Manipulation of Cancer Cell Spheroids by Photovoltaic Tweezers: Determination of Their Charge State

Photovoltaic optoelectronic tweezers (PVOT), based on the bulk photovoltaic effect presented by certain ferroelectric crystals, have emerged as a versatile tool to manipulate and/or trap micro‐ and nano‐objects. Very recently, an ingenious method has enabled to extend PVOT manipulation to aqueous solution droplets opening the door to applications in optofluidics and biotechnology. The photovoltaic crystal is placed on the bottom of a cuvette filled with paraffin oil while the droplet is hanging from the air–oil interphase where it is manipulated. Herein, this method is used to investigate the feasibility of manipulating hybrid droplets as carriers containing a cancer multicellular spheroid inside cell culture medium. Droplets of two cancer cell lines, MCF‐7 and U‐87 MG, have been successfully manipulated by light‐induced electrical fields generated in the photovoltaic platform. From the droplet dynamics, which includes attractive and repulsive behaviors from the illuminated crystal region, the charge state of the cell culture medium and the spheroid are investigated. The results show that they are positively and negatively charged, respectively. Moreover, analyzing the migration kinetics of water, cell culture medium, and hybrid droplets, to the light of a proposed theoretical model, the charge density of both cell line spheroids has been estimated.