Tough and tunable scaffold-hydrogel composite biomaterial for soft-to-hard musculoskeletal tissue interfaces

Abstract

Tendon inserts into bone via a fibrocartilaginous interface (enthesis) that reduces mechanical strain and tissue failure. Despite this toughening mechanism, tears occur because of acute (overload) or degradative (aging) processes. Surgically fixating torn tendon into bone results in the formation of a scar tissue interface with inferior biomechanical properties. Progress toward enthesis regeneration requires biomaterial approaches to protect cells from high levels of interfacial strain. We report an innovative tissue reinforcement strategy: a stratified scaffold containing osseous and tendinous tissue compartments attached through a continuous polyethylene glycol (PEG) hydrogel interface. Tuning the gelation kinetics of the hydrogel modulates integration with the flanking compartments and yields biomechanical performance advantages. Notably, the hydrogel interface reduces formation of strain concentrations between tissue compartments in conventional stratified biomaterials that can have deleterious biological effects. This design of mechanically robust stratified composite biomaterials may be appropriate for a broad range of tendon and ligament-to-bone insertions.

Fig. 1. PEG hydrogel cross-linking reaction and triphasic scaffold fabrication.

(A) Formation of a cross-linked PEG network via HRP catalyzed cross-linking. Initially, hydrogen peroxide (H₂O₂) reacts with HRP in its inactive state. Activated HRP oxidizes tyramine to form phenolic radicals that will oxidize thiol groups. Thiol groups on 4-arm PEG-thiol (PEG-SH) monomers are oxidized to thiol radicals that readily form disulfides over time to create a cross-linked polymer network. (B) A suspension-layering lyophilization method is used to incorporate a PEG hydrogel layer between tendinous (CG) and osseous (CGCaP) collagen-GAG compartments. First, CG and CGCaP liquid suspensions and the PEG hydrogel precursor solution are layered into a mold and allowed to mix diffusively at their interface as the PEG precursor solution gels. (C) Following lyophilization, structurally continuous triphasic scaffolds are generated with a distinct interfacial PEG hydrogel layer between tendinous and osseous tissue compartments.

Fig. 2. Interfacial hydrogel topology and width.

(A) Representative ESEM images of triphasic scaffolds show that incorporation of the interfacial hydrogel phase (green dashed line region) and (B) interface width are dependent on the hydrogel gelation. Groups not sharing a letter are significantly different (P < 0.05). Triphasic scaffolds are identified via unique interface hydrogel gelation parameters (t_cross:Δt_gel:G′_eq) that describe time to viscous-elastic transition (t_cross), time to complete gelation (Δt_gel), and final elastic properties (G′_eq).

Fig. 3. Bulk scaffold mechanical properties under uniaxial tension.

(A) Bulk scaffold toughness through the point of failure. (B) Averaged stress-strain curves for highest-toughness triphasic scaffold variants (left) and all other triphasic variants (right) versus biphasic scaffolds. (C) Maximum tensile stress and (D) strain at scaffold fracture. (E) Bulk elastic modulus of scaffolds. (F) Bulk scaffold toughness up to physiological levels of strain (3%). (G) Averaged stress-strain curves for highest-toughness triphasic scaffold variants (left) and all other triphasic variants (right) versus biphasic scaffolds up to physiological levels of strain (3%). Groups not sharing a letter are significantly different (P < 0.05). For stress-strain curves, linear interpolation was used to average multiple stress-strain curves for each scaffold. Triphasic scaffolds are identified via unique interface hydrogel gelation parameters (t_cross:Δt_gel:G′_eq) that describe time to viscous-elastic transition (t_cross), time to complete gelation (Δt_gel), and final elastic properties (G′_eq).

Fig. 4. Local strain profiles across scaffolds under uniaxial tension.

(A) Representative profiles of local strain across the entire scaffold and (B) within the middle regions of biphasic, med:long:high, fast:long:high, and fast:slow:low scaffolds at global applied strains of 0, 1.1, 2.2, and 3.3% (strains are with respect to the full-length scaffolds). See fig. S5 for individually scaled heatmaps of local strain across scaffolds. Triphasic scaffolds are identified via unique interface hydrogel gelation parameters (t_cross:Δt_gel:G′_eq) that describe time to viscous-elastic transition (t_cross), time to complete gelation (Δt_gel), and final elastic properties (G′_eq).