Fluorinated polyurethane scaffolds for ¹⁹F magnetic resonance imaging

Abstract

Polymers are increasingly employed in implant materials. To reduce the incidence of complications, which in the case of vascular grafts include incorrect placement and restenosis, materials are needed which allow for image-guided implantation, as well as for accurate and efficient postoperative implant imaging. We here describe amorphous fluorinated polymers based on thermoplastic polyurethane (¹⁹F-TPU), and show that are useful starting materials for developing tissue-engineered vascular grafts which can be detected using ¹⁹F MRI.

Non-invasive imaging plays an increasingly important role in regenerative medicine. The direct visualization of implants using magnetic resonance imaging (MRI) can provide longitudinal feedback on the position, functionality, resorption and remodeling of the material. It also helps to identify collapsed or damaged regions in vascular grafts, and facilitates decision-making with respect to pharmacological or surgical interventions.

Besides the advantages of standard ¹H MRI, which include high spatial resolution, very good soft-tissue contrast and lack of ionizing radiation, ¹⁹F MRI offers additional advantages.^1,2 First and foremost, since there are hardly any endogenous fluorine atoms in the body (high levels are only found in teeth), the background MRI signal is very low, enabling 'hot spot' imaging and highly specific implant detection. The absence of background signals also allows for accurate and efficient quantification. A further advantage of ¹⁹F MRI (over labeling with superparamagnetic iron oxide nanoparticles and standard ¹H MR imaging of implants^3,4) is the absence of susceptibility artifacts, which can disturb the analysis of adjacent tissues and of remodeling processes within the vascular graft. Fluorinated scaffold materials are consequently considered to be interesting starting materials for developing image-guided tissue-engineered vascular grafts.

Commonly used fluorinated polymers, such as polytetrafluoro-ethylene (PTFE; Teflon; GoreTex) and polyvinylidene fluoride (PVDF) cannot be employed for ¹⁹F MRI, because their high crystallinity compromises the molecular mobility of the signal-producing fluorine atoms. As a consequence, even at ultra-short echo times, hardly any MRI signals are generated and the signals decay very fast, leading to poor sensitivity and prohibiting image acquisition (Figure 1). To enable ¹⁹F MR imaging of scaffold materials, we generated a novel fluorinated polymer based on thermoplastic polyurethane (¹⁹F-TPU) which possesses distinct properties rendering it suitable for fluorine-based MRI: (i) it possesses a large number of magnetically equivalent ¹⁹F atoms, yielding strong resonance peaks; (ii) it has an amorphous structure, resulting in high mobility of the incorporated ¹⁹F atoms; and (iii) it has favorable relaxation properties to achieve relatively long echo times.

Figure 1.

Fluorinated polymers for ¹⁹F MRI. A: The amorphous structure of ¹⁹F-TPU results in high mobility of fluorine atoms and in long T₂ relaxation times, enabling efficient MR image acquisition. Conversely, the high crystallinity of PVDF leads to short echo times, fast signal decay and short T₂. B: ¹⁹F-TPU was used to prepare polymeric fibers, which were braided into vascular scaffold materials enabling ¹⁹F MRI.

The ¹⁹F-TPU is synthesized from a perfluorinated aromatic diisocyanate and butanediol. After polymerization, the polyurethane is N-alkylated to obtain a non-crystalline material with partially fluorinated alkyl side chains (Figure 2A and Scheme S1). The average size of the final polymeric material was 7.4*10³ Da (Mn; Mw=1.5*10⁴), and the polydispersity 2.0. It is known that PVDF exhibits high crystallinity (see DSC and XRD spectra in Figure S1 and S2). In order to model a spinning process, PVDF was heated to T = 210 °C, which is above T_m (~180 °C as determined by DSC in Figure S1), followed by subsequent cooling to room temperature. After this thermal treatment, PVDF retains its high crystallinity, which was verified using DSC and wide-angle X-ray scattering (WAXS) analysis (Figure 2D). Conversely, ¹⁹F-TPU remained largely amorphous after the same thermal treatment (Figure 2C and S1).

