Poly(gadodiamide fumaric acid): A Bioresorbable, Radiopaque, and

Article pubs.acs.org/journal/abseba

Poly(gadodiamide fumaric acid): A Bioresorbable, Radiopaque, and MRI-Visible Polymer for Biomedical Applications Amy C. Goodfriend,*,† Tre R. Welch,† Kytai T. Nguyen,‡ Jian Wang,† Romaine F. Johnson,§ Alan Nugent,∥ and Joseph M. Forbess† †

Division of Pediatric Cardiovascular and Thoracic Surgery, §Department of Otolaryngology, and ∥Department of Pediatrics, University of Texas Southwestern Medical Center, Dallas, Texas 75390, United States ‡ Department of Bioengineering, University of Texas Arlington, Arlington, Texas 76019, United States ABSTRACT: Bioresorbable medical devices once implanted into the body are “invisible” to imaging techniques such as Xray/fluoroscopy and magnetic resonance imagining (MRI). Prior attempts to produce radiopaque polymers have limited success due to their inability to generate homogeneous mixtures of polymer and contrast agent without subsequent alterations in polymer structure. Here we investigate a novel approach in which a MRI contrast medium, gadodiamide, can be used as a polymerization initiator in poly(propylene fumarate) (PPF) synthesis to achieve a radiopaque and MRI-visible polymer poly(gadodiamide fumaric acid) (PGFA). With this method polymer structure, thermal properties, and rheological behavior are conserved with no prior manipulation to monomer units necessary. This unique polymer in combination with poly(lactic-co-glycolic acid) (PLGA) can be formulated into MRI-visible nanoparticles with drug delivery potential. This novel polymer in both liquid and nanoparticle form enables new possibilities in medical device and drug delivery design. KEYWORDS: bioresorbable polymer, radiopaque, MRI-visible, polymer synthesis, polymer characterization, nanoparticles

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INTRODUCTION Synthetic and biologically derived polymers have been extensively researched as biodegradable materials for medical applications. Some important properties of biodegradable biomaterials for medical applications include: nontoxic degradation products, appropriate mechanical and thermal properties, acceptable shelf life, degradation time of material should match healing/regeneration timeline, and limited associated inflammation upon implantation.1 Biologically derived or natural polymers were the first biodegradable biomaterials used clinically. Some examples include chitosan, chitin, collagen, cellulose, dextran and other polyamino acids.2−4 These natural polymers possessed several advantages compared to synthetic polymers including bioactivity, inherent receptor-binding ligands for cells, enzymatic or hydrolysis driven degradation, and natural remodeling.5 However, the inherent bioactivity of natural polymers has its own disadvantages, which include difficulties in purification and potential for disease transmission. In the past few decades, a new generation of synthetic biodegradable polymers has been developed for biomedical applications. The emergence of new technologies and applications such as controlled drug delivery, tissue engineering, and regenerative medicine drove this paradigm shift. Synthetic manufacturing of biomaterials has a unique advantage to tailor the polymer properties for a particular application. Thus, allowing control over properties such as mechanical strength, © XXXX American Chemical Society

thermal transitions, and hydrophobicity to make synthetic polymers more desirable for implantable devices. Currently many families of synthetic polymers are available from manufacturers or have reputable published synthesis methods for laboratory fabrication. Some examples of polymer families include polyanhydrides, polyesters, and polyurethanes.1 The most commonly used synthetic bioresorbable polymers for medical applications are poly(lactic acid) (PLA), poly(glycolic acid) (PGA) and poly(lactic-co-glycolic acid) (PLGA).6−8 Regardless of polymer origin, polymers lack one key desirable property that denser materials such as metal and ceramics possess: radiopacity. With the advent of minimally invasive treatments and patient monitoring performed under fluoroscopy (DSA, CT), ultrasound, or MRI, there is a great need for the development of radiopaque and MRI-visible polymers. The radiological detectability of conventional polymers used as medical implants or inserts is limited by their density.9 To render a polymer visible via MRI or other fluoroscopic techniques heavy elements or contrast medium must be included. Current MRI contrast mediums rely on paramagnetic or superparamagnetic substances.10 These substances are attracted by an applied magnetic field and form an internally induced Received: February 24, 2015 Accepted: June 22, 2015

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DOI: 10.1021/acsbiomaterials.5b00091 ACS Biomater. Sci. Eng. XXXX, XXX, XXX−XXX

