Surface characteristics of spray-dried microspheres consisting of PLGA and PVP: relating the influence of heat and humidity to the thermal characteristics of these polymers.

In view of the increasing interest in injectable controlled release formulations for the treatment of chronic diseases, injectable polymeric microspheres consisting of a surface layer of poly(lactic-co-glycolic acid) (PLGA) and an underlying polyvinylpyrrolidone (PVP) layer were previously developed. The present study focuses on the influence of heat and humidity on the surface characteristics of these spray-dried PLGA/PVP microspheres. The response of the polymeric matrix to these factors will provide an insight into the expected release behavior and stability of the formulation. This should result in the development of a drug matrix with desired and tunable characteristics in terms of physicochemical stability and drug release profile, relevant in a later stage of research. Glass transition temperatures (Tgs) and miscibility behavior were analyzed by modulated differential scanning calorimetry (MDSC). Scanning electron microscopy (SEM) provided insight in particle morphology. Atomic force microscopy (AFM) was used to study the nanoscale topography and phase behavior of the samples. Time-of-flight secondary ion mass spectrometry (ToF-SIMS) and X-ray photoelectron spectroscopy (XPS) were utilized for surface chemical analysis and quantification respectively. It could be concluded that the surface characteristics (chemical composition, phase behavior, and topography) of spray-dried PVP/PLGA microparticles were affected by exposure to heat and humidity. When exposed to these conditions, a surface rearrangement occurs whereby an increase of PVP at the surface is observed, coupled with a decrease in PLGA. This phenomenon can be explained based upon the relative thermal characteristics and consequent molecular mobility of the two polymers.