Ultrasound Stimulation: Harnessing Non-Invasive Neuromodulation

Abstract

Ultrasound stimulation is promising as a non-invasive technique The present article examines various therapeutic techniques of ultrasound stimulation and its applications. These techniques are known not only as an effective tool in the treatment of diseases and health problems, but also for augmentation and rehabilitation.This article examines different types of ultrasound stimulations and their respective applications, the positive effects of these techniques and research results in different conditions and diseases.

Keywords: ultrasound stimulation; neuromodulation; non-invasive; therapeutic techniques; nervous system

Introduction

Ultrasound stimulation has various therapeutic applications. One of its applications in the field of neurology is neuromodulation. Neuromodulation represents a swiftly advancing domain within the field of neurotechnology, employing physical energy for the modulation of both the central and peripheral nervous systems. Neuromodulation techniques possess the capability to modify abnormal neural activity by employing electrical, magnetic, optical, or acoustic energy, consequently mitigating the disease condition and establishing stability within the system.  Neuromodulation can be categorized into two main divisions: peripheral versus central nervous system (CNS) modulation and invasive versus noninvasive methods of interface [1]. Ultrasound neuromodulation is a non-invasive technique that can overcome the drawbacks of other neuromodulation techniques such as deep brain stimulation (DBS), transcranial magnetic stimulation (TMS), and transcranial current stimulation (tCS). Unlike DBS, it does not require surgery, which reduces the risk of infection and immunological reactions. Also, unlike TMS and tCS, ultrasound has better spatial specificity and can penetrate deep-seated brain regions, making it more suitable for targeting specific neural circuits. Therefore, ultrasound neuromodulation has the potential to be a safer and more precise alternative to other neuromodulation techniques [2],[3].

For understanding how ultrasound may affect the electrical activity of neuronal cells, the potential effects of non-electric stimuli such as ultrasound on the action potential (electrical signals) are clarified by biophysical models and hypotheses, including the soliton model, the flexoelectricity hypothesis, and the neuromembrane cavity stimulation model.

• Flexoelectricity Hypothesis

Like piezoelectricity, flexoelectricity is a phenomenon in which vibrations or mechanical changes in matter produce an electrical signal that leads to the occurrence of electric fluxes and related electrical effects. These electrical changes can increase or decrease the activity of neurons[4].

• Soliton model

The soliton model is a theoretical framework that describes how action potentials in nerve cells are produced and spread. According to this model, an ionic depolarization pulse that passes down the axon causes the action potential. The soliton model postulates that the action potential is a soliton, a self-reinforcing wave that keeps its shape and speed as it moves along the axon [5],[6]. According to the soliton model, thermodynamic factors can affect the action potential and it is a stable soliton.

• Neuronal intramembrane cavitation excitation (NICE) model

According to the Neuronal Intramembrane Cavitation Excitation (NICE) paradigm, ultrasound can affect neuronal activity by causing intramembrane cavitation to occur. When nanobubbles exist between the two leaflets of the lipid bilayer that make up the membrane of a neuron, this is referred to as intramembrane cavitation. According to this theory, the ultrasound’s mechanical energy is changed into electrical energy inside the neuron, causing modifications to the membrane’s electrical potential and the start of the action potential.

Finally, sound stimulation can have a variety of direct or indirect impacts on the function of neurons depending on the electrical processes and neurophysiology of neurons. Changes in membrane potential, action potential activity, and the transmission of nerve impulses in neural networks are only a few examples of these modifications [7]. We explained the basics of ultrasound in the previous article, you can visit our article [8] for more information about the biological effects of ultrasound, including cavitation. In this article,we will review the different techniques of ultrasound stimulation and the different applications of each technique in various fields, especially neuromodulation.

Types of ultrasound stimulation techniques and applications

Focused ultrasound Stimulation (FUS)

One prominent technique is focused ultrasound stimulation (FUS), which utilizes ultrasonic waves to target specific locations while minimizing pressure in surrounding areas. FUS can exert both thermal and mechanical effects, with the extent depending on the ultrasound exposure. Recent studies have demonstrated that both focused and unfocused ultrasound can modulate neuronal activity, although the patterns and outcomes of modulation differ. Focused ultrasound provides more effective energy delivery compared to unfocused ultrasound, which requires higher frequency and longer exposure for therapeutic impact. Transcranial focused ultrasound (tFUS), which is also a form of FUS, involves stimulating the brain through the skull in a non-invasive manner [2],[9],[10].