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ACS Biomaterials Science & Engineering magnetic field in the direction of the applied magnetic field.11 In an MRI scanner, a strong magnetic field followed by a radiofrequency pulse is applied causing a change in the net magnetization generated from protons (mostly from water). In time, relaxation mechanisms return the protons to their equilibrium magnetization and the change (the signal) is detected. Gadolinium (Gd) is the most commonly used compounds for contrast enhancement followed by iodine, iron oxide and manganese (Mn).10 The heavy metal compounds (Gd and Mn) often are used in a chelate form in which the heavy metal is the central atom bound or in close proximity to a multiple bonded ligand.12 Iron oxide and other iron conjugations used as contrast agents are typically in the form of injectable nanoparticles or micelles.13 Incorporation of heavy elements and contrast medium into polymeric materials has been investigated for orthopedic and dental applications.14−18 Many limitations exists in these radiopacifying polymer formulations. Nonhomogeneous distribution of radiopacifying agents and agent leaching can lead to potential toxicity. Also, cracking or failure of polymeric device at the interface between polymer and additive is common because of moisture or bacteria penetration.17,18 The study aims to investigate a novel polymer synthesis in which a contrast medium can be used as an initiator to create a radiopaque and MRI-visible polymer. In the synthesis, a gadolinium-based contrast medium, Gadodiamide, was substituted as an initiator in the transesterification of diethyl fumarate and propylene glycol. We hypothesized that the polymer product could be blended with PLGA and formulated stable MRI-visible drug delivery nanoparticles. This study reports the synthesis and characterization of novel poly(gadodiamide fumaric acid) (PGFA). Chemical, thermal, and rheological characteristics of PGFA were compared to a control polymer synthesis; poly(propylene fumarate) (PPF). Feasibility of PLGA/PGFA nanoparticles for drug delivery and MRI was also examined.

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Scheme 1. Synthesis of Poly(gadodiamide fumaric acid) (PGFA); Two-Step Synthesis from Deithyl Fumarate and Propylene Glycol Facilitated by Gadodiamide Anhydrous

spectrophotometer.21 The samples were then run on the FTIR scanning 500 to 4000 cm−1. Spectra can be compared to He et al.20 To confirm polymer composition using proton nuclear magnetic resonance (1H-NMR), PGPF was dissolved in deuterated chloroform (CDCl3) (Sigma-Aldrich 151823), and analyzed by Varian Unity Inova 500 MHz 1H-NMR. 1H-NMR spectra of PPF was confirmed from He et al.20 Polymer Thermal Property Assessment. Small aliquot of polymer was pipetted into a TZero aluminum pan, sealed, and analyzed via Q20 differential scanning calorimeter (TA Instruments, New Castle, DE). Samples were equilibrated at 10 °C, ramped to 75 °C at 10 °C/min, held isothermal for 1 min then cooled to 10 °C at 50 °C/min. Samples followed the temperature sweep twice, reporting data on the second cycle. Heating curves were analyzed via TA Universal Analysis software (TA Instruments, New Castle, DE). Polymer Rheological Behavior Determination. Fluid behavior of PPF and PGFA was assessed via AR G2 rheometer (TA Instruments, New Castle, DE) and analyzed with TA Universal Software Analysis (TA Instruments, New Castle, DE.22 For all experiments, a gap size of 1000 μm was used with the temperature set at 37 °C and data recording at 10 points per decade. To analyze viscosity (η), we used a constant frequency (ω) of 1 rad/s with a broad torque range of 0.1−1000 μN m. To analyze storage (G′) and loss (G″) modulus, a constant frequency (ω) of 1 rad/s was used with a strain range of 0.1−30%. Polymer Degradation Kinetics. One-half a gram of polymer was measured into a 2 mL tube and distilled water (pH 7.4) was added to fill remaining volume in the tube. Tubes were placed on a shaker (120 rpm) at 37 °C. Distilled water was removed from each sample tube at days 2 and 4 and then weekly, leaving only polymer remaining. Ten microliters of polymer was removed and analyzed following the method described above for molecular weight determination. Tubes were then refilled with distilled water and placed on a shaker. Nanoparticle Formulation and Characterization. PGFA polymer was formulated into particles by solvent displacement technique using surfactant and sonication.23 PLGA (17kD) (Corbion,

