Citric-acid-derived photo-cross-linked biodegradable elastomers

Abstract

Citric-acid-derived thermally cross-linked biodegradable elastomers (CABEs) have recently received significant attention in various biomedical applications, including tissue-engineering orthopedic devices, bioimaging and implant coatings. However, citric-acid-derived photo-cross-linked biodegradable elastomers are rarely reported. Herein, we report a novel photo-cross-linked biodegradable elastomer, referred to as poly(octamethylene maleate citrate) (POMC), which preserves pendant hydroxyl and carboxylic functionalities after cross-linking for the potential conjugation of biologically active molecules. Pre-POMC is a low-molecular-mass pre-polymer with an average molecular mass between 701 and 1291 Da. POMC networks are soft and elastic with an initial modulus of 0.07 to 1.3 MPa and an elongation-at-break between 38 and 382%. FT-IR-ATR results confirmed the successful surface immobilization of type-I collagen onto POMC films, which enhanced in vitro cellular attachment and proliferation. Photo-polymerized POMC films implanted subcutaneously into Sprague-Dawley rats demonstrated minimal in vivo inflammatory responses. The development of POMC enriches the family of citric-acid-derived biodegradable elastomers and expands the available biodegradable polymers for versatile needs in biomedical applications.

Figure 1

Schematics of POMC synthesis. Step 1: Synthesis of pre-POMC by reacting citric acid, maleic acid and 1,8-octanediol. Ratios of maleic acid and citric acid were varied as 10:0, 8:2, 6:4, 4:6 and 2:8, respectively, whereas the ratio of overall acid to diol was kept as 1:1. All the monomers were melted at 160°C under a nitrogen blanket. Further polymerization was continued at 135°C for 125 min to obtain a low-molecular-weight pre-polymer. Step 2: Photo-cross-linking of pre-POMC was performed under 365 nm UV light for 3 min in the presence of a photoinitiator (Igacure 2959).

Figure 2

Structural characterization of pre-POMC. (A) MALDI-MS spectra of pre-POMC8/2. (B) ¹H-NMR spectra of pre-POMC8/2. (C) FT-IR spectra of pre-polymers: (a) pre-POMC4/6, (b) pre-POMC6/4, (c) pre-POMC8/2 and (d) pre-POMC 10/0. This figure is published in colour in the online edition of this journal, that can be accessed via http://www.brill.nl/jbs

Figure 3

Thermal characterization of POMC. (A) TGA graph of (a) POMC4/6, (b) POMC8/2 and (c) POMC10/0 films indicating the decomposition temperature (T_d, at 10% weight loss) at 363°C, 310°C and 344°C, respectively. (B) DSC thermograph of (a) POMC4/6, (b) POMC8/2 and (c) POMC10/0 films with respective T_g at −9°C, −28°C and −36°C. This figure is published in colour in the online edition of this journal, that can be accessed via http://www.brill.nl/jbs

Figure 4

Mechanical properties of photo-cross-linked POMC with varying polymerization conditions. (A) Effects of concentration of photoinitiator while cross-linking prePOMC8/2 (50 wt% in DMSO) on polymer initial modulus and elongation (n = 6). (B) Effect of polymer concentration in DMSO while cross-linking pre-POMC8/2 with 2% of photoinitiator concentration on polymer initial modulus and elongation (n = 6).

Figure 5

Swelling and sol content of photo-cross-linked POMC with varying polymerization conditions. (A) Effect of maleic acid and citric acid ratio cross-linked with 2% photoinitiator and 50% polymer in DMSO. (B) effect of concentration of photoinitiator while cross-linking pre-POMC8/2 with 50% polymer in DMSO and (C) effect of polymer concentration in DMSO while cross-linking pre-POMC8/2 with 2% photoinitiator. Swelling agents are used as DMSO and PBS. Sol content was leached out from the film in DMSO (n = 6).

Figure 6

In vitro degradation of POMC in PBS (pH 7.4, 37°C). (a) POMC 10/0, (b) POMC8/2, (c) POMC4/6 and (d) POMC6/4. This figure is published in colour in the online edition of this journal, that can be accessed via http://www.brill.nl/jbs

Figure 7

The FT-IR–ATR spectra of the POMC8/2 film and collagen-immobilized POMC 6/4 film. Amide I, amide II and amine, representing peaks at 1648, 1544 and 3450 cm⁻¹ in collagen-immobilized POMC8/2 film, illustrate the successful conjugation of collagen to POMC surface. This figure is published in colour in the online edition of this journal, that can be accessed via http://www.brill.nl/jbs

Figure 8

(a, b) Photomicrograph of NIH-3T3 cell culture for 48 h on (a) POMC8/2 film and (b) collagen-immobilized POMC8/2 film. (c, d) HASMCs cell culture for 48 h on (c) POMC8/2 film and (d) collagen-immobilized POMC8/2 film. (e) MTT assay for HASMC cell growth on POMC 8/2, collagen-immobilized POMC8/2 and PLLA control for 2, 5 and 7 days (n = 7), ^*P < 0.05. Scale bar = 100 μm.

Figure 9

In vivo host response to photo-cross-linked POMC8/2 implanted subcutaneously in Sprague–Dawley rats. Implants and surrounding tissues were harvested 1 week after (a) POMC8/2, (b) PLLA and 4 weeks after (c) POMC8/2 and (d) PLLA implantation. No tissue necrosis was found. P, polymer; F, fibrocapsule; M, muscle. Scale bar = 100 μm. This figure is published in colour in the online edition of this journal, that can be accessed via http://www.brill.nl/jbs

Figure 10

SEM pictures of the (a) surface and (b) cross-section of POMC8/2 scaffolds fabricated using the salt-leaching method.