Species and density of implant surface chemistry affect the extent of foreign body reactions

Abstract

Implant-associated fibrotic capsule formation presents a major challenge for the development of long-term drug release microspheres and implantable sensors. Since material properties have been shown to affect in vitro cellular responses and also to influence short-term in vivo tissue responses, we have thus assumed that the type and density of surface chemical groups would affect the degree of tissue responses to microsphere implants. To test this hypothesis, polypropylene particles with different surface densities of -OH and -COOH groups, along with the polypropylene control (-CH2 groups) were utilized. The influence of functional groups and their surface densities on fibrotic reactions were analyzed using a mice subcutaneous implantation model. Our comparative studies included determination and correlation of the extents of fibrotic capsule formation, cell infiltration into the particles, and recruitment of CD11b+ inflammatory cells for all of the substrates employed. We have observed major differences among microspheres coated with different surface functionalities. Surfaces with -OH surface groups trigger the strongest responses, while -COOH-rich surfaces prompt the least tissue reactions. However, variation of the surface density of either functional group has a relatively minor influence on the extent of fibrotic tissue reactions. The present results show that surface functionality can be used as a powerful tool to alter implant-associated fibrotic reactions and, potentially, to improve the efficacy and function of drug-delivery microspheres, implantable sensors, and tissue-engineering scaffolds.

Figure 1

FT-IR transmission spectra of EO2V polymer films produced as a function of the average power input during the pulsed plasma depositions. Spectra are arranged in order of decreasing power input, reading top to bottom. The exact plasma conditions employed are shown in Table 1.

Figure 2

C(1s) high resolution XPS spectra of EO2V polymer films arranged in order of decreasing power input (top to bottom) employed during the deposition process.

Figure 3

FT-IR transmissionspectra of VAA polymer films produced at different average power input conditions during the pulsed plasma depositions. Spectra are arranged in order of decreasing power input, reading top to bottom. The exact plasma conditions employed are shown in Table 2.

Figure 4

C(1s) high resolution XPS spectra of VAA films arranged in order of decreasing average power input (topto bottom) employed during the deposition process.

Figure 5

Implant-induced tissue responses were assessed based on capsule thickness and cellular infiltration. Polypropylene microspheres with –OH and –COOH functionalities at high, medium and low densities along with control –CH2 functionality were subcutaneously injected in to Balb/C mice. H&E stain of subcutaneous tissue 2 weeks post implantation shows the tissue reaction to different surface functionalities. ( ) shows the trend followed by capsule thickness and the extent of cell infiltration in to the implant across functional groups (Magnification 20x).

Figure 6

Extent of tissue responses to polypropylene microspheres with –OH, -COOH and control –CH2 functionality were assessed based on (A) capsule thickness and (B) cell infiltration distance into particle implants. Polypropylene (PP) microspheres coated with high, medium and low densities of –OH and –COOH groups along with control –CH2 were subcutaneously implanted in Balb/C mice. The animals were sacrificed at 2 weeks post implantation. Values shown reflect the average thickness of the capsule ± SD (n= 3 for -OH, -COOH and –CH₂).

Figure 7

The deposition of collagen along the implant surrounding capsule was observed by Masson Trichrome stain. Polypropylene microspheres with –OH and –COOH functionalities at high, medium and low densities along with control –CH2 functionality were subcutaneously injected in to Balb/C mice. Masson Trichrome stain of subcutaneous tissue 2 weeks post implantation shows the tissue reaction to different surface functionalities which leads to deposition of collagen. ( ) shows the extent of collagen deposition. (Magnification 20x).

Figure 8

Immunohistochemical analyses of the recruitment of CD11b+ inflammatory cells to particle implants. Polypropylene particles coated with different surface functionalities were subcutaneously implanted in Balb/C mice. The animals were sacrificed at 2 weeks post implantations. Accumulation of CD11b + cells at the capsule highest for particles implant with –OH group. Values shown reflect the average number of CD11b+ cells seen at the capsule for different surface functionalities. ± SD (n= 4 for -OH, -COOH and –CH₂).

Figure 9

The linear correlations among capsule thickness, CD11b+ cell number, and cell infiltration depth. (A) The capsule thicknesses are graphed against cell infiltration depth. (B) The capsule thicknesses are graphed against CD11b+ cell numbers. (C) The cell infiltration depths are graphed against CD11b+ cell numbers.