MATERIALS AND METHODS

Synthesis and Molecular Weight Determination of PPF. PPF synthesis protocol is derived from Kasper et al. 200919 and He et al. 2001.20 Briefly, PPF is synthesized by a two-step reaction of diethyl fumarate and propylene glycol through a bis(hydroxypropyl) fumarate diester intermediate facilitated by zinc chloride. After synthesis PPF is dried in a vacuum oven for 24 h to remove residual solvent. To determine molecular weight, the polymer was dissolved in tetrahydrofuran (THF) and analyzed using Ultimate 3000 High Pressure Liquid Chromatography (HPLC) system (Thermo Scientific Dionex, Chicago, IL) and the I-MBLMW column, I-OLIGO (Viscotek, 10 μm, 7.8 × 30 cm). The mobile phase was 100% tetrahydrofuran (THF) with the column oven at 35 °C. The flow rate was 1.0 mL/min with an injection volume of 30 μL. Polymer was detected with Refractive Index detector (VE3580 Malvern, Houston, TX) and analyzed with OmniSEC 4.7 software (Malvern, Houston, TX). Molecular weight calibration was determined using polystyrene standards. Synthesis and Molecular-Weight Determination PGFA. PGFA was synthesized by a two-step reaction of diethyl fumarate and propylene glycol through a bis(hydroxypropyl) fumarate diester intermediate facilitated by gadodiamide anhydrous (see Scheme 1). Propylene glycol is at an excess to diethyl fumarate in a molar ratio of 3:1 and 0.003 mol of gadodiamide anhydrous are used. Molecular weight was determined by method describe for PPF. Polymer Chemical Structure Analysis of Polymers. Fourier transform infrared spectroscopy (FTIR) analysis of PPF and PGFA was conducted using a PerkinElmer Spectrum 1000 FT-IR B


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ACS Biomaterials Science & Engineering Netherlands) with a glass transition temperature (Tg) of 42.0 ± 0.9 °C was used for nanoparticle formulation. 1g PLGA was dissolved in 5 mL of THF. Then 1g of PGFA is dissolved into PLGA solution followed by the addition of 5 mL surfactant solution (0.35% Pluronic F127) and sonication for 60 min. Solvent is then removed via evaporation and particles are washed by centrifugation in deionized water three times. Particle effective hydrodynamic diameter and zeta potential was determined via ZetaPALS dynamic light scattering (DLS) apparatus (Brookhaven Instruments Corporation, Novato, CA, USA). Glass transition temperature (Tg) was determined by pipetting 50 μL of nanoparticle suspension into aluminum pan and placed in desiccator for 48 h. Pans were sealed and analyzed as described above. Nanoparticle MRI Phantom Study. T1 and T2 maps were constructed with a phantom containing six concentrations of PLGA/ PGFA particles utilizing Agilent (Varian) 4.7T MR imaging system (Figure 1 and Table 1). Briefly, a resolution phantom was constructed

