Atom-economical in situ synthesis of BaSO₄ as imaging contrast agents within poly(N-isopropylacrylamide) microgels using one-step droplet microfluidics

A novel in situ synthesis of BaSO₄ as an X-ray imaging contrast agent within poly(N-isopropylacrylamide) (PNIPAM) microgels has been accomplished by one-step droplet microfluidics with interlinking reactions. A one-step droplet microfluidics device with wavy channels is employed to produce monodispersed PNIPAM microgels (∼40 μm) via free radical polymerization by the initiation of the ammonium persulfate (APS) and N,N,N′,N′- tetramethylethylenediamine (TEMED) complex. The resulting PNIPAM microgels are subsequently dripped into a collecting bath containing an aqueous BaCl ₂ solution. For the in situ synthesis of BaSO₄, Ba ²⁺ ions would diffuse into the PNIPAM microgels and further react with SO₄^2- ions originated from the decomposed by-product of the APS-TEMED complex during the initiation, yielding almost 100% atom efficiency of persulfate ions. The morphologies and the composition of the PNIPAM microgels containing in situ synthesized BaSO₄ (BaSO ₄-PNIPAM microgels) are characterized by scanning electron microscopy combined with energy-dispersive X-ray spectroscopy (SEM-EDX) and X-ray diffraction (XRD). These results show that 14 nm BaSO₄ crystallites have been in situ synthesized in the PNIPAM microgels. The resulting BaSO ₄-PNIPAM microgels are also visible under X-ray radiation by an in vitro fluoroscopic evaluation. This droplet microfluidics based method provides a green and atom-economical reaction approach to synthesize functionalized microgels with a uniform distribution of BaSO₄ nano-crystallites. Moreover, the radiopaque BaSO₄-PNIPAM microgels also possess great potential in the fields of endovascular embolization, diagnostic imaging, and visible implantation materials.