Figure 2.

Chemical structure and wide-angle X-ray scattering (WAXS) diffractograms of ¹⁹F-TPU (A and C) and PVDF (B and D), exemplifying the amorphous character of ¹⁹F-TPU and the high crystallinity of PVDF.

Due to their amorphous nature, ¹⁹F-TPU polymers have a much higher chain mobility than crystalline PVDF polymers. Therefore, the fluorine atoms in ¹⁹F-TPU are less confined and can more efficiently tumble within the amorphous structure of the polymer. As a result, the fluorine atoms in ¹⁹F-TPU generate two resonance peaks at -83.6 ppm and -123.5 ppm with favorable relaxation times (T₁= 225±6 ms; T₂= 0.76±0.04 ms; Figure 3A-C). The separation of both resonances enables undisturbed excitation and subsequent image readout of a single resonance, preventing the occurrence of artifacts. The generation of strong and specific ¹⁹F MRI signals with slow signal decay permits background-free imaging of ¹⁹F-TPU at clinically relevant settings, i.e. in a 3T patient scanner, at a relatively short acquisition time (<3 min), with a good signal-to-noise ratio (>10) and with a relatively high image resolution (<1 mm isotropic voxel size) (Figure 3D).

Figure 3.

MRI characterization of ¹⁹F-TPU. A: ¹⁹F-NMR spectra of the polymeric solution with deuterated chloroform as solvent, recorded using a Bruker DPX-400 FT-NMR spectrometer at 376.5 MHz. B-C: T₁ and T₂ measurements. Please note that only the strongest ¹⁹F-peak has been used for imaging due to the large spectral separation (~40 ppm) of the two ¹⁹F peaks (A) and acquisition bandwidth of 8 kHz of the clinical 3T MRI scanner. D: ¹⁹F MR imaging of ¹⁹F-TPU polymeric pellets in a 3T scanner using an UTE sequence.

To provide proof-of-principle for the usefulness of ¹⁹F-TPU for developing MRI-detectable scaffold materials, it was processed into monofilaments via melt spinning. Therefore, ¹⁹F-TPU pellets were introduced into a twin-screw extruder and spun into monofilaments at a process temperature of 225-230°C. Subsequently, a vascular graft with a diamond braid and twelve monofilaments was produced. The braided structure had a braiding angle of 33° and its inner diameter was 6 mm. The resulting scaffold was imaged in a clinical 3T MRI scanner using a dual-tuned (¹⁹F/¹H) volume coil and ultra-short echo time (UTE) sequences with an echo/repetition time of 0.12/100 ms and a flip angle of 67°. Afterwards, the ¹⁹F imaging data was 3D-rendered and overlaid with a ¹H image to demonstrate ‘hot spot’ MR imaging (Figure 4).

Figure 4.

¹⁹F-TPU-based vascular scaffold, enabling ‘hot spot’ MRI and 3D rendering. A: Photographic image of the braided vascular scaffold structure. Scale bar: 2 mm. B: ¹⁹F MRI of the scaffold in sagittal and coronal planes (top) and upon 3D rendering (bottom). C: ¹H MRI (top) and ¹H /¹⁹F MRI overlay images (bottom) of the ¹⁹F-TPU scaffolds.

These findings indicate that ¹⁹F-TPU can be utilized to develop MR imageable cardiovascular implants, enabling a detailed visualization of the architecture of vascular grafts and their unambiguous identification upon in vivo implantation. In recent years, several other fluorine-containing materials have been developed for ¹⁹F MRI, but thus far only for functional and molecular imaging applications^6–11, for monitoring drug delivery¹², for cell tracking^13,14 and for visualizing encapsulated cellular therapeutics^15,16. We here for the first time provide proof-of-principle for the generation of ¹⁹F MRI-detectable polymeric scaffolds, which can be used as a starting material to develop imageable vascular grafts, and which may serve as a basis for theranostic tissue engineering.

Supporting information

Supporting information is available online. This includes Materials and Methods, plus 1 additional scheme and 2 figures.