Low-frequency sonophoresis: ultrastructural basis for stratum corneum permeability assessed using quantum dots

Microbubble-Enhanced Transdermal Drug Delivery Sonoelectric Patch

Transdermal drug delivery systems are highly appealing as a convenient drug delivery manner applicable to a wide variety of drugs. While most delivery relies on only passive diffusion and suffers low transdermal efficiencies. Ultrasound motivation promotes drug transdermal penetration but still calls for improvement, because only a thin proportion of the ultrasound energy is applied on the drug delivery patch and most ultrasound energy is wasted in deeper portions of biotissues. In this work, we develop a transdermal patch for enhanced drug delivery. The combination of microsized air pockets and the piezoelectric soft structure enable the conversion of an intended proportion of ultrasound energy into electric energy. The intensified drug flow and synergistic ultrasound pressure and electric field function simultaneously to enhance drug transdermal delivery. The delivery efficacy is related to the power of the ultrasound motivation, the size of the microscopic air pockets, and the chemical structure of the drug molecules. The temperature of the patch within the delivery process remains in the safe range, and the mild temperature elevation causes color changes of the thermochromic patch, used to indicate effective ultrasound-patch matching. A model delivery patch for pain release is constructed, and animal experiments indicate that the drug blood concentrations are 100% higher than the delivery using only ultrasound and even more remarkably enhanced when compared to only electric-field-motivated delivery or static delivery without external motivations.

Progress and Challenges of Topical Delivery Technologies Meditated Drug Therapy for Osteoarthritis

Osteoarthritis (OA) is a degenerative disease commonly seen in middle-aged and elderly people. Multiple cytokines are involved in the local tissue damage in OA. Currently, non-pharmacologic and surgical interventions are the main conventional approaches for the treatment of OA. In terms of pharmaceutical drug therapy, NSAIDs and acetaminophen are mainly used to treat OA. However, it is prone to various adverse reactions such as digestive tract ulcer, thromboembolism, prosthesis loosening, nerve injury and so on. With the in-depth study of OA, more and more novel topical drug delivery strategies and vehicles have been developed, which can make up for the shortcomings of traditional dosage forms, improve the bioavailability of drugs, and significantly reduce drug side effects. This review summarizes the immunopathogenesis, treatment guidelines, and progress and challenges of topical delivery technologies of OA, with some perspectives on the future pharmacological treatment of OA proposed.

Miniaturized therapeutic systems for ultrasound-modulated drug delivery to the central and peripheral nervous system

Ultrasound is a promising technology to address challenges in drug delivery, including limited drug penetration across physiological barriers and ineffective targeting. Here we provide an overview of the significant advances made in recent years in overcoming technical and pharmacological barriers using ultrasound-assisted drug delivery to the central and peripheral nervous system. We commence by exploring the fundamental principles of ultrasound physics and its interaction with tissue. The mechanisms of ultrasonic-enhanced drug delivery are examined, as well as the relevant tissue barriers. We highlight drug transport through such tissue barriers utilizing insonation alone, in combination with ultrasound contrast agents (e.g., microbubbles), and through innovative particulate drug delivery systems. Furthermore, we review advances in systems and devices for providing therapeutic ultrasound, as their practicality and accessibility are crucial for clinical application.

Comparative study of permeation effects between vibrating microneedle and low-frequency sonophoresis systems

Microneedle transdermal administration and low-frequency ultrasound represent two important physical penetration-promoting methods for enhancing drug penetration. This article aims to investigate and compare the effects of drug penetration enhancement through transdermal administration using vibrating microneedles versus low-frequency sonophoresis. In Vitro permeation studies were conducted using Valia-Chien double chamber diffusion cells to evaluate the transdermal delivery of tetramethylpyrazine hydrochloride (TMPH). The TMPH concentration in the receiving compartment was determined using high-performance liquid chromatography (HPLC). Several combinations of microneedles and ultrasound settings were investigated, including different needle heights, vibration frequencies, exposure times, and assorted distances of ultrasound horn and skin. The results revealed the vibrating microneedle system as the most efficacious treatment to increase the TMPH permeability into the rat skin. The combination of a larger needle, higher frequency, and a 3-min exposure led to a 41.92-fold increase in cumulative permeability compared to the control group. The ultrasound treatment exhibited a moderate enhancement effect on TMPH skin penetration. Using a horn-to-skin distance of 3 mm and a 3-min exposure resulted in a 4.34-fold increase in TMPH cumulative permeation compared to the control group. It could be concluded that while both the vibrating microneedle and the low-frequency ultrasound systems act as penetration enhancers for promoting the TMPH permeation through the skin, the vibrating microneedle system notably demonstrates a more effective penetration-promoting effect.

