Effects of electrical stimulation on skin surface

The development, use, and challenges of electromechanical tissue stimulation systems

BACKGROUND

Tissue stimulations greatly affect cell growth, phenotype, and function, and they play an important role in modeling tissue physiology. With the goal of understanding the cellular mechanisms underlying the response of tissues to external stimulations, in vitro models of tissue stimulation have been developed in hopes of recapitulating in vivo tissue function.

METHODS

Herein we review the efforts to create and validate tissue stimulators responsive to electrical or mechanical stimulation including tensile, compression, torsion, and shear.

RESULTS

Engineered tissue platforms have been designed to allow tissues to be subjected to selected types of mechanical stimulation from simple uniaxial to humanoid robotic stain through equal-biaxial strain. Similarly, electrical stimulators have been developed to apply selected electrical signal shapes, amplitudes, and load cycles to tissues, lending to usage in stem cell-derived tissue development, tissue maturation, and tissue functional regeneration. Some stimulators also allow for the observation of tissue morphology in real-time while cells undergo stimulation. Discussion on the challenges and limitations of tissue simulator development is provided.

CONCLUSIONS

Despite advances in the development of useful tissue stimulators, opportunities for improvement remain to better reproduce physiological functions by accounting for complex loading cycles, electrical and mechanical induction coupled with biological stimuli, and changes in strain affected by applied inputs.

Baroreceptor-Inspired Microneedle Skin Patch for Pressure-Controlled Drug Release

Objective: We have developed a baroreceptor-inspired microneedle skin patch for pressure-controlled drug release. Impact Statement: This design is inspired by the skin baroreceptors, which are mechanosensitive elements of the peripheral nervous system. We adopt the finger touching to trigger the electric stimulation, ensuring a fast-response and user-friendly administration with potentially minimal off-target effects. Introduction: Chronic skin diseases bring about large, recurrent skin damage and often require convenient and timely transdermal treatment. Traditional methods lack spatiotemporal controllable dosage, leaving a risk of skin irritation or drug resistance issues. Methods: The patch consists of drug-containing microneedles and stretchable electrode array. The electrode array, integrated with the piezoconductive switch and flexible battery, provides a mild electric current only at the spot that is pressed. Drugs in microneedles will then flow along the current into the skin tissues. The stretchable feature also provides the mechanical robustness and electric stability of the device on large skin area. Results: This device delivers Cy3 dye in pig skin with spatiotemporally controlled dosage, showing ~8 times higher fluorescence intensity than the passive delivery. We also deliver insulin and observe the reduction of the blood glucose level in the mouse model upon pressing. Compared with passive delivery without pressing, the dosage of drugs released by the simulation is 2.83 times higher. Conclusion: This baroreceptor-inspired microneedle skin patch acts as a good example of the biomimicking microneedle device in the precise control of the drug release profile at the spatiotemporal resolution.

Bioelectric medicine: unveiling the therapeutic potential of micro-current stimulation

Bioelectric medicine (BEM) refers to the use of electrical signals to modulate the electrical activity of cells and tissues in the body for therapeutic purposes. In this review, we particularly focused on the microcurrent stimulation (MCS), because, this can take place at the cellular level with sub-sensory application unlike other stimuli. These extremely low-level currents mimic the body's natural electrical activity and are believed to promote various physiological processes. To date, MCS has limited use in the field of BEM with applications in several therapeutic purposes. However, recent studies provide hopeful signs that MCS is more scalable and widely applicable than what has been used so far. Therefore, this review delves into the landscape of MCS, shedding light on the multifaceted applications and untapped potential of MCS in the realm of healthcare. Particularly, we summarized the hierarchical mediation from cell to whole body responses by MCS including its physiological applications. Our final objective of this review is to contribute to the growing body of literature that unveils the captivating potential of BEM, with MCS poised at the intersection of technological innovation and the intricacies of the human body.

