Cellulose nanocrystal-based enhancement of ultrasound microbubbles for increased tolerance of mechanical index values

Abstract

Enhancements in the echo intensity and mechanical index (MI) tolerance of ideal ultrasound contrast agents (UCAs) are urgently needed to realize precise clinical ultrasound (US). Solid cellulose nanocrystals (CNCs) have the advantages of imparting strong sound wave propagation and echo intensity when irradiated by US. This study aimed to fabricate improved UCAs via CNC-modified SonoVue ultrasound microbubbles (MBs), characterize the resulting SonoVue@CNC MBs, and evaluate the performance of the SonoVue@CNC MBs in US imaging in vitro and in vivo. SonoVue@CNC MBs were successfully prepared by incorporating CNCs into SonoVue MBs via self-assembly. The SonoVue@CNC MBs were spherical in shape with mean particle sizes of (5.50 ± 1.60) μm. The CNCs were mainly distributed on the polymeric shells of the SonoVue@CNC MBs. When the MI was 0.4, the echo intensity of the SonoVue@CNC MBs was 1.5-fold greater than that of the SonoVue MBs in vitro. The CNC concentration regulated the quality of SonoVue@CNC-promoted contrast-enhanced ultrasound (CEUS) images. Enhanced US images of both the livers and kidneys of Sprague–Dawley (SD) rats showed that the CNC-loaded UCA led to a higher echo intensity than SonoVue. In CCK-8 assays using MCF-10A cells, both SonoVue@CNC MBs and CNCs exhibited weak toxicities against MCF-10A cells. Ultimately, the spherical SonoVue@CNC MBs with a uniform particle size displayed an enhanced safety profile and improved the imaging quality of US in vivo and in vitro. Next, drugs and targeting moieties will be loaded onto the SonoVue@CNC MBs via the CNCs to develop UCA-based selective ultrasonic theranostic methods.Graphical abstractAs shown in the graphical abstract, the white bubble in the left-middle position symbolized SonoVue microbubble (MB), the green rod-like particle signified CNC nanorod. The big white bubble decorated with green rod-like nanorod next to the mouse was SonoVue@CNC MB which was successfully prepared by incorporating CNCs into SonoVue MB via self-assembly. The purple square frame on the upper left displayed CNC, SonoVue MB, SonoVue@CNC MB and their relative diagrammatic drawings. Picture (I) displayed the transmission electron microscopy results for CNCs (a) and SonoVue@CNC MBs (b, c and d) of morphological study. Picture (II) displayed the in vitro ultrasound (US) imaging of SonoVue (a) MB and SonoVue@CNC (b, c and d) MBs with different CNCs concentrations. Picture (III and IV) displayed the in vivo US imaging of SonoVue (a) and SonoVue@CNC (b, c and d) MBs with different CNCs concentrations for SD mouse liver and kidney, respectively.