Sonosensitive Cavitation Nuclei-A Customisable Platform Technology for Enhanced Therapeutic Delivery

Ultrasound-mediated cavitation shows great promise for improving targeted drug delivery across a range of clinical applications. Cavitation nuclei-sound-sensitive constructs that enhance cavitation activity at lower pressures-have become a powerful adjuvant to ultrasound-based treatments, and more recently emerged as a drug delivery vehicle in their own right. The unique combination of physical, biological, and chemical effects that occur around these structures, as well as their varied compositions and morphologies, make cavitation nuclei an attractive platform for creating delivery systems tuned to particular therapeutics. In this review, we describe the structure and function of cavitation nuclei, approaches to their functionalization and customization, various clinical applications, progress toward real-world translation, and future directions for the field.

Evolution of Transdermal Drug Delivery Devices and Novel Microneedle Technologies: A Historical Perspective and Review

The history of transdermal drug delivery is as old as humankind. Transdermal drug delivery has undergone three generations of development; the third generation has involved the use of medical devices and instruments. This review provides a historical perspective on the primary approaches employed in the three generations of transdermal drug delivery. In addition, we explore some of the recently developed transdermal techniques that are deemed promising in the field of drug delivery. We discuss how advances in these techniques have led to the development of devices for the delivery of a therapeutically effective amount of drug across human skin and highlight the limitations of the first- and second-generation drug delivery tools. As such, a review of the performance of these techniques and the toxicity of the devices used in transdermal drug delivery are considered. In the last section of the review, a discussion of the fabrication and operation of different types of microneedles is presented. The applications of microneedles in the sensing and delivery of various therapeutic agents are described in detail. Furthermore, an overview of the efficacy of microneedles as emerging tools for the controlled release of drugs is presented.

A Conformable Ultrasound Patch for Cavitation-Enhanced Transdermal Cosmeceutical Delivery

Increased consumer interest in healthy-looking skin demands a safe and effective method to increase transdermal absorption of innovative therapeutic cosmeceuticals. However, permeation of small-molecule drugs is limited by the innate barrier function of the stratum corneum. Here, a conformable ultrasound patch (cUSP) that enhances transdermal transport of niacinamide by inducing intermediate-frequency sonophoresis in the fluid coupling medium between the patch and the skin is reported. The cUSP consists of piezoelectric transducers embedded in a soft elastomer to create localized cavitation pockets (0.8 cm² , 1 mm deep) over larger areas of conformal contact (20 cm² ). Multiphysics simulation models, acoustic spectrum analysis, and high-speed videography are used to characterize transducer deflection, acoustic pressure fields, and resulting cavitation bubble dynamics in the coupling medium. The final system demonstrates a 26.2-fold enhancement in niacinamide transport in a porcine model in vitro with a 10 min ultrasound application, demonstrating the suitability of the device for short-exposure, large-area application of sonophoresis for patients and consumers suffering from skin conditions and premature skin aging.

Fabrication of High-Resolution Fine Microneedles Derived from Hydrolyzed Hyaluronic Acid Gels in Vacuum Environment Imprinting Using Water Permeable Mold

Hydrolyzed hyaluronic acid high-resolution fine microneedles of 13 µm in diameter and 24 µm in height were fabricated from hydrolyzed hyaluronic acid gels made in mixtures of water using vacuum environment imprint lithography processes with a water permeable mold. The gas traps of water and volatile solvents in the imprint materials cause transfer failure in the conventional water impermeable molds of quartz and metal. However, the water permeable mold allows the use of 67 wt% dilution water with high solubility to increase the fluidity of the hydrolyzed hyaluronic acid during the patterning of high-resolution fine microneedles for cosmetics and pharmaceuticals. This demonstration sets a new paradigm of functional pure gels for high-resolution nano-patterning applications with various cosmetic and pharmaceutical materials containing dilution water using a water permeable mold.

Quantitative Analysis of Acoustic Pressure for Sonophoresis and Its Effect on Transdermal Penetration

Ultrasound facilitates the penetration of macromolecular compounds through the skin and offers a promising non-invasive technique for transdermal delivery. However, technical difficulties in quantifying ultrasound-related parameters have restricted further analysis of the sonophoresis mechanism. In this study, we devise a bolt-clamped Langevin transducer-based sonophoresis device that enables us to measure with a thin lead zirconate titanate (PZT) sensor. One-dimensional acoustic theory accounting for wave interaction at the skin interface indicates that the acoustic pressure and cavitation onset on the skin during sonophoresis are sensitive to the subcutaneous support, meaning that there is a strong need to perform the pressure measurement in an experimental environment replacing the human body. From a series of the experiments with our new device, the transdermal penetration of polystyrene, silica and gold nanoparticles is found to depend on the size and material of the particles, as well as the hardness of the subcutaneous support material. We speculate from the acoustic pressure measurement that the particles' penetration results from the mechanical action of cavitation.

