Monodispersed Sirolimus-Loaded PLGA Microspheres with a Controlled Degree of Drug-Polymer Phase Separation for Drug-Coated Implantable Medical Devices and Subcutaneous Injection

Abstract

Monodispersed sirolimus (SRL)-loaded poly(lactic-co-glycolic acid) microspheres with a diameter of 1.8, 3.8, and 8.5 μm were produced by high-throughput microfluidic step emulsification─solvent evaporation using single crystal silicon chips consisted of 540-1710 terraced microchannels with a depth of 2, 4, or 5 μm arranged in 10 parallel arrays. Uniform sized droplets were generated over 25 h across all channels. Nearly 15% of the total drug was released by the initial burst release during an accelerated drug release testing performed at 37 °C using a hydrotropic solution containing 5.8 M N,N-diethylnicotinamide. After 24 h, 71% of the drug was still entrapped in the particles. The internal morphology of microspheres was investigated by fluorescence microscopy using Nile red as a selective fluorescent stain with higher binding affinity toward SRL. By increasing the drug loading from 33 to 50 wt %, the particle morphology evolved from homogeneous microspheres, in which the drug and polymer were perfectly mixed, to patchy particles, with amorphous drug patches embedded within a polymer matrix to anisotropic patchy Janus particles. Janus particles with fully segregated drug and polymer regions were achieved by pre-saturating the aqueous phase with the organic solvent, which decreased the rate of solvent evaporation and allowed enough time for complete phase separation. This approach to manufacturing drug-loaded monodisperse microparticles can enable the development of more effective implantable drug-delivery devices and improved methods for subcutaneous drug administration, which can lead to better therapeutic treatments.

Keywords: biodegradable polymer; controlled drug release; drug delivery; drug-eluting medical devices; poly(lactic-co-glycolic acid); step microfluidic emulsification.

Figure 1

Schematics of the microfluidics rig: (a) stainless steel module with two syringe pumps, a collection syringe, and a reflected-light microscope for observation of droplet generation; (b) silicon chip with 5 cross-flow channels and 10 parallel rows of terraced microchannels (MCs) (red labels are terrace numbers): A = inlet hole for the dispersed phase, B = inlet holes for the continuous phase (CP), and C = outlet hole for the emulsion. Red regions are dead-end channels for the dispersed phase, and blue regions are cross-flow channels for the CP; (c) magnified top view of a single dead-end channel in section D–D; (d) magnified side view of the dead-end channel with two terrace walls; (e) terrace wall with two MCs on the top and a deep well at each side.

Figure 2

Droplet generation in different chips: (a) CMS6-1 chip after 1 h of continuous operation; (b) CMS6-1 chip after 24 h of continuous operation; (c) CMS6-3 chip after 2 h of continuous operation; (d) CMS6-2 chip after 2 h of continuous operation. Samples S1–S3 in Table 2.

Figure 3

(a) Mean droplet diameter in the CMS6-1 chip at different terraces over 25 h; (b) mean droplet diameter (bars) and CV (solid line) in the CMS6-2 chip over 24 h; (c) mean droplet dimeter (bars) and CV (solid line) in the CMS6-3 chip over 20 h; (d) confocal laser scanning microscopy (CLSM) images of SRL-loaded PLGA particles fabricated in runs S1 (i), S2 (ii), and S3 (iii) in Table 2.

Figure 4

Effect of the number of washing cycles on the purity of the supernatant and particle morphology for the sample S2: (a) UV–visible absorption spectra of the supernatant after each washing cycle and for 1 wt % PVA solution; (b) equivalent PVA concentration of the supernatant as a function of the number of washing cycles (the PVA concentrations in the plot are estimated from the heights of the UV–vis peak at 280 nm based on the assumption that the supernatant is a pure PVA solution); (c) SEM image of SRL-loaded PLGA particles without washing; and (d,e) SEM images of the same sample after eight washes.

Figure 5

Effect of saturating the CP with the organic solvent on the particle morphology. (a1,a2): CLSM images of particles before freeze drying; (b1,b2): CLSM images of particles after freeze drying; (c1,c2): SEM images of intact particles and the schematic view of their internal structure; (d1,d2): SEM images of particles cross-sectioned by FIB. The CP was 1.5 wt % PVA solution saturated by IPAc in (a1–d1) and pure 1.5 wt % PVA solution, in (a2–d2). In (a1,b1), the brighter particle parts are rich in SRL and the darker parts are rich in PLGA. The drug loading in all particles was 33 wt %.

Figure 6

Effect of drug loading on the particle morphology. The drug loading was: (i) 33 wt % (S4); (ii) 42 wt % (S5); (iii) 44 wt % (S6); (iv) 45 wt % (S7); (v) 46 wt % (S8); and (vi) 50 wt % (S9). The experimental conditions and emulsion formulations are shown in Table 2. The fluorescence microscopy images are taken at 20× (A) and 60× (B) magnification. The SEM images of the particles are shown in (C), and the schemes of particle morphologies are shown in (D).

Figure 7

(a) XRD patterns of raw SRL powder, raw PLGA powder, SRL-loaded PLGA particles (sample S4), and blank PLGA particles; (b) ATR–FTIR spectra of raw SRL powder, raw PLGA powder, and SRL-loaded particles (sample S4).

Figure 8

Percentage of SRL released from PLGA particles as a function of time under accelerated conditions for two replicate determinations on sample S3 (Table 1). After 24 h, all particles in the sample were dissolved with ACN.