The Institute of Microelectronics of Barcelona (IMB-CNM-CSIC) and the Universitat Autònoma de Barcelona (UAB) have developed a new generation of wireless piezoelectric microdevices capable of electrically stimulating living cells on an individual level. The study, recently published in the journal Small and chosen as the cover image, demonstrates how these microdevices can convert mechanical forces, whether produced by the cells themselves or applied externally via ultrasound, into electrical signals that enable the controlled and noninvasive activation of cellular processes.

This work falls within the field of electroceuticals, therapies based on the application of electrical stimuli to modulate cellular or tissue processes, which, in turn, enable the control of signals or tissue repair. To assess the scope of this work, it is necessary to clarify what is meant by “cell activation.” This concept refers to a cell’s response to a stimulus, whether internal or external, that triggers processes at the cellular level, such as proliferation or differentiation. In this study, the stimulus is the local electric field produced by zinc oxide piezoelectric nanogenerators in response to mechanical stimulation. This action produces an increase in cellular activation via calcium signaling pathways at the moment of stimulation.

“The microdevices developed are capable of applying stimuli to individual cells, and because they measure tens of micrometers in size, they would enable highly precise and targeted therapy. Furthermore, this stimulation is wireless and minimally invasive, as it is performed remotely using ultrasound in the biomedical range,” explains Laura Lefaix, a researcher at IMB-CNM-CSIC in the NEMESYS Lab group and the study’s first author.

Piezoelectricity for cell stimulation

The IMB-CNM Clean Room has played a key role in the manufacturing process for these devices. Silicon-based microfabrication is the technological foundation that enables the development of silicon dioxide microparticles onto which these zinc oxide nanogenerators are integrated. This piezoelectric material is responsible for generating a local electric field when subjected to mechanical deformation, in this case, by ultrasound.

The cell-based assays that confirmed the functionality of the microdevices were conducted at the UAB’s Department of Cellular Biology, Physiology, and Immunology, in the group led by Andreu Blanquer and Carme Nogués, BioTEn Lab. The research team used various indicators to assess activation, including changes in intracellular calcium levels and variations in membrane potential, both of which are fundamental to cell signaling, the process by which cells receive and respond to signals. In this regard, Andreu Blanquer notes that “the results obtained have demonstrated that these microdevices are capable of inducing changes at the cell membrane level and have revealed the mechanism by which cells are activated. This type of bioelectric modulation is key to understanding how physical stimuli can be translated into biological signals.”

The study combined simulation and experimentation, which allowed researchers to understand the mechanisms underlying the stimulation and to validate the performance of the microdevices. One of the main challenges was to evaluate the parameters of ultrasound stimulation to achieve optimal electrical stimulation of the cells, as well as to understand the fundamental mechanisms by which this interaction occurs.

Laura Lefaix notes that “the results validate the idea that the size of piezoelectric devices can be reduced to the order of tens of micrometers while maintaining their functionality,” as evidenced by the fact that “using this method, up to 58% of the cells in the sample were activated in a controlled manner.” The microdevices were tested on bone cells (Saos-2 line), since they serve as a robust model for this type of experiment due to the presence of voltage-gated ion channels in their membranes.

Gonzalo Murillo, a researcher at the IMB-CNM, coordinator of the study, and winner of the 2023 “Ángela Ruiz Robles” National Research Award for Young Researchers, adds that, in addition to its effectiveness, “the microdevice offers significant technological advantages, as it is biocompatible, allows for wireless and localized stimulation, and its manufacturing is scalable and adaptable, which facilitates the mass production of microdevices with different configurations.”

Continuation of an emerging line of research

Entitled Suspendable and Scalable Ultrasound-Actuated ZnO-Nanosheet-Based Piezoelectric Microdevices for Wireless Electrical Stimulation of Cells, the study features extensive participation by researchers from IMB-CNM-CSIC, including Laura Lefaix, Gonzalo Murillo, Marc Navarro, and Jaume Esteve. Also involving participation from the UAB, Andreu Blanquer and Carme Nogués, and from the Institute of Physiology of the Czech Academy of Sciences, Lucie Bacakova. The results have been published in the journal Small, where the article was selected as the cover story, highlighting the relevance and innovative nature of the work.

Building on the team’s previous research, the study goes a step further in demonstrating the ability of these microdevices to activate cells through electromechanical interactions with the material. In this study, the concept is expanded by introducing remote ultrasound-based stimulation, thereby consolidating a line of research with great potential in the field of bioelectronics.