Dexamethasone Sodium Phosphate Penetration During Phonophoresis at 2 Ultrasound Frequencies

CONTEXT

The effect of ultrasound frequency on phonophoresis drug delivery in humans is unknown.

OBJECTIVE

To determine if a low (45-kHz) or high (1-MHz) frequency delivered a higher dexamethasone (Dex) concentration through the skin.

DESIGN

Controlled laboratory study.

SETTING

Laboratory.

PATIENTS OR OTHER PARTICIPANTS

A total of 40 healthy men between the ages of 18 and 45 years (age = 23.1 ± 2.6 years, height = 176.1 ± 7.2 cm, mass = 88.5 ± 19.4 kg, posterior calf subcutaneous thickness measured using musculoskeletal ultrasound imaging = 0.6 ± 0.2 cm).

INTERVENTION(S)

Participants were randomly assigned to 1 of 4 groups (ultrasound frequency at microdialysis probe depth): (1) 45-kHz frequency at 1 mm, (2) 45-kHz frequency at 4 mm, (3) 1-MHz frequency at 1 mm, or (4) 1-MHz frequency at 4 mm (n = 10 in each group). Three linear microdialysis probes were inserted at the desired tissue depth. We rubbed dexamethasone sodium phosphate (Dex-P) into the skin and then applied a 15-minute phonophoresis treatment.

MAIN OUTCOME MEASURE(S)

Dialysate was collected during the treatment and 60 minutes posttreatment and analyzed for Dex-P, Dex, and the metabolite form of Dex. The sum of the 3 analytes was calculated as total dexamethasone (Dex-total), and differences between the 45-kHz and 1-MHz treatment groups were determined by a repeated-measures analysis of variance.

RESULTS

At 1 mm, 3 (30%) participants in the 45-kHz and 4 (40%) participants in the 1-MHz group had measurable levels of Dex-P. Total dexamethasone increased after the treatment ceased, independent of ultrasound frequency (P < .001), with a trend of the 45-kHz treatment to produce a greater increase in drug concentration (P = .006). At 4 mm, 5 (50%) participants in the 45-kHz and 1 (10%) participant in the 1-MHz group had measurable levels of Dex-P. We observed no difference in Dex-total concentration between treatment groups at 4 mm (P = .72).

CONCLUSIONS

Phonophoresis provided a mechanism for Dex-total delivery at the 1- and 4-mm tissue depths. However, the effectiveness of the ultrasound frequencies varied between the 2 measured tissue depths.

Circulation Cooling in Continuous Skin Sonoporation at Constant Coupling Fluid Temperatures

Exposure of the skin to low-frequency ultrasound in the Franz diffusion cell has been found to increase the permeability of the skin to molecular transport. In many cases, significant heating of the coupling fluid requires the use of duty cycles that extend the total experimental time. This is a methodological study in which the coupling fluid is circulated between a modified Franz diffusion cell and a heat exchanger to allow for the continuous application of low-frequency ultrasound while the coupling fluid temperature is held constant. Dermatomed porcine skin was exposed to continuous ultrasound at 20 kHz for 10 min at an intensity of 55 W/cm² while the coupling fluid was maintained at one of three target temperatures (13°C, 33°C or 46°C). Foil pitting and passive cavitation detection revealed that inertial cavitation activity decreased with increasing coupling fluid target temperature. Transport measurements revealed an increase in mean donor calcein concentration with increasing coupling fluid temperature, though these were not statistically significant. Taken together these findings suggest that the weakened stratum corneum lipid structure at higher temperatures is more susceptible to the introduction of defects from the jetting of cavitation.

