3.2.1. Neutral Medium

Doxorubicin release behaviors from sericin nanoparticle formulations were investigated by in vitro tests (PBS: pH 7.45, 37 ◦C). All nanoparticle formulations had relatively similar behavior with significant differences in release efficiency and time. The results are shown in Figure 3. Both the efficiency and the release time increased for sericin nanoparticles prepared from lower concentrations. The release profile showed a specific behavior with a faster release for the first 100 min with an efficiency of 35%. This behavior cannot be associated with a specific burst release. The release behavior followed a slower release for the next 180 min for SER 1% and for the next 360 min for SER 0.1%. Formulations with SER 0.25% and SER 0.5% followed a slower release time placed within this interval. All nanoparticles SER 0.1%–SER 1% revealed a high entrapment efficiency of 90–95%. Morphological and dimensional analyses showed that the size of the nanoparticle aggregates decreased for those obtained from lower sericin concentrations. These results may suggest that small-size aggregates reached higher efficiency but for a longer release time. The maximum cumulative release efficiency was 74%. This means there was still some entrapped doxorubicin within the mass of sericin nanoparticles. The obtained results in terms of release and encapsulation efficiency can be correlated with other sericin formulations containing low soluble agents [46,47]. Most probably, this behavior was influenced by the fact that the nanoparticles were self-assembled in the presence of doxorubicin. This approach facilitated the formation of sericin–doxorubicin conjugates which are driven by multiple physical bonding [47]. In this situation, the doxorubicin remained entrapped, and only specific environments could act as release stimulus. Therefore, specific media such as pH decrease or enzymatic activity may facilitate further release.

**Figure 3.** Doxorubicin release profile from sericin nanoparticles in neutral medium.