FUS has a wide range of applications in neurological and psychiatric conditions, such as Parkinson’s disease, ALS, epilepsy, reducing depression symptoms, enhancing memory performance in healthy individuals, chronic pain, and cognitive rehabilitation. For example, according to a recent study [11], FUS also is a promising method for non-invasive deep brain stimulation since it may have a good impact on the motivational and cognitive aspects of decision making. Applications in neurological and mental illnesses now cover the full clinical spectrum, and preclinical studies and human trials continue to yield new information. Figure 1, taken from the paper [9]  illustrates the various intracranial applications of FUS in humans. Additionally, tFUS has been shown to activate specific brain regions, such as the hippocampus and the prefrontal cortex, which are involved in memory consolidation and decision-making, respectively.

In the following paragraphs, we will delve elaborate on some selected application of FUS.

• Open the Blood-Brain Barrier

FUS has generated a great deal of interest as a safe treatment alternative because of its interactions with brain tissue, which range from blood-brain barrier opening (BBBO) to neuromodulation. In BBB, for example; the barrier is an obstacle to the passage of foreign bodies and actually keeps toxic substances away from the brain. As a result, it is also a barrier to keeping the drug away; here, FUS can be used to temporarily and safely open the blood-brain barrier to increase the transfer of drugs to the brain.  This process is shown in Figure 2 [12].

• Parkinson

Modification of brain activity in patients with Parkinson’s disease (PD) using focused ultrasound stimulation (FUS) is described in study [13]. According to the paper, FUS is safe and has shown beneficial effects on motor behavior in PD patients. Moreover, FUS has sharper spatial resolution and non-invasive deep penetration, which makes it a promising tool for neuromodulating deep brain structures relevant to PD, such as the striato-pallido-thalamic network or deep structures including the STN and Gpi, as shown in Figure 3.

• Nerve pain control

One kind of subjective experience that involves various functional brain circuits for processing is pain. Four ascending routes are involved in pain perception at the level of the central nervous system. These pathways carry nociceptive information from the dorsal root ganglions to the cerebral cortex [14]. Depending on the stimulation parameters and paradigm, tFUS modulation has been observed to produce both excitatory and inhibitory effects in the brain. tFUS has a higher spatial resolution than classic noninvasive brain stimulation methods like magnetic or electric stimulations and could stimulate deep brain areas. Because of this property, tFUS is an ideal tool for pain neuromodulation because it can almost target any region of the peripheral or central nervous system.

Pain associated with sickle cell disease (SCD) is an important model for chronic neuropathic pain syndromes in the central nervous system (CNS), due to its widespread nature and non-specific location. The ACC, S1, and insula are the most consistent locations, while the thalamus, ACC, and S1 are the most connected to pronociceptive areas in SCD patients with significant levels of pain. As shown in Figure 4, tFUS can be applied to these brain regions with specialized ultrasonic parameters and temporal sequences to suppress sensory input and promote emotional output. It is possible to increase neuromodulation effects while keeping spatial specificity by combining tFUS with pain-relieving therapies. Examples include endothelium selective transfection or ultrasound-enhanced therapeutic delivery by microbubble expansion, which provide spatially targeted medication or gene delivery. By incorporating pharmacological effects into the ultrasound neuromodulation framework, low-intensity tFUS can unlock and activate potential anesthetic agents, improving pain management capabilities [1].

Low-intensity focused ultrasound (LIFU)

Low-intensity focused ultrasound (LIFU) is a non-invasive technique that holds significant potential for neuromodulation in various clinical applications. It stands out due to its ability to penetrate the skull bone, concentrate in a targeted volume, and interact with biological tissues, making it a powerful tool for neuromodulation [15]. LIFU can also deliver low-intensity, pulsed mechanical waves to tissues like bone, cartilage, and tendon, resulting in regenerative and anti-inflammatory effects [2]. Its highly focused acoustic energy ensures a therapeutic effect only at the geometric focus of the transducer, making it precise and safe for treating neurological disorders [16],[17].