Biosynthesis of size-controlled gold nanoparticles using M. lucida leaf extract and their penetration studies on human skin for plastic surgery applications

Recently, majority of the studies were focusing on the nanoparticles (NPs) and their abilities of penetrating Stratum Corneum (SC), as they can be prominently utilized in the plastic surgeries. In the current work, we demonstrated the penetrating abilities of gold NPs (AuNPs) through anthropological skin with diameters of 10 and 15 nm, varying in sizes, with the help of Multiphoton Microscopy. In addition, we also demonstrated a rapid facile environment friendly process of synthesizing AuNPs of adjustable sizes with the help of aqueous M. lucida leaf extract. Surface plasmon resonance was performed to confirm the synthesis of AuNPs at 530 nm with the help of UV-vis spectrophotometer. By differentiating the quantities of M. lucida leaf aqueous extracts, we studied the reduction time, morphological differences and size of the AuNPs. By performing Fourier Transformation Infrared Spectroscopy (FTIR), UV-vis spectroscopy, Transmission Electron Microscopy (TEM), Powder X-ray Diffraction (XRD), Energy Dispersive X-ray Spectroscopy (EDAX) and Selected Area Electron Diffraction (SAED), we characterized the fabricated AuNPs. The further aggregation and growth of AuNPs was protected by the polyphenols in the oxidised form by having a coordination with the surface of AuNPs. Moreover, the experiments of skin penetration showed an effort to deeply examine the factors leading to the penetration of particles into the human skin. These responses indicate that NPs at the determined size ranges penetrate the SC in the same pattern of the drug molecules, mostly by the intercellular paths. These responses attained were essential for developing a unique transdermal transporter as well as for understanding the basic interaction of skin-NPs for the application of plastic surgeries.

Controlled Ultrasound Erosion for Transdermal Delivery and Hepatitis B Immunization

Although ultrasound is effective for transdermal delivery, it remains difficult to control the position, shape and size of localized skin transport regions. We developed an ultrasound erosion protocol to generate a single-site, circular delivery region with controlled size at the center of patched skin. We found that (i) shorter ultrasound pulses (25 cycles) with higher pulse repetition frequency (4 kHz) and higher peak negative pressure (17.0 MPa) resulted in larger (0.995 mm²) and deeper (∼300 µm) skin delivery regions with a higher success rate (94.44%); and (ii) temperature elevation of the skin increased with ultrasound exposure time, with a 30-s safety threshold. Furthermore, we found that hair follicles decreased the delivery controllability of ultrasound erosion. Therefore, we selected the skin of the hind legs of mice without dense hair follicles to deliver more than 1 μL of vaccine solution and successfully elicit immune responses against hepatitis B surface antigen.

A Thermoelectric Device for Coupling Fluid Temperature Regulation During Continuous Skin Sonoporation or Sonophoresis

During skin sonoporation and sonophoresis, time-consuming duty cycles or fluid replacement is often required to mitigate coupling fluid temperature increases. This study demonstrates an alternative method for temperature regulation: a circulating, thermoelectric system. Porcine skin samples were sonoporated continuously for 10 min at one of three intensities (23.8, 34.2, 39.4 W/m²). A caffeine solution was then applied to the skin and left to diffuse for 20 h. During sonoporation, the system was able to maintain the temperature between 10 and 16°C regardless of the intensity. No increase in transdermal transport was achieved with an intensity of 23.8 W/m². Intensities of 34.2 and 39.4 W/m² resulted in 3.5-fold (p < 0.05) and 3.7-fold (p < 0.05) increases in mean transport, relative to a control case with no ultrasound. From these results, it is concluded that a significant transport increase can be achieved with a system that circulates and cools the coupling fluid during ultrasound application. Relative to the previous methods of temperature control (duty cycles and fluid replacement), use of this circulation system will lead to significant time savings in future experimental studies.

Acoustic-transfection for genomic manipulation of single-cells using high frequency ultrasound

Efficient intracellular delivery of biologically active macromolecules has been a challenging but important process for manipulating live cells for research and therapeutic purposes. There have been limited transfection techniques that can deliver multiple types of active molecules simultaneously into single-cells as well as different types of molecules into physically connected individual neighboring cells separately with high precision and low cytotoxicity. Here, a high frequency ultrasound-based remote intracellular delivery technique capable of delivery of multiple DNA plasmids, messenger RNAs, and recombinant proteins is developed to allow high spatiotemporal visualization and analysis of gene and protein expressions as well as single-cell gene editing using clustered regularly interspaced short palindromic repeats (CRISPR)-associated protein-9 nuclease (Cas9), a method called acoustic-transfection. Acoustic-transfection has advantages over typical sonoporation because acoustic-transfection utilizing ultra-high frequency ultrasound over 150 MHz can directly deliver gene and proteins into cytoplasm without microbubbles, which enables controlled and local intracellular delivery to acoustic-transfection technique. Acoustic-transfection was further demonstrated to deliver CRISPR-Cas9 systems to successfully modify and reprogram the genome of single live cells, providing the evidence of the acoustic-transfection technique for precise genome editing using CRISPR-Cas9.

