Structural transitions and encapsulation selectivity of thermoresponsive polyelectrolyte complex micelles

Area of Science:

• Polymer Science
• Materials Science
• Nanotechnology

Background:

• Polyelectrolyte complex (PEC) micelles encapsulate charged molecules, with neutral blocks protecting cargo.
• Thermosensitive poly(N-isopropyl acrylamide) (pNIPAM) exhibits a lower critical solubility temperature (LCST) transition.
• Leveraging pNIPAM's thermosensitivity in PEC micelles for controlled drug delivery is explored.

Purpose of the Study:

• To characterize PEC micelles formed with pNIPAM-based block copolymers and oppositely charged polymers.
• To investigate the effect of pNIPAM's thermosensitivity on micelle structure and stability.
• To evaluate the encapsulation and release of charged therapeutics by these micelles.

Main Methods:

• Dynamic light scattering (DLS)
• Small-angle X-ray scattering (SAXS)
• Absorbance and fluorescence spectroscopy
• Transmission electron microscopy (TEM)

Main Results:

• Mixing pNIPAM-b-poly(acrylic acid) with poly(lysine) formed worm-like micelles; with poly(ethylene glycol)-b-poly(lysine) formed spherical micelles.
• Worm-like micelles lost structure above the pNIPAM LCST (~35 °C), while spherical micelles retained structure but aggregated.
• Methylene blue was not encapsulated, but rhodamine-labeled polylysine was, with minimal release above LCST and potential for temperature-triggered protection.

Conclusions:

• Thermosensitive pNIPAM-based PEC micelles can be engineered into different morphologies (worm-like vs. spherical).
• Spherical micelles offer greater structural stability above the LCST, while both aggregate.
• Encapsulation of polyions is confirmed, with evidence suggesting temperature-induced changes in micelle quenching may enhance cargo protection.