The effect of electrical stimulation on skin vulnerability to irritants

PURPOSE

Electrical stimulation (ES) is a widely used technique in the medical field for various purposes. The effect of ES on several skin properties has been investigated; however, its effect on skin vulnerability to irritants remains unknown. This study aimed to investigate the effects of ES application on skin vulnerability to external irritants.

MATERIALS AND METHODS

An experimental study on 12 healthy male subjects (Mean ± SD, 22.9 ± 3.6 years) who completed the study. The subjects were free of skin abnormalities in the volar aspect of both forearms. Three areas were allocated to each forearm and marked as areas 1, 2, and A in the treated forearm, and areas 3, 4, and B in the control forearm. ES was applied to the volar aspect of the treated forearm for 30 min three times a week, for 2 weeks. The effect of ES on skin vulnerability was investigated using 5% and 0.5% sodium lauryl sulfate (SLS) patches applied to both treated and control forearms. The skin response to irritants was evaluated using transepidermal water loss (TEWL) and a visual erythema score 24 h after patch removal.

RESULTS

Compared to the control forearm, ES increased skin permeability and erythema in response to external irritants (SLS), as measured by the visual analog score (Z = 2.75, p = 0.006) and TEWL (p < 0.05), respectively.

CONCLUSIONS

ES escalates skin reactions to low concentrations of irritant substances, such as SLS, in the area between the two electrodes. This emphasizes the use of this substance, and similar irritants should be avoided in areas treated with ES.

Exploring sensory, motor, and pain responses as potential side or therapeutic effects of sub-2 mA, 400 Hz transcranial pulsed current stimulation

BACKGROUND

Various brain stimulation devices capable of generating high-frequency currents are readily available. However, our comprehension of the potential side or therapeutic effects associated with high-frequency transcranial pulsed current stimulation (tPCS), particularly concerning the new 400 Hz tPCS device, AscenZ-IV Stimulator, developed by AscenZion Neuromodulation Co. Pte. Ltd. in Singapore, remains incomplete.

OBJECTIVE

This study examines preliminary parameters for the safe and comfortable application of 400 Hz tPCS at intensities below 2 mA.

METHODS

In a cross-sectional study, 45 healthy participants underwent sub-2 mA 400 Hz tPCS to assess sensory, motor, and pain thresholds on the dominant side. Study 1 (N = 15) targeted the primary motor cortex of the right-hand area, while study 2 (N = 30) focused on the back of the right forearm.

RESULTS

Study one showed that increasing the current intensity gradually resulted in no responses at sub-0.3 mA levels, but higher intensities (p < 0.001) induced sensory perception and pain responses. Study two replicated these findings and additionally induced motor responses along with the sensory and pain responses.

CONCLUSION

Despite the theoretical classification of tPCS as a subsensory level of stimulation, and the expectation that individuals receiving this type of current should not typically feel its application on the body, this high-frequency tPCS device generates different levels of stimulation due to the physiological phenomenon known as temporal summation. These novel levels of stimulation could be viewed as either potential "side-effects" of high frequency tPCS or as additional "therapeutic benefits". This dual capacity may position the device as one that generates both neuromodulatory and neurostimulatory currents. Comprehensive comprehension of this is vital for the development of therapeutic protocols that incorporate high-frequency tPCS.

Kilohertz-frequency interferential current induces hypoalgesic effects more comfortably than TENS

Recent research on transcutaneous electrical stimulation has shown that inhibiting nerve conduction with a kilohertz frequency is both effective and safe. This study primarily aims to demonstrate the hypoalgesic effect on the tibial nerve using transcutaneous interferential-current nerve inhibition (TINI), which injects the kilohertz frequency produced by the interferential currents. Additionally, the secondary objective was to compare the analgesic effect and comfort of TINI and transcutaneous electrical nerve stimulation (TENS). Thirty-one healthy adults participated in this cross-over repeated measures study. The washout period was set to 24 h or more. Stimulus intensity was set just below the pain threshold level. TINI and TENS were applied for 20 min each. The ankle passive dorsiflexion range of motion, pressure pain threshold (PPT), and tactile threshold were measured at the baseline, pre-test, test (immediately before ceasing intervention), and post-test (30 min after ceasing intervention) sessions. After the interventions, the participants evaluated the level of discomfort for TINI and TENS on a 10 cm visual analog scale (VAS). As the results, PPT significantly increased compared to baseline in test and posttest sessions of TINI, but not in those of TENS. Also, participants reported that TENS was 36% more discomfort than TINI. The hypoalgesic effect was not significantly different between TINI and TENS. In conclusion, we found that TINI inhibited mechanical pain sensitivity and that the inhibitory effect persisted long after electrical stimulation ceased. Our study also shows that TINI provides the hypoalgesic effect more comfortably than TENS.

