Biomacromolecules 2003, 4, 1335-1342
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Photoinitiated Cross-Linking of the Biodegradable Polyester Poly(propylene fumarate). Part II. In Vitro Degradation John P. Fisher,†,‡ Theresa A. Holland,†,‡ David Dean,§ and Antonios G. Mikos*,† Department of Bioengineering, Rice University, P.O. Box 1892, MS 142, Houston, Texas 77251-1892, and Department of Neurological Surgery and the Research Institute, University Hospitals of Cleveland and the Department of Neurological Surgery, Case Western Reserve University, 10900 Euclid Avenue, Cleveland, Ohio 44106-5042 Received April 2, 2003; Revised Manuscript Received June 10, 2003
This study investigated the in vitro degradation of both solid PPF networks and porous PPF scaffolds formed by photoinitiated cross-linking of PPF polymer chains. Three formulations of scaffolds of differing porosity and pore size were constructed by varying porogen size and content. The effects of pore size and pore volume on scaffold mass, geometry, porosity, mechanical properties, and water absorption were then examined. Throughout the study, the solid networks and porous scaffolds exhibited continual mass loss and slight change in length. Porogen content appeared to have the greatest effect upon physical degradation. For example, scaffolds initially fabricated with 80 wt % porogen content lost approximately 30% of their initial PPF content after 32 weeks of degradation, whereas scaffolds fabricated with 70 wt % porogen content lost approximately 18% after 32 weeks of degradation. For all scaffold formulations, water absorption capacity, porosity, and compressive modulus were maintained at constant values following porogen leaching. These results indicate the potential of photo-cross-linked PPF scaffolds in tissue engineering applications which require maintenance of scaffold structure, strength, and porosity during the initial stages of degradation. Introduction Recent tissue engineering research has focused on designing polymer scaffolds to provide temporary structural support and to promote cell growth in defective or degenerative tissues.1,2 These three-dimensional, porous structures may be coated with bioactive and selectively binding compounds and then implanted into a tissue defect to facilitate the ingrowth of cells from surrounding tissue. Alternatively, scaffolds may be seeded with cells prior to implantation to promote preferred cell migration and the growth of these cells within the composite. As tissue growth occurs, the polymer matrix degrades, replacing the defect with healthy tissue.3,4 To be considered for tissue engineering applications, polymer scaffolds must be biocompatible to avoid inflammatory response from host tissue and to provide a suitable substrate for cell attachment and proliferation. Additionally, porosity and pore size should be sufficient for the transport of cells and nutrients through the material. The composite should possess mechanical properties similar to the native tissue, provide structural strength sufficient to prevent pore collapse, and protect surrounding tissues from injury during the ingrowth of new tissue. Furthermore, because polymer scaffolds serve a temporary purpose, the composite material should biodegrade into nontoxic products that can be * To whom correspondence should be addressed. Dr. Antonios G. Mikos, Department of Bioengineering, Rice University, P.O. Box 1892, MS 142, Houston, Texas 77251-1892, Phone: (713) 348-5355. Fax: (713) 348-5353. E-mail:
[email protected]. † Rice University. ‡ These authors contributed equally to this manuscript. § Case Western Reserve University.
eliminated from the body through natural pathways. Ideally, polymer scaffolds would degrade at the rate of tissue ingrowth to allow for maintenance of scaffold porosity, structure, and mechanical integrity in the initial stages of tissue formation.1,5 Recent investigations have demonstrated the potential of the aliphatic polyester poly(propylene fumarate) (PPF) in constructing degradable scaffolds which meet the design criteria for tissue engineering.6-8 PPF’s repeating unit contains one unsaturated bond that permits covalent crosslinking and two ester groups that allow for hydrolysis of the polymer into the principal, nontoxic degradation products of fumaric acid and propylene glycol.9 Previous investigations have shown thermally cross-linked PPF to be biocompatible and biodegradable.7,8,10 However, photo-cross-linking techniques have the advantage of greater temporal and spatial control of polymerization and greater flexibility during scaffold implantation than chemical cross-linking methods.11 Therefore, we have recently reported the photo-cross-linking of PPF using the photoiniatior bis(2,4,6-trimethylbenzoyl) phenylphosphine oxide (BAPO) and long wavelength ultraviolet (UV) light.12,13 Using this photo-cross-linking technique with the incorporation of a porogen, PPF scaffolds with an interconnected pore network and promising mechanical properties have been reproducibly fabricated.12 To further assess the usefulness of this material in tissue engineering applications, a detailed examination of the physical and chemical structure of scaffolds during long-term degradation is required.
10.1021/bm0300296 CCC: $25.00 © 2003 American Chemical Society Published on Web 07/11/2003
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Biomacromolecules, Vol. 4, No. 5, 2003
Previous studies have analyzed the in vitro degradation of PPF thermally cross-linked with the monomer N-vinyl pyrrolidinone (NVP), initiator benzoyl peroxide (BP), and accelerator N,N-dimethyl-p-toluidine (DMT).8 However, because the composition and cross-linking chemistry of thermal initiated and photointiated cross-linked PPF scaffolds differ significantly, their degradation behavior can also be expected to differ. This study examined changes in scaffold mass, geometry, porosity, mechanical properties, and water absorption throughout 32 weeks of in vitro degradation to address the following questions: (1) How does the incorporation of a pore network into photo-cross-linked PPF composites effect material degradation? (2) How does degradation alter scaffold structure? (3) How does pore size and/or pore volume effect scaffold degradation? Materials and Methods Poly(propylene fumarate) Synthesis. Poly(propylene fumarate) was synthesized by a two step procedure.14 In the first step, 1 mol of diethyl fumarate (DEF) (Acros Organics, NJ) and 3 mol of 1,2 propanediol (PG) (Acros Organics) were reacted using 0.01 mol of the catalyst ZnCl2 (Fisher Chemicals, Fair Lawn, NJ) and 0.002 mol of hydroquinone (Acros) as a radical inhibitor. As the reaction stoichiometry calls for a 1:2 mole ratio of DEF to PG, excess PG was used to drive the reaction to completion. The reaction was run for approximately 8 h under a nitrogen blanket and at atmospheric pressure, producing bis(hydroxypropyl) fumarate as the main product and ethanol as a byproduct. The vapor temperature was maintained at approximately 10 °C above the boiling point of ethanol (78 °C) in order to remove the byproduct by distillation. In the second step, bis(hydroxypropyl) fumarate was transesterified at low pressure (
