Fabrication of Polymeric Coatings with Controlled Microtopographies Using an Electrospraying Technique

Abstract

Surface topography of medical implants provides an important biophysical cue on guiding cellular functions at the cell-implant interface. However, few techniques are available to produce polymeric coatings with controlled microtopographies onto surgical implants, especially onto implant devices of small dimension and with complex structures such as drug-eluting stents. Therefore, the main objective of this study was to develop a new strategy to fabricate polymeric coatings using an electrospraying technique based on the uniqueness of this technique in that it can be used to produce a mist of charged droplets with a precise control of their shape and dimension. We hypothesized that this technique would allow facile manipulation of coating morphology by controlling the shape and dimension of electrosprayed droplets. More specifically, we employed the electrospraying technique to coat a layer of biodegradable polyurethane with tailored microtopographies onto commercial coronary stents. The topography of such stent coatings was modulated by controlling the ratio of round to stretched droplets or the ratio of round to crumped droplets under high electric field before deposition. The shape of electrosprayed droplets was governed by the stability of these charged droplets right after ejection or during their flight in the air. Using the electrospraying technique, we achieved conformal polymeric coatings with tailored microtopographies onto conductive surgical implants. The approach offers potential for controlling the surface topography of surgical implant devices to modulate their integration with surrounding tissues.

Fig 1. An electrospraying setup for stent-coating.

(A) Schematic of an electrospraying setup for stent-coating using a circular shielding electrode placed right underneath the needle and above the aluminum plate. Continuous coating on Express coronary stent was achieved by exposed to the electrospraying mist for 30 min: (B) bare metal stent and (C) coated stent. Scale bar: 500 μm.

Fig 2. Deposited droplet size histogram obtained from different eletrospraying modes.

Four eletrospraying modes were analyzed: (A) micro-dripping mode at electric field of 1.3 kV/cm, (B) spindle mode at electric field of 1.4 kV/cm, (C) cone-jet mode at electric field of 1.5 kV/cm, and (D) precession mode at electric field of 1.6 kV/cm. The optical microscopy images of the deposited droplets are shown in the inset Figs with scale bars of 100 μm.

Fig 3. Formation of primary electrosprayed droplets.

(A) Schematic diagram of the droplets formed from a jet through Rayleigh’s capillary breakup under a cone-jet mode during electrospraying process. (B) The diameter of deposited droplets vs. the square root of flow rate using the improved electrospraying setup with circular shielding electrode as shown in Fig 1A.

Fig 4. Breakup of electrosprayed droplets due to Coulombic fission.

Schematic (left) and optical microscopy (right) images of deposited droplets collected from electrspraying mist at different tip-to-collector distance of (A) 3 cm, (B) 6 cm and (C) 9 cm, using a same scale bar of 200 μm. Fragmented droplets were formed in (B) and (C) due to Coulombic fission. The electrospraying process was performed using a constant tip-to-plate distance of 9 cm.

Fig 5. Coating morphology obtained at different electrospraying modes.

SEM images of Express coronary stents coated by electrosprayed droplets obtained at different electrospraying modes: (A) cone-jet mode, (B) precession mode, and (C) multi-jet mode. These electrospraying modes were achieved at an increasing electric field of (A) 1.5 kV/cm, (B1) 1.6 kV/cm, (B2) 1.7 kV/cm, and (C) 1.8 kV/cm, respectively. Round and stretched deposited droplets were highlighted in empty yellow triangles (A) and solid yellow triangles (B), respectively.

Fig 6. SEM image of a stent coated with large electrosprayed droplets (dia. ~120 μm).

A non-uniform coating with large pores was observed on the stent due to dewetting of the liquid coating caused by ineffective evaporation and solidification of the deposited droplets during coating process.

Fig 7. Coating morphology controlled by Coulombic fission.

SEM images of Express coronary stents coated by electrosprayed droplets obtained at different flow rates with varying degree of Coulombic fission: (A) 0.5 mL/h, (B) 0.4 mL/h, (C) 0.3 mL/h, and (D) 0.2 mL/h.