Injectable and thermosensitive hydrogels mediating a universal macromolecular contrast agent with radiopacity for noninvasive imaging of deep tissues

Abstract

It is very challenging to visualize implantable medical devices made of biodegradable polymers in deep tissues. Herein, we designed a novel macromolecular contrast agent with ultrahigh radiopacity (iodinate content > 50%) via polymerizing an iodinated trimethylene carbonate monomer into the two ends of poly(ethylene glycol) (PEG). A set of thermosensitive and biodegradable polyester-PEG-polyester triblock copolymers with varied polyester compositions synthesized by us, which were soluble in water at room temperature and could spontaneously form hydrogels at body temperature, were selected as the demonstration materials. The addition of macromolecular contrast agent did not obviously compromise the injectability and thermogelation properties of polymeric hydrogels, but conferred them with excellent X-ray opacity, enabling visualization of the hydrogels at clinically relevant depths through X-ray fluoroscopy or Micro-CT. In a mouse model, the 3D morphology of the radiopaque hydrogels after injection into different target sites was visible using Micro-CT imaging, and their injection volume could be accurately obtained. Furthermore, the subcutaneous degradation process of a radiopaque hydrogel could be non-invasively monitored in a real-time and quantitative manner. In particular, the corrected degradation curve based on Micro-CT imaging well matched with the degradation profile of virgin polymer hydrogel determined by the gravimetric method. These findings indicate that the macromolecular contrast agent has good universality for the construction of various radiopaque polymer hydrogels, and can nondestructively trace and quantify their degradation in vivo. Meanwhile, the present methodology developed by us affords a platform technology for deep tissue imaging of polymeric materials.

Keywords: Block copolymers; In vivo degradation; Non-invasive deep tissue imaging; Radiopacity; Thermosensitive hydrogels.

Graphical abstract

Fig. 1

Schematic presentation of design of macromolecular contrast agent, construction of radiopaque and thermosensitive PEG/polyester copolymer hydrogels, and their nondestructive imaging in vivo via Micro-CT. CL: ε-caprolactone, GA: glycolide, ITMC: 5,5-bis(iodomethyl)-1,3-dioxan-2-one, LA: d,l-lactide, PEG: poly(ethylene glycol), ROP: ring-opening polymerization, TEA: triethylamine, THF: tetrahydrofuran.

Fig. 2

Characterization of PI, the novel macromolecular contrast agent. (A) ¹H NMR spectrum of PI in CDCl₃. (B) Full XPS survey spectra for PI and PEG1000. (C) SEM image, EDS distribution maps and analysis results (wt%) for carbon (red dots), oxygen (green dots) and iodine (blue dots) of PI.

Fig. 3

Thermogelation behaviors of aqueous polymer solutions with or without the macromolecular contrast agent PI. (A) Photographs showing sol (25 °C) and gel (37 °C) states of the various aqueous polymer solutions. (B) Storage modulus G′ and loss modulus G″ of the aqueous polymer solutions containing PI as a function of temperature. PI1 means the 1/1 (w/w) mixture of PI and P1, and the same is for PI2 and PI3.

Fig. 4

X-ray opacity of various aqueous polymer solutions in vitro. (A) The linear relationship between the average grayscale indices/Hounsfield units of iohexol aqueous solutions and their iodine amounts (n = 3, P_max = 0.1). (B) The optical photographs, the corresponding Micro-CT images on the x-axis and the radiopacities of the indicated samples using the grayscale unit. The results are expressed as mean ± standard deviation (SD) (n = 3, P_max = 0.1). (C) Design, modeling and 3D printing of “FDU” pattern. (D) The X-ray opacity of “FDU” pattern containing P1 or PI1 solution using Micro-CT. (E) The X-ray opacity of “FDU” pattern containing P1 or PI1 solution covered with different layers of pork using X-treme.

Fig. 5

In vitro cytotoxicity of the polymer mixtures, PI1, PI2 and PI3, as a function of polymer concentration against MC3T3-E1 cells. The viability of cells with the treatment of culture medium only was set as 100% and each point is represented as the mean ± SD (n = 4).

Fig. 6

(A) Cross-sectional views and 3D reconstructed Micro-CT images of the indicated samples 30 min after subcutaneous injection into female ICR mice. The dotted coils represent the PI-free hydrogels. (B) Cross-sectional views and 3D reconstructed Micro-CT images of the indicated samples 30 min after intraperitoneal injection into female ICR mice. (C) Grayscale indices of the indicated samples after subcutaneous injection. (D) Volume of the indicated samples after subcutaneous injection. (E) Grayscale indices of the indicated samples after intraperitoneal injection. (F) Volume of the indicated samples after intraperitoneal injection. (n = 3, P_max = 0.1).

Fig. 7

The in vivo degradation of polymer hydrogels with or without the macromolecular contrast agent PI. (A) Changes in the volume and grayscale index of PI1 hydrogel over time after subcutaneous injection into female ICR mice. (B) Representative cross-sectional views of the remaining PI1 hydrogels at the predetermined time points. (C) Representative photographs of the remaining hydrogels in ICR mice that received the subcutaneous administration of 15 wt% P1 hydrogel. (D) Changes in the weight fraction of 15 wt% P1 hydrogel and the corrected volume fraction of 30 wt% PI1 hydrogel over time. The results are expressed as mean ± SD (n = 3).