complex tissues, is vital for spatially influencing cellular behavior and inducing proper tissue regeneration. Towards this aim, we have developed a magnetic electrospinning technique to selectively and precisely control gradients in fiber alignment and chemistry.

METHODS: Multiple different types of polymers were electrospun onto a flat plate collector containing one or more magnet(s) in a range of configurations. Scanning electron microscopy was performed to quantify fiber diameter and alignment. Mesenchymal stromal cells (MSCs) were seeded onto fibrous scaffolds and cell aspect ratio evaluated as a function of scaffold region. To spatially control fiber surface chemistry, thiol containing peptides were conjugated to photoreactive polymers (i.e. norbornene-modified polymers) using a sliding photomask with a velocity between 0-100 mm/s. Unconjugated peptides were removed via washing prior to analysis. Fluorescence microscopy was used to quantify peptide fluorescence intensity as a function of scaffold length. Of particular interest, YGNAE-Glu₈ peptide, was tethered to spatially control mineralization after incubated with simulated body fluid (SBF). Lastly, gradient and non-gradient scaffolds were evaluated using digital image correlation (DIC) to evaluate local strain during tensile testing.

RESULTS: Magnetically-assisted electrospinning resulted in fiber alignment over the magnet surface and transitioned to unaligned fibers away from the magnet. Moreover, the transition length between aligned and unaligned fibers occurred over a scale of 1 mm, similar to the scale observed in native tendon-bone junctions. MSCs seeded onto gradient scaffolds showed higher aspect ratios and alignment on the aligned fibers compared to MSCs on the unaligned fibers. To show the versatility of this magnetic electrospinning apparatus, various fiber alignment configurations were tested. Magnets placed close together resulted in a lengthening of the aligned fiber region. Magnets placed far away (1 cm) result in fibers aligning (region M1), unaligning (region C), and then realigning again (region M2) (Fig. 1D). Other complex shapes, such as “T” and “L”, and an interdigitated interface were also created. To create scaffolds with gradients in chemistry, peptides were conjugated to photoreactive electrospun scaffolds using a sliding photomask (Fig. 2A). Using this technique and fluorescent molecules for visualization, a dual, opposing, gradient in chemistry was manufactured (Fig. 2B). Additionally, tethering YGNAE-Glu₈ peptide drastically increased mineralization. Ongoing work includes tethering YGNAE-Glu₈ peptide gradients to spatially control mineralization on fibrous scaffolds.

DISCUSSION: Magnetic electrospinning allows for precise spatial control over fiber alignment that can replicate the structure of musculoskeletal interfacial tissues, such as the tendon-bone junction. Conjugating molecules onto fibrous scaffolds using a sliding mask can easily generate multiple chemistry gradients at a high resolution with a similar scale compared to native interfacial tissues. Ongoing work is combining these two systems into a single material platform with controllable gradients in both fiber alignment and chemistry. Additional current work is analyzing MSC behavior as a function of fiber alignment and chemistry under static and dynamic loading conditions.

REFERENCES: [1] Tellado, et al. Advanced Drug Delivery Reviews, 94:2015. [2] Mozdzen, et al. Journal of the Mechanical Behavior of Biomedical Materials, 66:2017.