Synthesis of magnetic resonance–, X-ray– and ultrasound-visible alginate microcapsules for immunoisolation and noninvasive imaging of cellular therapeutics

Abstract

Cell therapy has the potential to treat or cure a wide variety of diseases. Non-invasive cell tracking techniques are, however, necessary to translate this approach to the clinical setting. This protocol details methods to create microcapsules that are visible by X-ray, ultrasound (US ) or magnetic resonance (MR) for the encapsulation and immunoisolation of cellular therapeutics. Three steps are generally used to encapsulate cellular therapeutics in an alginate matrix: (i) droplets of cell-containing liquid alginate are extruded, using an electrostatic generator, through a needle tip into a solution containing a dissolved divalent cation salt to form a solid gel; (ii) the resulting gelled spheres are coated with polycations as a cross-linker; and (iii) these complexes are then incubated in a second solution of alginate to form a semipermeable membrane composed of an inner and an outer layer of alginate. The microcapsules can be rendered visible during the first step by adding contrast agents to the primary alginate layer. Such contrast agents include superparamagnetic iron oxide for detection by 1H MR imaging (MRI); the radiopaque agents barium or bismuth sulfate for detection by X-ray modalities; or perfluorocarbon emulsions for multimodal detection by 19F MRI, X-ray and US imaging. The entire synthesis can be completed within 2 h.

Cell encapsulation: technical and clinical advances

“…Materials can also be designed in ways that enable realtime imaging to confirm implant location and function of cells encapsulated into those materials. The precise location of the implanted cell-loaded capsules may be determined by traditional imaging modalities, including magnetic resonance imaging (MRI) [21][22][23][24] or ultrasound [25]. For example, microcapsules can be coated with selfassembled gold nanoparticles, which enable them to be monitored by X-ray micro-CT scanners [26,27].…”

Trimodal Gadolinium-Gold Microcapsules Containing Pancreatic Islet Cells Restore Normoglycemia in Diabetic Mice and Can Be Tracked by Using US, CT, and Positive-Contrast MR Imaging

“…Two types of alginate microcapsules were prepared: conventional alginatepoly-L -lysine-alginate (APLLA) capsules ( 25,26,33 ) and new alginate-protamine sulfate-alginate (APSA) capsules ( Fig 1, A ; Appendix E1 [online]). To prepare labeled capsules, GG nanoparticles were cosuspended with human islets in lowviscosity high-guluronate alginate before bead formation.…”

Recent progress in the use and tracking of transplanted islets as a personalized treatment for type 1 diabetes

“…In comparison to direct labeling of naked islets, it is much easier to label microencapsulated islets: the contrast agent or label is simply added to the alginate, and then automatically embedded within the hydrogel (Figure 1B) [79, 80, 81]. A key difference is that the label is not inside the transplanted islet cell but outside in the gel matrix, ameliorating concerns about potential toxicity.…”