Bijel-templated implantable biomaterials for enhancing tissue integration and vascularization

Abstract

Mitigation of the foreign body response (FBR) and successful tissue integration are essential to ensuring the longevity of implanted devices and biomaterials. The use of porous materials and coatings has been shown to have an impact, as the textured surfaces can mediate macrophage interactions with the implant and influence the FBR, and the pores can provide space for vascularization and tissue integration. In this study, we use a new class of implantable porous biomaterials templated from bicontinuous interfacially jammed emulsion gels (bijels), which offer a fully percolating, non-constricting porous network with a uniform pore diameter on the order of tens of micrometers, and surfaces with consistent curvature. We demonstrate that these unique morphological features, inherent to bijel-templated materials (BTMs), can enhance tissue integration and vascularization, and reduce the FBR. Cylindrical polyethylene glycol diacrylate (PEGDA) BTMs, along with PEGDA particle-templated materials (PTMs), and non-templated materials (NTMs), were implanted into the subcutaneous space of athymic nude mice. After 28 days, implants were retrieved and analyzed via histological techniques. Within BTMs, blood vessels of increased size and depth, changes in collagen deposition, and increased presence of pro-healing macrophages were observed compared to that of PTM and NTM implants. Bijel templating offers a new route to biomaterials that can improve the function and longevity of implantable devices. STATEMENT OF SIGNIFICANCE: All implanted biomaterials are subject to the foreign body response (FBR) which can have a detrimental effect on their efficacy. Altering the surface chemistry can decrease the FBR by limiting the amount of proteins adsorbed to the implant. This effect can be enhanced by including pores in the biomaterial to allow new tissue growth as the implant becomes integrated in the body. Here, we introduce a new class of self-assembled biomaterials comprising a fully penetrating, non-constricting pore phase with hyperbolic (saddle) surfaces for enhanced tissue integration. These unique morphological characteristics result in dense blood vessel formation and favorable tissue response properties demonstrated in a four-week implantation study.

Keywords: Bijel; Foreign body response; Microstructure; Porous implants; Vascularization.

Fig. 1.

Bijel formation schematic. (a) Particles are dispersed in a critical composition of water and 2,6-lutidine (mole fraction of 2,6-luitidine, x_lut = 0.064). (b) Heating above the lower critical solution temperature (34.1 °C) prompts spinodal decomposition and particle adsorption at the coarsening interface. (c) The system arrests as the interface becomes completely occupied by particles.

Figure 2.

Template materials and scanning electron microscopy micrographs of polymer implants. Pictured are (a) BTM, (b) PTM, and (c) NTM polyethylene glycol diacrylate implants. Scale bar, 100 μm. Superimposed red circle diameter in panels (a) and (b), 32 μm.

Figure 3.

Histology and second harmonic generation (SHG) of BTM (a), PTM (b), and NTM (c) implants. Histology sections stained with hematoxylin and eosin (H&E) shown in row 1 and Masson’s trichrome (MT) shown in row 2. Scale bar, 100 μm. Sections imaged using second harmonic generation (SHG) shown in row 3. Filled arrows and open arrows denote blood vessels and collagen networks, respectively. White dashed line denotes tissue-implant boundary with implant oriented on top. Scale bar, 50 μm.

Figure 4.

Vessel immunohistochemistry and quantification. Shown are CD31 (red), αSMA (green), and DAPI (blue) labeling in BTM (a) and PTM (b) implants. DAPI counterstaining shown in blue. Dashed lines denote implant boundary, an arrow denotes a thin vessel extending through pore-pore windows, a diamond denotes a αSMA+ cell, and stars denote CD31+ cells. Scale bar, 50 μm. Vessel area versus distance to nearest implant boundary in BTM (c) and PTM (d) implants. Vessel area versus distance to boundary (200 μm bin width), plotted for all vessels across all mice (e), and as mean ± standard error of the mean of total vessel area per mouse (n=4) (f). *p < 0.05

Figure 5.

Macrophage immunohistochemistry and quantification. Shown are F4/80 (green) and CD206 (red) labeling in BTM (a) and PTM (b) implants. DAPI counterstaining shown in blue. Dashed lines denote implant boundary. Scale bar, 50 μm. Percent CD206+ cells relative to total F4/80+ cells for both implant types in each mouse (c-d). *p < 0.05.