LIFU has been proven safe and capable of modulating neural tissue non-invasively in both animal and human subjects, targeting regions including the motor cortex, sensory cortex, visual cortex, frontal eye fields, and hippocampus. It can affect neuronal activity by causing the release of neurotransmitters like dopamine and glutamate and modulating the activity of ion channels [18],[2],[19]. It has been used to improve neuronal firing, suppress cortical and epileptic discharges, and induce behavioral changes in cortical and subcortical mammalian brain regions [20]. LIFU has shown effectiveness in thrombolysis in cerebral vasculature, urological surgery, neurosurgery, analgesic therapy, and other applications [15],[21]. The effects of LIFU on neuromodulation are believed to be attributed to thermal effects, radiation force, and acoustic cavitation [20].

• Blood-tumor barrier (BTB)

LIFU presents a promising alternative technique for functional neurosurgery, offering potential applications in tumor ablation [22],[23]. The combination of LIFU with intravenously administered oscillating microbubbles (MBs) has shown promise for systemic glioblastoma treatment. By enhancing the permeability of the blood-brain barrier (BBB) and blood-tumor barrier (BTB), LIFU+MBs facilitate the delivery of therapeutic agents directly to the brain or tumor parenchyma, potentially improving tumor control and overall survival [24].

Figure 5 illustrates the targeting and effects of low-intensity focused ultrasound (LIFU) during high and low pressures in the context of treating glioblastomas. At low pressure (stable cavitation), the red blood cells (RBCs) and immune cells are confined to the blood vessels, disrupting tight junctions (TJs) and creating openings with low levels of therapeutic extravasation. At higher pressure (inertial cavitation), wider openings are created between the endothelial cells, allowing for greater concentration of drugs delivered and extravasation of RBCs, immune cells, and therapeutic agents. The effects of LIFU targeting are visualized in Figure 5, which shows how external transducers target glioblastomas in practice and the possible scenarios at low and high pressures.

• Depression

Anxiety and depressive symptoms frequently coexist, and comorbidity is linked to more severe symptoms than either condition alone. Only 50% of patients who get standard first-line therapy experience remission, including those who receive psychotherapy and medication. Patients with depression have elevated levels of peripheral inflammatory biomarkers, and the degree of inflammation is correlated with the severity of particular symptoms. Multiple brain areas in depression patients exhibit neuroinflammation, according to research using positron emission computed tomography (PET) imaging and the 18 kDa translocator protein (TSPO) as a microglia biomarker (Fig.6). There is evidence that pharmacological therapy and neuromodulation interventions for psychiatric disorders can lower inflammation while providing symptomatic relief. TUS is one of the promising neuromodulation techniques that offers significant adjunctive therapy for the treatment of anxiety and depressive disorders [25]. Ultrasound stimulation has excellent therapeutic effects on depression in preclinical trials. Depressed model rats’ depressive behaviors were lessened by LIFUS stimulation of either the prefrontal cortex or the ventromedial prefrontal cortex (vmPFC), which was accompanied by higher levels of brain-derived neurotrophic factor (BDNF), whose downregulation is strongly associated with depression. An important finding was that LIFUS increased BDNF levels in the hippocampus of normal mice, pointing to a typical mechanism of BDNF signaling induced by ultrasonic stimulation. In addition, LIFUS improved adult hippocampus neural stem cells’ proliferation and neurogenesis [26]. Recent research by Sha-Sha Yi et al. [27] revealed that LIFUS can improve the behaviors of mice that have been depressed due to lipopolysaccharide. Additionally, LIFUS greatly decreased the increase in inflammatory cytokines caused by lipopolysaccharide. After around 30 minutes, healthy people’s moods were improved by tFUS targeting the right ventrolateral prefrontal cortex, according to research by Joseph L. Sanguinetti et al. [28]. Similar to this, research has indicated that LIFUS may elevate mood in healthy individuals [29]. These trials offer proof that LIFUS can be used to treat depression, but further research is still required to confirm that LIFUS actually improves the symptoms of depression.

High-Intensity Focused Ultrasound (HIFU)

This method involves the application of high-intensity ultrasound waves focused on a target location, raising the temperature to 70-80 °C. High-Intensity Focused Ultrasound (HIFU) is a technique used for non-invasive thermal or mechanical ablation of benign and malignant tissue. The strong heat generated by HIFU leads to the destruction of tissue through various mechanisms, including heat shock, tissue coagulation necrosis, and cavitation [30],[31],[32]. One notable advantage of HIFU is its ability to eliminate tissue with fewer adverse effects. Focal thermal ablation of deep-brain circuits, guided by magnetic resonance imaging (MRI), has shown promise in ameliorating movement abnormalities and psychiatric illnesses. Although the increase in temperature induced by ultrasound may be less visually prominent, temperature changes within the physiological range can still modulate neuronal activity [33].