NMR (Figure 1). The control synthesis showed chemical structure different than prior described.19,24,25 Polymer product from PPF synthesis was also a viscous liquid, unlike prior described in which a solid is achieved. Further examination of the 1H-NMR spectra reveals the presence of propylene glycol (PG) thus the synthesized polymer product is poly(fumaric acid) (PFA) with associated PG. PFA and PG are discernible from PPF with all noted peaks associated with PG and a significant hydroxyl peak that are not present in PPF literature (labels 1−4, Figure 1) . This is further confirmed via FTIR with a large hydroxyl group peak at 3448 cm−1. In the synthesis methoxyethane is lost as a waste product and some excess PG not only ethanol. Methoxyethane in the reaction environment loses two protons and condenses to form denatured propylene alcohol. Hence the actual waste products of the reaction are ethanol, denature propylene alcohol, and PG (data not shown). It is indiscernible if 2-butenedioic acid (2E) or fumaric acid or a combination there of is polymerized because they are stereoisomers. Because of the starting reactants, it is most probable that fumaric acid is polymerized. Comparative to the PFA spectra, PGFA has signature peaks at 736 cm−1 indicating the presence of gadolinium and a dual peak around 3500 cm−1 indicating the presence of amide and hydroxyl bonds (Figure 1). 1H-NMR was also used to confirm structure, finding only mild peak shifts of signature peaks in PGFA compared to PFA because of the presence of gadolinium (Figure 1). The molecular weight (Mw) of the synthesized PGFA was 1347 Da and PFA was 612 Da.. At this molecular weight, the polymer is a low viscosity liquid that may be suitable for injection purposes. DSC shows no significant difference (p < 0.05) between Tg of PGFA (−38.17 ± 2.2 °C) and PFA (−34.83 ± 2.6 °C) (Figure 3). PGFA and PFA viscosity showed nonlinear behavior within a shear rate range of 0.1− 1000 μN·m (Figure 4A). The viscosity of these materials is dependent on shear rate and is not a constant coefficient thus PGFA and PFA are Non-Newtonian fluids. These polymers do not exhibit a yield stress and display shear thinning (viscosity decreased with increased stress) within this range. The behavior of storage modulus (G′) and loss modulus (G″) for PGFA and PFA (Figure 4B, C) are independent of strain rate which classifies this material as a pseudoplastic. G′ describes the elastic properties and G″ the viscous properties of the system. At 37 °C, the value of G″ is always greater than G′. Therefore, the viscous properties dominate the elastic properties or the material behaves more like a viscous fluid than an elastic solid. When a load is applied, energy is lost (G″) in the form of heat. The amount of stored energy (G′) cannot compensate for the amount of energy lost (G″) therefore plastic deformation occurs.26 Polymer Degradation Kinetics. The degradation of PGFA and PFA are linear. Normalizing the data, PGFA degraded at a lower rate than PFA in aqueous environment at 37 °C (Figure 5). The inclusion of gadodiamide in the synthesis reaction resulted in a polymer with significantly higher molecular weight. Using equivalent molar ratios of reactants, there is a difference in degradation kinetics between polymers. The presence of gadodiamide in the polymer chain may hinder hydrolytic degradation, slowing overall degradation kinetics. PLGA/PGFA Nanoparticle Characterization and MRI Phantom Study. Using a solvent displacement technique, hybrid particles of size 250 ± 50 nm were formulated (Figure

Figure 1. MRI phantom setup.

Table 1. Concentration Values of Nanoparticles (NP) and Gadolinium (Gd) in Phantom Setup phantom number

NP concentration (mg/mL)

Gd concentration (mM)

1 2 3 4 5 6

0.000 2.000 4.250 6.250 12.500 25.000

0.000 0.007 0.014 0.021 0.041 0.082

using modified plastic syringe barrels containing the nanoparticles concentrations. A set pulse sequence (spin−echo inversion recovery, SE-IR) is introduced by the magnetic and the phantom is imaged. A 2D transverse slice from the center of the phantom is then chosen. Then the signal from the water is removed in the area that contains the nanoparticles. Using the MRI software, the signal from the slice is converted a signal map. The map appears pixelated due to the image resolution when being processed in the fitting procedure to generate the map. The map is then was analyzed with ImageJ (U.S. National Institutes of Health, Bethesda, MD). Linear regression was performed with GraphPad Prism 6 (GraphPad Software, La Jolla, CA) to calculate relaxivity coefficients (r1, r2). Statistical Analysis. Molecular weight and Tg of polymers (n = 4 per group) were compared using one-tailed student t test (p < 0.05) where the molecular weight and Tg of PGFA was hypothesized to be greater than PPF. Linear regression analysis of rheological behavior (n = 3) and degradation kinetics (n = 4) were performed. Slopes were compared using two-tailed student t test (p < 0.05) where PGFA rheological properties and degradation kinetics were hypothesized to be different in than PPF. Relaxivity coefficients were determined via linear regression of phantom study (n = 6). All statistical analysis was performed via GraphPad v.6 (GraphPad Software, La Jolla, California).

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RESULTS Chemical, Thermal, and Rheological Characterization. Polymer chemical structures were analyzed via FTIR and 1HC


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ACS Biomaterials Science & Engineering

Figure 2. Chemical structure characterization of PGFA utilizing FTIR and 1H-NMR. FTIR spectra of PGFA with signature gadolinium peak of 736 cm−1 and split peak indicating hydroxyl bonds at 3448 cm−1 and amide bonds at 3512 cm−1 not present in PFA spectra. One H NMR shows comparable peaks between PGFA and PFA with a significant shift of peaks in PGFA. Most notable peaks in 1 H NMR are from PG still present in synthesis final product.

= 1.33) within the range of literature reported values for gadolinium in refs 27−29.

