Info
Background:
Polymers are used in tissue engineering for their tunable, application-specific properties, including radiolucency, which minimizes medical imaging artifacts. However, radiolucent polymers are often radiologically indistinguishable from biological tissues and cannot be monitored using biomedical imaging like computed tomography (CT) – critical for assessing implant integrity, migration, or failure. [1] In this study, a radiolucent, porous polymer was modified with iodine contrast – a well-known clinical contrast – and in vivo monitoring, degradation and toxicity were assessed.
Hypothesis/Objective:
The addition of molecular iodine will improve in vivo visualization of formerly radiolucent polymers, affording the ability to non-invasively monitor the degradation and positioning of polymeric implants.
Methods:
10wt% iodinated PEO-PEI30 was mixed with 90wt% 50:50 poly(lactide co-glycolide) (PLGA) to create porous discs that served as the experimental implant. [2] 12 male Sprague-Dawley rats (10 weeks old) were randomly assigned to three groups: Naïve (n=3), Sham (n=3), and Implant (n=6). Naïve animals did not receive surgery or implant and served as a control for blood collections. Sham animals received the surgical incision, but no implant, to control for immune/inflammatory response to surgery itself. Implant animals received surgery and implant. PEO-PEI30 discs with diameter of 5mm and thickness of 1.5mm were implanted into the deep right quadriceps muscle and secured with suture. Implant animals received micro-CT scans at post-operative days (POD) 0, 7, 14, 28, and 42, and serial tail vein blood collections were obtained for implant and shams. Histology was performed on organs collected at terminal day 42, as well as mass spectrometry to assess iodine content. Micro-CT was analyzed for change in implant size and intensity, using Fiji ImageJ and ITK-SNAP.
Results:
The iodinated polymer implants were visible with micro-CT for the duration of the study, distinguishable from surrounding tissues with resolution affording signal and volume quantification. Iodinated polymer was incorporated into the implant via the polyester at a low wt%, maintaining the polyester’s properties, such as degradability and mechanics. The blend used is additionally capable of being 3D-printed, permitting scaling to a range of relevant applications. Micro-CT shows a decrease in both gross and matrix volume over time. As the volume decreases, X-ray attenuation increases, due to densification of the structure (and iodinated PEO). The implant is undergoing compressive forces possibly due to the mechanically active environment of the quadriceps implantation site. Iodine content, as calculated from the initial average mass of iodine per implant and average signal intensity over a given volume, showed no significant amount of iodine leaving the implant over 42 days. As with nanoparticle contrast agents, clearance of the contrast post implant degradation requires fine-tuning [3].
No toxicity of liver or kidneys was caused from the device and its iodine levels. Triglycerides, urea nitrogen, Na/K ratio, and creatinine levels showed anticipated fluctuations that returned near-baseline levels by day 42. White blood cells (WBCs) show no significant changes, validating the safety of our monitoring procedure, as WBCs are the first to get ablated by radiation. However, there is an increase in monocytes at day 42 in the implant group, indicating a biological response to the implant.
Conclusion:
This study highlights benefits of iodinated composites, including visualization via micro-CT, allowing characterization of the implant degradation profile. Minimal iodine leaving the implantation site leads us to infer that PEO-PEI30 is not degrading as quickly as PLGA, and is an appropriate contrast for permanent or semi-permanent implants where in situ monitoring is desired – such as stents or drug delivery systems. This work advances polymer-based tissue engineering through noninvasive, post-implantation monitoring, validating safe and effective preclinical performance of implanted iodinated polymers.
Image/Figure:
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Image/Figure Caption:
By incorporating free molecular iodine into a poly(ethylene oxide) (PEO) and blending with 50:50 poly(lactic-co-glycolic) acid (PLGA), porous radiopaque implants can be created and implanted into a rat model to evaluate the relationship between iodine content and polymer radiopacity during in vivo degradation, shown in a schematic. (a) Study timeline showing serial tail vein blood collections and micro-CT scans with time of polymer implantation at Post-Op Day (POD) 0. Terminal day 42 shows organ collection to assess iodine distribution. (b) White blood cell counts were evaluated to validate the safety of the imaging protocol, as white blood cells are the first to get ablated by radiation. No significant changes between baseline and POD42 are indicative of an intact immune system. (c1) Segmented monocytes show an interesting increase at POD42 in response to the device, suggesting an implant specific biological response. (c2) Longitudinal micro-CT scans show a significant increase in signal (Hounsfield Units, HU) over time, visually shown in (d) with scale bar:1mm and quantitatively shown in (e). A decrease in gross volume was observed as shown in (f1), calculated by implant thickness and diameter, seen schematically in (f2). A decrease in matrix volume over time was inversely related to the increase in X-ray attenuation (HU) (g). The polymer blend contained 2.95wt% iodine, with an average iodine mass of 0.1531mg in implants. The iodine mass, related to the change in intensity (HU) and change in matrix volume (mm3), was relatively constant over time (h). Data are reported as mean and standard deviation, analyzed with Repeated-Measures One-Way ANOVA and Tukey Post-hoc (* p < 0.05 ** p < 0.001).
Author
Emma Rene Wessels
Michigan State University