• Tissue repair

In the context of wound healing, researchers have investigated the effects of High-Intensity Ultrasound (HIUS) on cellular processes involved in tissue regeneration. Their study utilized in vitro wound models and human dermal fibroblasts. The results showed that short-term ultrasound treatment significantly enhanced fibroblast proliferation, migration, and the production of crucial components for wound healing, such as fibronectin and collagen. Furthermore, HIUS promoted the contraction of a three-dimensional collagen matrix embedded with fibroblasts, particularly in the presence of TGF-β. RNA-sequencing and bioinformatics analyses revealed changes in gene expression and the activation of specific signaling pathways (p38 and ERK1/2 MAPK) in ultrasound-stimulated fibroblasts. These findings suggest that HIUS can activate human dermal fibroblasts, indicating its potential to improve wound healing and facilitate skin regeneration [33].

• Cancer

(HIFU) is a clinically applied technique for treating malignant and benign tumors. It offers several advantages over traditional cancer treatments such as chemotherapy, radiotherapy, and open surgery. HIFU is a non-invasive and non-ionizing modality that can precisely target tumors without harming the overlying skin and tissues. This is achieved by focusing the ultrasound energy at the tumor site, causing coagulative necrosis and effectively destroying the tumor tissue Figure 7. The focused energy causes coagulative necrosis of the tumor, effectively heating and destroying the tumor tissue. HIFU has been proven effective for treating both primary solid tumors and metastatic diseases, and its efficacy has been demonstrated in various cancer treatments. In liver cancer, HIFU has been utilized to selectively ablate tumor nodules, resulting in improved symptoms and extended survival periods for patients Figure 8 [34].  For breast cancer, HIFU offers a non-surgical approach, particularly beneficial for high-risk surgical patients. It provides advantages such as local tumor necrosis, reduced anesthesia requirements, shorter recovery times, lower infection risks, and no scar formation, with high rates of coagulation necrosis and disease-free survival observed. In prostate cancer, HIFU has shown promise by significantly reducing prostate-specific antigen levels and exhibiting positive survival rates, especially for patients who are not eligible for surgery. Similarly, HIFU ablation therapy presents an attractive alternative to surgery for small renal tumors, producing positive outcomes such as irreversible damage to treated areas, tumor necrosis, tumor shrinkage, and pain relief. HIFU therapy has also demonstrated potential efficacy in treating localized esophageal tumors, exhibiting significant tumor necrosis and improvement in dysphagia. However, when it comes to pancreatic cancer, although HIFU treatment has shown reductions in tumor size and response rates when combined with chemotherapy, complications and side effects have been reported, necessitating further research [35].   In the realm of brain cancer, HIFU has been studied for its feasibility in treating glioblastoma, the most common malignant brain tumor. It has demonstrated the ability to induce focal heating in brain tumors and possesses immunomodulatory effects that could potentially enhance immunotherapy response. Additional trials are required to validate these findings. Lastly, in bone cancer, HIFU has been used to destroy tumor microvessels, provide pain palliation for metastatic tumors, and treat primary bone malignancies [36],[37]. Treated patients have exhibited high survival rates, tumor regression, and pain reduction, highlighting the efficacy of HIFU in this domain.

Peripheral Focused Ultrasound Stimulation (pFUS)

