## 3 Results and Discussion  
### Morphology of the CS/PCL MPs  
The variation in microscopic morphology in the obtained CS/PCL bowl-shaped particles with solution concentration and solute ratio is shown in Figure 1. When spherical particles begin to dry, because the droplet movement process of the outside of the contact with air flow velocity is faster, the outer layer of the droplet first dries and solidifies, and then forms a solid \"shell\", so that the volume of the internal changes is not immediately reflected in the external shape. However, as the interior of the droplet also begins to dry, the solute left behind by the drying of the solution inside is not enough to fill the internal structure, resulting in the formation of a cavity inside the droplet. Eventually, the cavity collapses and the whole sphere collapses into a bowl shape [36; 37]. At a solution concentration of 3% (Figure 1a-c), due to the low solute content, low solution viscosity, and small diameter of the atomised droplets, the drying rate inside and outside the droplets is similar, and no cavities are formed, resulting in fewer and smaller-diameter bowl-shaped particles. At a solute concentration of 4% (Figure 1d-f), a large number of regular bowl-shaped particles are produced. This is due to the moderate droplet diameter and the large difference in drying rates between the inside and outside; the particles are highly collapsed and flattened with one side concave. When the solute concentration is increased to 5% (Figure 1g-i), the mass of the solute in the atomised droplets increases with the increase in mass volume fraction, resulting in larger particle diameters and thicker walls.  
The mass ratio of PCL to CS in the solution is one of the most important factors in particle generation. When the mass ratio of PCL:CS was 7:1, the number of particles  
Figure 1: SEM images and diameter distribution of MPs prepared at different solute concentrations (\(w/v\)): (**a–c**) 3%, (**d–f**) 4%, (**g–i**) 5%. SEM images and diameter distribution of MPs prepared with different PCL and CS ratios at the concentration of 4% \(w/v\): (**j–I**) PCL and CS ratio of 7:1; (**m–o**) PCL and CS ratio of 1:1.  
produced by the spraying process was very small, which can be inferred from the images (Figure 1j-l), due to the fact that PCL is easily soluble in the aqueous solution of THF and glacial acetic acid; a large amount of PCL in the large mass ratio of PCL is sprayed directly onto the receiver plate to solidify and does not bind with CS to form particles, and the small amount of particles formed by CS also forms irregular concave cake-like particles in the process. When the ratio of PCL-CS was reduced to 3:1 (Figure 1d-f), the state of the microparticles changed significantly and bowl-shaped MPs with regular morphology were formed, and when the mass ratio was reduced to 1:1 (Figure 1m-o), although a few of the formed microparticles had the morphology of bowl-shaped microparticles, adhesion of particles caused by a large amount of chitosan and the direct formation of particles by chitosan were very obvious.  
The elemental distribution of CS/PCL MPs is shown in Figure 2, where the three elements C, N, and O are uniformly distributed; since only CS contains nitrogen in PCL ((C\({}_{6}\)H\({}_{10}\)O\({}_{2}\))n) and CS ((C\({}_{6}\)H\({}_{11}\)NO\({}_{4}\))n), it can be demonstrated that chitosan and polycaprolactone are uniformly distributed in the particles.