Physiological effects of microcurrent and its application for maximising acute responses and chronic adaptations to exercise

Microcurrent is a non-invasive and safe electrotherapy applied through a series of sub-sensory electrical currents (less than 1 mA), which are of a similar magnitude to the currents generated endogenously by the human body. This review focuses on examining the physiological mechanisms mediating the effects of microcurrent when combined with different exercise modalities (e.g. endurance and strength) in healthy physically active individuals. The reviewed literature suggests the following candidate mechanisms could be involved in enhancing the effects of exercise when combined with microcurrent: (i) increased adenosine triphosphate resynthesis, (ii) maintenance of intercellular calcium homeostasis that in turn optimises exercise-induced structural and morphological adaptations, (iii) eliciting a hormone-like effect, which increases catecholamine secretion that in turn enhances exercise-induced lipolysis and (iv) enhanced muscle protein synthesis. In healthy individuals, despite a lack of standardisation on how microcurrent is combined with exercise (e.g. whether the microcurrent is pulsed or continuous), there is evidence concerning its effects in promoting body fat reduction, skeletal muscle remodelling and growth as well as attenuating delayed-onset muscle soreness. The greatest hindrance to understanding the combined effects of microcurrent and exercise is the variability of the implemented protocols, which adds further challenges to identifying the mechanisms, optimal patterns of current(s) and methodology of application. Future studies should standardise microcurrent protocols by accurately describing the used current [e.g. intensity (μA), frequency (Hz), application time (minutes) and treatment duration (e.g. weeks)] for specific exercise outcomes, e.g. strength and power, endurance, and gaining muscle mass or reducing body fat.

Cell Secretome Strategies for Controlled Drug Delivery and Wound-Healing Applications

There is significant interest in using stem cells in the management of cutaneous wounds. However, potential safety, efficacy, and cost problems associated with whole-cell transplantation hinder their clinical application. Secretome, a collective of mesenchymal stem-cell-stored paracrine factors, and immunomodulatory cytokines offer therapeutic potential as a cell-free therapy for the treatment of cutaneous wounds. This review explores the possibility of secretome as a treatment for cutaneous wounds and tissue regeneration. The review mainly focuses on in vitro and in vivo investigations that use biomaterials and secretome together to treat wounds, extend secretome retention, and control release to preserve their biological function. The approaches employed for the fabrication of biomaterials with condition media or extracellular vesicles are discussed to identify their future clinical application in wound treatment.

Effect of Microgravity on Endothelial Cell Function, Angiogenesis, and Vessel Remodeling During Wound Healing

Wound healing is a complex phenomenon that involves different cell types with various functions, i.e., keratinocytes, fibroblasts, and endothelial cells, all influenced by the action of soluble mediators and rearrangement of the extracellular matrix (ECM). Physiological angiogenesis occurs in the granulation tissue during wound healing to allow oxygen and nutrient supply and waste product removal. Angiogenesis output comes from a balance between pro- and antiangiogenic factors, which is finely regulated in a spatial and time-dependent manner, in order to avoid insufficient or excessive nonreparative neovascularization. The understanding of the factors and mechanisms that control angiogenesis and their change following unloading conditions (in a real or simulated space environment) will allow to optimize the tissue response in case of traumatic injury or medical intervention. The potential countermeasures under development to optimize the reparative angiogenesis that contributes to tissue healing on Earth will be discussed in relation to their exploitability in space.