Peripheral focused ultrasound (pFUS) is another approach in ultrasound stimulation, providing precise stimulation to peripheral nerves, neural endings, or sub-organs. This method has emerged as a promising technique in neuromodulation and human-computer interaction. Two categories of pFUS exist: body-coupled ultrasound stimulation, commonly used for therapeutics or neuromodulation, and air-coupled contactless ultrasound haptic systems, enabling human-computer interaction paradigms Figure 9 [38]. Peripheral focused ultrasound (pFUS), offers high spatial and temporal resolution for targeted and localized stimulation without affecting surrounding tissues. This technique directly targets specific anatomical sites within organs, allowing for selective modulation of sub-organ physiology. Recent studies have investigated the effectiveness of pFUS in modulating neural signaling in peripheral organs such as the spleen and liver. These organs play crucial roles in regulating inflammation and blood glucose levels. The experiments have shown that pFUS can achieve similar outcomes compared to implant-based vagus nerve stimulation (VNS) [39]. Figure 10 visually illustrates the distinction between VNS and precision ultrasound (U/S) neuromodulation. In VNS Figure 10a, a device is implanted in the cervical vagus nerve, stimulating both target and non-target pathways. However, achieving precise stimulation by implementing smaller stimulators or advanced electrode designs closer to the target organ remains challenging. On the other hand, precision ultrasound neuromodulation Figure 10b directly targets specific anatomical sites within organs that are innervated by known axonal populations. In this case, the spleen and liver are targeted. The U/S technique allows for selective modulation of sub-organ physiology, such as the anti-inflammatory pathway in the spleen and metabolic pathways in the liver.

Low-intensity pulsed ultrasound ( LIPUS)

LIPUS (Low-intensity pulsed ultrasound) is a minimally invasive therapeutic technology that delivers acoustic pulsed energy to promote healing and regeneration in various parts of the body. It operates at a frequency in the low megahertz range, typically between 1 and 3 MHz. LIPUS has been widely recognized as an essential method for tissue repair and regeneration, particularly in the treatment of wounded tissues, bone healing, and wound healing [40],[41].

Research studies have shown that LIPUS has a significant impact on various cell types. By generating minor biomechanical interactions with cells, LIPUS triggers intracellular biological effects that ultimately contribute to tissue repair and regeneration. For instance, LIPUS stimulation has been found to positively affect stem cell-based tissue regeneration. Several types of postnatal mesenchymal stem cells (MSCs), including adipose-derived stem cells, bone marrow mesenchymal stem cells (BMSCs), periodontal ligament-derived stem cells (PDLSCs), and human umbilical cord-derived MSCs, have demonstrated increased viability, proliferation, and multilineage differentiation when exposed to LIPUS [41],[42].

In the dental field, LIPUS has shown promise in stimulating dental tissue regeneration. In vitro and in vivo research indicates that dentinogenesis, rapid periodontal tissue repair, and dental implant osseointegration can be facilitated by exposing dental tissues to LIPUS. One study specifically explored the therapeutic effect of LIPUS on dental and pulp tissue regeneration, utilizing mesenchymal stem cells from dental tissues. The researchers found that dental MSCs responded to LIPUS, leading to increased proliferation across various sources of MSCs. As well as, another study focused on the use of LIPUS to enhance the osteogenic potential of ligament stem cells extracted from the tooth’s side, showing increased osteogenic stage through pulsed ultrasound. These findings highlight the potential of LIPUS to control osteogenic capacity in an inflammatory environment through the unfolded protein response (UPR) in periodontal ligament stem cells (PDLSC) both in vitro and in vivo [43],[44].

LIPUS has also demonstrated effectiveness in improving bone fracture healing by inducing molecular, biochemical, and biomechanical changes at the fracture site. Compared to other fracture therapies, LIPUS therapy is considered a safe adjuvant therapy to accelerate bone healing in cases of new fractures, delayed fractures, and nonunions [45]. One specific study investigated the effects of LIPUS on the growth and osteogenic differentiation of human alveolar bone-derived mesenchymal stem cells (hABMSCs) for tooth tissue engineering. The research examined different duty cycles (20% and 50%) of LIPUS treatment and assessed cell viability, alkaline phosphatase (ALP) activity, gene expression, and mineralized nodule formation. The findings revealed that hABMSCs exposed to LIPUS exhibited higher cell viability, increased ALP activity, and upregulation of CD29, CD44, COL1, and OCN gene expressions compared to the control group. Moreover, LIPUS treatment enhanced mineralized nodule formation, suggesting its potential as an effective treatment for periodontal healing and regeneration [46].

Conclusion

We investigated the efficiency and appropriateness of ultrasound stimulation as a therapeutic, augmentation and rehabilitation approach. Due to its advantages, ultrasound stimulation can be a better alternative to other neuromodulation methods. According to the studies, we see that this non-invasive technique has a high potential in the development of neuroscience research and new treatments for psychiatric and neurological disorders. Also, the need for more research in this field is felt in order to examine the effectiveness and long-term effects of these techniques and the safety of these techniques on patients in more detail.

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