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DISCUSSION New Class of Radiopaque and MRI-Visible Polymers Utilizing Contrast Medium as Synthesis Initiator. Current radiopaque polymer systems can be categorized into three classes: heterogeneous polymer blends with radiopacifiers, radiopaque polymer-salt complexes, and polymers composed of radiopaque monomers. A direct covalent bond between the radiopacifying agent and the polymer is not formed in the first two classes. Without a direct bond these systems generate nonhomogeneous mixtures susceptible to heavy element leaching, imaging artifacts, material failure.30 Early development of dental and orthopedic applications used gold gauze, lead foil, and fine metal wires inserted into poly(methyl methacrylate) (PMMA).18 The inconsistency of material mixing resulted in imaging artifacts and material failure. Thus, sifting fine grains of metals into the mixture were sought to improve the homogeneity of the system. Bowen and Cleek, 1972 examined blending powdered glasses with high content of barium, lead, or bismuth forming a polymer slurry prior polymerization.31 Although early success of their work was promising, difficulty of homogeneous mixing of polymer slurries led to failures at the interface of the polymer and additive.17,18 Further investigation to improve polymer homogeneity were performed by modifying monomers with radiopacifying agents prior to polymerization. Early approaches relied on the addition of halogen groups such as iodine and bromine.18 For example, a radiopaque material composed of methyl methacrylate (MMA)

Figure 3. Thermal characterization of PGFA and PFA via DSC. Average DSC curves of PGFA (black line) and PFA (blue line) plotted (n = 5). Tg and specific heat are not significantly different (p < 0.05).

6). The addition of PLGA increased the Tg to 26.1 ± 0.3 °C and generated relatively stable nanoparticles with a zeta potential of −12.5 ± 3.5 mV. These particles were further investigated for MRI applications. T1 and T2 measurements of the phantom were taken using the Agilent (Varian) 4.7T MR imaging system. Utilizing the grayscale gradients of the T1 and T2 maps, we obtained relaxivity coefficients r1 and r2 via linear regression analysis (Figure 7). A clearly visible concentration gradient is observed with relaxivity coefficients (r1 = 4.85 and r2 D


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Figure 4. Rheological characterization of PGFA and PFA. (A) Assessment of polymer viscosity using a broad torque range of 0.1−1000 μN m. (B) Assessment of storage modulus (G′) and loss modulus (G″) using a strain range of 0.1−30%.

Figure 5. Degradation kinetics of PGFA and PFA in deionized water at 37 °C. Raw data with computed linear regression (n = 10).

Figure 7. MRI phantom of PLGA/PGFA nanoparticles. MRI T1 and T2 maps were generated (top). Using the grayscale gradient, linear regression was performed to determine Gd relaxivity coefficients; r1 = 4.58 and r2 = 1.33.

To preserve polymer system properties and homogeneity of the system, using a contrast medium containing a heavy metal as a polymerization initiator is a viable option. By definition an initiator starts a chemical reaction by undergoing a chemical change to provide free radicals. In our synthesis, gadodiamide can provide free radicals from four available carboxylic acid groups. The generation of free radicals initiates transesterification and renders gadodiamide able to bind to the polymer chain. Gadodiamide has four available binding sites, thus possibly forming a network between linear chains of fumaric acid and branches of gadodiamide. Covalent bonds lock gadodiamide between linear polymer chains preventing leaching as well as preserving polymer structure of PFA. Our results confirm the structure of PGFA. Signature bonds associated with fumaric acid and PG are detected via 1H-NMR and FTIR with addition of associated gadodiamide signature bonds (Figure 1). It is possible that polymer chains polymerize in a strictly linear fashion with gadodiamide. PGFA degrades linearly and there are not significant differences in rheological behavior from PFA. However, because of the configuration of gadodiamide bonds, it is more logical that a network of linear

Figure 6. SEM image of PLGA/PGFA nanoparticles. Nanoparticles have a smooth, round surface morphology with a size distribution of 250 ± 50 nm.

with bromine had equivalent radiopacity to pure aluminum.18 Other approaches relied on the polymerization of heavy metal (i.e., bismuth, tin, lead) containing monomers.16 Although heavy metal approaches improved the radiopacity of polymers, oxidation of the added heavy metals led to inflammation and foreign body response upon implantation.32,33 Using a heavy metal in the polymer is plausible for generating a homogeneous distribution of radiopacifying agents. However, modifying the monomers prior to polymerization can significantly alter polymer system properties (i.e., viscosity, mechanical strength, glass transition temperature). E


