Ultrasonic neuromodulation is a rapidly growing field, in which low-intensity ultrasound (US) is delivered to nervous system tissue, resulting in transient modulation of neural activity. This review summarizes the findings in the central and peripheral nervous systems from mechanistic studies in cell culture to cognitive behavioral studies in humans. The mechanisms by which US mechanically interacts with neurons and could affect firing are presented. An in-depth safety assessment of current studies shows that parameters for the human studies fall within the safety envelope for US imaging. Challenges associated with accurately targeting US and monitoring the response are described. In conclusion, the literature supports the use of US as a safe, non-invasive brain stimulation modality with improved spatial localization and depth targeting compared with alternative methods. US neurostimulation has the potential to be used both as a scientific instrument to investigate brain function and as a therapeutic modality to modulate brain activity.

Transcranial ultrasound stimulation (TUS) is emerging as a potentially powerful, non-invasive technique for focal brain stimulation. Recent animal work suggests, however, that TUS effects may be confounded by indirect stimulation of early auditory pathways. We aimed to investigate in human participants whether TUS elicits audible sounds and if these can be masked by an audio signal. In 18 healthy participants, T1-weighted magnetic resonance brain imaging was acquired for 3D ultrasound simulations to determine optimal transducer placements and source amplitudes. Thermal simulations ensured that temperature rises were <0.5 °C at the target and <3 °C in the skull. To test for non-specific auditory activation, TUS (500 kHz, 300 ms burst, modulated at 1 kHz with 50% duty cycle) was applied to primary visual cortex and participants were asked to distinguish stimulation from non-stimulation trials. EEG was recorded throughout the task. Furthermore, ex-vivo skull experiments tested for the presence of skull vibrations during TUS. We found that participants can hear sound during TUS and can distinguish between stimulation and non-stimulation trials. This was corroborated by EEG recordings indicating auditory activation associated with TUS. Delivering an audio waveform to participants through earphones while TUS was applied reduced detection rates to chance level and abolished the TUS-induced auditory EEG signal. Ex vivo skull experiments demonstrated that sound is conducted through the skull at the pulse repetition frequency of the ultrasound. Future studies using TUS in humans need to take this auditory confound into account and mask stimulation appropriately.

Cellular membranes are, in general, impermeable to macromolecules (herein referred to as macrodrugs e.g. recombinant protein, expression plasmids or mRNA), which is a major barrier for clinical translation of macrodrug-based therapies. Encapsulation of macromolecules in lipid nanoparticles (LNPs) can protect the therapeutic agent during transport through the body and facilitate the intracellular delivery via a fusion based pathway. Furthermore, by designing LNPs responsive to stimuli, can make their delivery more localised, thus limiting the side effects. However, the principles and criteria for designing such nanoparticles remain unclear. We show that the thermodynamic state of the lipid membrane of the nanoparticle is a key design principle for acoustically responsive fusogenic nanoparticles. We have optimized a cationic lipid nanoparticle (designated LNPLH) with two different phase transitions near physiological conditions for delivering mRNA. A bicistronic mRNA encoding a single domain intracellular antibody fragment and green fluorescent protein (GFP) was introduced into a range of human cancer cell types using LNPLH and the protein expression was measured via fluorescence corresponding to GFP expression. The LNPLH/mRNA complex demonstrated low toxicity and high delivery, which was significantly enhanced when the transfection occurred in the presence of acoustic shock waves. The results suggest that the thermodynamic state of LNPs provides an important criterion for stimulus responsive fuso-genic nanoparticles to deliver macrodrugs to the inside of cells.

The dorsal raphe nucleus (DRN) is an important source of serotonin in the brain, but fundamental aspects of its function remain elusive. Here, we present a combination of minimally invasive recording and disruption studies to show that DRN brings about changes in motivation states. We use recently developed methods for identifying temporal patterns in behavior to show that monkeys change their motivation depending on the availability of rewards in the environment. Distinctive patterns of DRN activity occur when monkeys transition between a high-motivation state occupied when rewards are abundant, to a low-motivation state engendered by reward scarcity. Disrupting DRN diminishes sensitivity to the reward environment and perturbs transitions in motivational states.

The rheological properties of porcine heart, kidney, liver and brain were measured using dynamic oscillatory shear tests over a range of frequencies and shear strains. Frequency sweep tests were performed from 0.1 Hz to a maximum of 9.5 Hz at a shear strain of 0.1%, and strain sweep tests were carried out from 0.01% to 10% at 1 Hz. The effect of pre-compression of samples up to 10% axial strain was considered. The experimental measurements were fit to a Semi-Fractional Kelvin Voight (S-FKV) model. The model was then used to predict the stress relaxation in response to a step strain of 0.1%. The prediction was compared to experimental relaxation data for the porcine organ samples, and the results agreed to within 30%. In conclusion, this study measured the rheological properties of porcine organs and used a fractional viscoelastic model to describe the response in frequency and time domain.