Recombinant DNA technology allows the modification of organisms on the genetic level and transferring a manifold of functions that have evolved in nature. An essential step in many genetic engineering workflows is the delivery of exogenous genetic material into a bacterial cell, a process in the course of which the receiving bacterium is transformed. For genetic material to enter the cell, it must first pass the cell envelope, which in the case of the model bacterium Escherichia coli (E.coli) consists of two layers, the outer membrane and the cytoplasmic membrane. To cross this envelope, cells need to be conditioned to form holes in the envelope, or "made competent", which is typically achieved by ‡ These authors contributed equally to this work transferring cells out of growth medium and into specific aqueous solutions, which either contain calcium ions or are of particularly low conductivity. In the case of a chemical transformation, pores in the cells' membrane are formed through the interplay of chemicals and a heat shock, whereas during electroporation pores form in the cell membrane during an electric shock. Once the pores have formed, the genetic material can enter the cell, where it can either propagate independently from the chromosome in the form of a plasmid or is introduced into the chromosome and in the process causes specific mutations (e.g. recombineering). To be successfully electroporated, cells are briefly (5 -10ms) exposed to a strong (∼ 12.5 -15kV cm -1 ) electric field and thus need to be suspended in a low conductivity medium, to prevent strong currents and subsequent Joule heating that would impact cell viability. This requirement is in strong contrast to growth media, where dissolved salts and other charged molecules required for cell growth lead to high electric conductivity. Therefore, medium exchange is an indispensable preparatory step for each form of transformation, and protocols for preparing competent cells usually include three cumbersome and time-consuming centrifugation-based washing steps. After each centrifugation, the cell-free supernatant is removed, and cells are resuspended in the respective specific solutions. For the highly efficient electroporation, this is typically ultrapure water. Given its crucial role in transformation, a procedure to reliably, and in an automated and ideally miniaturized fashion, condition cells for DNA uptake would represent a breakthrough for a plethora of microbiological and molecular engineering protocols, including such transformative methods as multiplexed automated genome engineering (MAGE). Recently, several research groups have developed methods for continuous medium exchange in microfluidic systems. These methods are based on particle trapping, dragging particles through a liquid interface, continuous buffer exchange, droplet merging, and fluid relocation. The forces utilized to achieve medium exchange are manifold. The most commonly used forces are dielectric, magnetic, hydrodynamic, and acoustic. However, each of these methods exhibits fundamental limitations, such as the requirement of low flow rates or particle labelling. In this work, medium exchange is achieved by first trapping bacterial cells by acoustic forces and subsequently flushing the channel with a new medium such as ultrapure water. The contactless manipulation of particles using acoustic forces, acoustophoresis, is one of the most used techniques for particle manipulation because it is non-invasive, label-free, flexible in design, and biocompatible. Acoustophoresis is based on the acoustic radiation force and the drag force from acoustic streaming. The acoustic radiation force results from scattering of ultrasonic waves on the surface of a particle. Its magnitude mainly depends on the particle size, the density and compressibility difference between the particle and its surrounding medium, and the amplitude of the acoustic pressure. In bulk acoustic wave (BAW) devices, an acoustic standing wave is generated in the microfluidic channel by matching the acoustic wavelength in the fluid to the channel's dimensions. Particles with a positive acoustic contrast, which is typical for biological samples, accumulate in the pressure nodes of a 1D standing wave. There are several ways to trap particles with acoustic forces. One way is to adjust the shape of a microfluidic chamber such that radiation forces are generated that point against the flow direction. Further, two 1D standing fields can be generated perpendicular to each other in a squareshaped cavity. Finally, acoustic modes inside glass capillaries can be utilized to retain particles against a flow at a specific position. In this work, we developed a new approach to trap particles in a BAW device. By etching two microfluidic channels close to each other, we get a thin silicon wall between the two channels. This wall can freely vibrate upon excitation and attracts particles to its displacement antinode. In contrast to all methods discussed before, we press the particles against an interface which has the potential to increase the trapping efficiency.