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decrease of pH in and around these new microstructures due to degradation product accumulation. Some biopolymers such as PLLA degrade by bulk degradation in which autocatalysis of the center of the polymer degrades prior to the outer surface. Others such as PLGA, PFA, and PGFA undergo surface degradation in which the polymer chains exposed at the exterior surface degrade at a rate that exceeds the degradation of the center.41 The release of oligomers and monomers leads to a decrease in molecular weight and weight loss of the polymer structure. Polymer degradation kinetics is directly related to the polymer architecture. As shown in prior works with polyanhydrides, Mw of branched polymers were significantly higher than linear polymers of the same monomers.42 It was also noted that there was no noticeable changes in the physiochemical or thermal properties in the branched polymer compared to the linear polymers.42 However, a difference in polymer degradation existed with the branched polyanhydride degraded significantly faster than the linear polymers.42 In this study, we observed similar results with PGFA and PFA. It was most probable that PGFA was synthesized into a polymer network and can be directly compared to PFA which is a linear polymer. There was no significant difference in Tg or rheological behavior but PGFA has a higher Mw than PFA. Unlike results observed in the polyanhydrides, the linear polymer (PFA) degraded at a faster rate than the network (PGFA) likely due to the inclusion of gadodiamide. Gadodiamide is very hydrophilic with four potential binding carboxylic acid side groups. These bonds will be hydrolyzed prior to fumaric acid chains thus hindering hydrolytic degradation of fumaric acid. The established network generated by the gadodiamide must be cleaved before the linear chain scission of fumaric acid. Radiopaque and MRI-Visible Polymer Applications in Medicine. Clinicians continue to widen the ever-expanding scope of MRI imaging techniques that can be used for clinical diagnoses, characterization, monitoring, and treatment of various illnesses. X-ray and other fluoroscopic techniques dominated the clinic in prior years. The development of radiopaque polymers was initiated for the design of nonmetal medical devices. Extensive research has been concentrated on radiopaque polymers, particularly idio-polymer compounds for medical purposes due to the reliance upon X-ray imaging.9 Now with the shift from fluoroscopic imaging to MRI, there is a great need in the development of MRI compatible materials. Limited published research is available in regards to MRI-visible devices; some examples include catheters, the REVA stent, and radiation dosimetry gels.43−45 This creates a new paradigm for completely bioresorbable medical devices with imaging capabilities. The unique composition of PGFA allows a multitude of applications including coatings, injections, nanoparticles, and device design. Synthesizing PGFA at a higher Mw increasing the Tg could result in an in situ cross-linking, visible polymer as shown in prior studies with PPF.24,25,46,47 In this study we show the development and MRI visualization of PGFA/PLGA nanoparticles that have theranostic potential. Future studies of these nanoparticles will incorporate therapeutic agents (such as dexamethasone) for local delivery. If controlled therapeutic agent delivery can be achieved, this system can provide a traceable drug delivery vehicle. Biodegradability of this formulation is the primary advantage. There have been many concerns raised regarding the biodistribution and clearance of

chains is formed. This novel synthesis method has created a new generation of polymers visible via MRI and fluoroscopic techniques. Contrast medium initiated polymerization offers a solution for homogeneous distribution of heavy elements and preservation of gross polymer properties. Rheological Characterization and Its Importance in the Design of Polymers for Medical Use. Understanding of the rheological behavior of a polymer is useful in the evaluation of a polymer for their suitability in processing environments and applications. In the construction of polymeric medical devices, thermally processing of polymer (such as injection molding, extrusion, annealing, etc.) is essential. Liquid polymers also can be used as injectable materials. Rheological properties, especially shear viscosity (η), have important effects on thermal and other processes.34 Rheological behavior of amorphous and semicrystalline polymers is assessed in one of three ways: melt, shear, or extensional (acoustic) rheology. Liquid polymers (like PGFA and PFA) are tested on a shear rheometer without the addition of a solvent. Solid polymers are typically examined via melt rheometer but can also be examined via shear rheometer if dissolved in a solvent. PGFA and PFA both exhibit Non-Newtonian pseudoplastic system behavior (Figure 3). These polymers do not exhibit a yield stress and polymer viscosity decreased with increased stress (shear thinning). Storage (G′) and loss moduli (G″) indicated PGFA and PFA behave more like a viscous fluid than an elastic solid. Many conventionally used polymers for biomedical devices are Non-Newtonian pseudoplastic systems. Polycaprolactone (PCL), polyhydroxybutyrate-co-hydroxyvalerate (PHBV), polystyrene (PS), polyethylene (PE), polyamide (PA), chitosan, PMMA, and PLA are all non-Newtonian pseudoplastics.34−38 The majority of these biomedical polymers also have higher G″ values than G′ meaning they behave more like a viscous fluid than an elastic solid. PCL and PLA show slightly more elastic solid behavior than the other polymers.34,35 The behavior of chitosan in solution is very similar to PGFA; however, it is highly concentration dependent.38 The behavior of G′ and G″ in PE and PA are frequency dependent unlike PGFA (data not shown).37 All of these polymers except for chitosan in solution are solid at 37 °C. Chitosan did exhibit similar rheological properties as a liquid polymer such as PGFA. The values for viscosity, storage and loss moduli for the solid structure polymers are much greater than the liquid polymers. Thus, thermal processing is necessary in order to form these polymers into particular shapes and designs. PGFA does not require heat or solvent to flow but heat could be used to thermally cross-link PGFA into a solid structure. Future studies investigating thermal cross-linking and ultraviolet (UV) crosslinking agents are also being consider for a nonthermal crosslinking technique. PGFA Degradation Kinetics and Thermal Properties. The process of polymer degradation describes the chain scission process during which polymer chains are cleaved to form oligomers and finally form monomers.39 There are many different ways in which polymer degradation can occurs: photo-, thermal-, mechanical, and chemical degradation.40 All biodegradable polymers contain hydrolyzable bonds. Chemical degradation via hydrolysis or enzyme catalyzed hydrolysis is the most important degradation mechanism. The degradation of biodegradable polymers is complex. Water enters the polymer and can induce swelling. Chemical degradation is then initiated by hydrolysis leading to progressive changes in the microstructure of the bulk polymer (cracks, pores, etc.) and a F



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contrast mediums and nanoparticles.48 This system without a therapeutic agent can serve as a research tool for biodistribution and clearance research. A polymeric theranostic system can overcome the disadvantages associated with conventional MRI contrast mediums. Advanced MRI imaging techniques are leading to the discovery and development of new therapeutic agents and therapies in humans and animals. Combining the benefits of diagnostics with the ability to treat a disease has pioneered a new field of research known as theranostics. Theranostic systems may be the long awaited answer for researchers and clinicians to meet the needs of the everincreasing need to provide clinical efficacy and costeffectiveness of new therapies while driving drug discovery and commercialization.

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CONCLUSIONS This preliminary characterization shows feasibility of a novel bioresorbable, imaging capable polyester, PGFA for various medical applications. PGFA is a contrast medium initiated polymerization formed polymer, novel to currently available radiopaque polymers. This synthesis technique allows for the inclusion of a contrast medium with preservation of polymer structure. The synthesis of novel radiopaque and MRI-visible polymers such as PGFA has the potential to influence medical device design and could enable new bioresorbable, noninvasively imaging visible biomedical devices and technologies in the future.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: 214-6484742. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Electron Microscopy Core, the Biological Chemistry Core, Dr. Fred Grinnell with rheology, and the Animal Imaging Resource Center (AIRC) at UT Southwestern.

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ABBREVIATIONS CDCl3, deuterated chloroform CT, computed tomography DLS, dynamic light scattering DSA, digital subtraction angiography DSC, differential scanning calorimetry FDA, Food and Drug Administration FTIR, Fourier transform infrared spectroscopy H NMR, proton nuclear magnetic resonance HPLC, high pressure liquid chromatography MMA, methyl methacrylate Mw, molecular weight PGPF, poly(gadodiamide propylene fumarate) PLGA, poly(lactic-co-glycolic acid) PMMA, poly(methyl methacrylate) PPF, poly(propylene fumarate) SEM, scanning electron microscopy Tg, glass transition temperature THF, tetrahydrofuran G



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Article

NOTE ADDED AFTER ASAP PUBLICATION This paper was published ASAP on June 30, 2015 with incorrect Figures 4 and 5 due to a production error. The corrected version was reposted on July 1, 2015.

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Poly(gadodiamide fumaric acid): A Bioresorbable, Radiopaque, and

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