Deformable Nanovesicles Synthesized through an Adaptable Microfluidic Platform for Enhanced Localized Transdermal Drug Delivery

Phospholipid-based deformable nanovesicles (DNVs) that have flexibility in shape offer an adaptable and facile method to encapsulate diverse classes of therapeutics and facilitate localized transdermal delivery while minimizing systemic exposure. Here we report the use of a microfluidic reactor for the synthesis of DNVs and show that alteration of input parameters such as flow speeds as well as molar and flow rate ratios increases entrapment efficiency of drugs and allows fine-tuning of DNV size, elasticity, and surface charge. To determine the ability of DNV-encapsulated drug to be delivered transdermally to a local site, we synthesized, characterized, and tested DNVs carrying the fluorescently labeled hydrophilic bisphosphonate drug AF-647 zoledronate (AF647-Zol). AF647-Zol DNVs were lyophilized, resuspended, and applied topically as a paste to the calvarial skin of mice. High-resolution fluorescent imaging and confocal microscopy revealed significant increase of encapsulated payload delivery to the target tissue-cranial bone-by DNVs as compared to nondeformable nanovesicles (NVs) or aqueous drug solutions. Interestingly, NV delivery was not superior to aqueous drug solution. Our studies show that microfluidic reactor-synthesized DNVs can be produced in good yield, with high encapsulation efficiency, reproducibility, and stability after storage, and represent a useful vehicle for localized transdermal drug delivery.

Hydrogel increases localized transport regions and skin permeability during low frequency ultrasound treatment

Low frequency ultrasound (LFU) enhances skin permeability via the formation of heterogeneous localized transport regions (LTRs). In this work, hydrogels with different zeta potentials were used as the coupling medium for LFU to investigate their contribution to LTR patterns and to the skin penetration of two model drugs, calcein and doxorubicin (DOX). When hydrogels were used, LTRs covering at least a 3-fold greater skin area were observed compared to those resulting from traditional LFU treatment and sodium lauryl sulfate. More LTRs resulted in an enhancement of calcein skin permeation. The zeta potential of the hydrogels affected the skin penetration of the positively charged DOX; the cationic coupling medium decreased the DOX recovered from the viable epidermis by 2.8-fold, whereas the anionic coupling medium increased the DOX accumulation in the stratum corneum by 4.4-fold. Therefore, LFU/hydrogel treatment increases LTRs areas and can target ionized drugs to specific skin layers depending on the zeta potential of the coupling medium.

Impact of Cosmetic Lotions on Nanoparticle Penetration through ex vivo C57BL/6 Hairless Mouse and Human Skin: A Comparison Study

Understanding the interactions of nanoparticles (NPs) with skin is important from a consumer and occupational health and safety perspective, as well as for the design of effective NP-based transdermal therapeutics. Despite intense efforts to elucidate the conditions that permit NP penetration, there remains a lack of translatable results from animal models to human skin. The objectives of this study are to investigate the impact of common skin lotions on NP penetration and to quantify penetration differences of quantum dot (QD) NPs between freshly excised human and mouse skin. QDs were mixed in 7 different vehicles, including 5 commercial skin lotions. These were topically applied to skin using two exposure methods; a petri dish protocol and a Franz diffusion cell protocol. QD presence in the skin was quantified using Confocal Laser Scanning Microscopy. Results show that the commercial vehicles can significantly impact QD penetration in both mouse and human skin. Lotions that contain alpha hydroxyl acids (AHA) facilitated NP penetration. Lower QD signal was observed in skin studied using a Franz cell. Freshly excised human skin was also studied immediately after the sub-cutaneous fat removal process, then after 24 hours rest ex vivo. Resting human skin 24 hours prior to QD exposure significantly reduced epidermal presence. This study exemplifies how application vehicles, skin processing and the exposure protocol can affect QD penetration results and the conclusions that maybe drawn between skin models.

Potential of combined ultrasound and microneedles for enhanced transdermal drug permeation: a review

Transdermal drug delivery (TDD) is limited by the outer layer of the skin, i.e., the stratum corneum. Research on TDD has become very active in the recent years and various technologies have been developed to overcome the resistance of the stratum corneum to molecular diffusion. In particular, researchers have started to consider the possibility of combining the TDD technologies in order to have further increase in drug permeability. Both microneedles (MNs) and ultrasound are promising technologies. They achieve enhancement in drug permeation via different mechanisms and therefore give a good potential for combining with each other. This review will focus on discussing the potential of this combinational technique along with other important issues, e.g., the mechanisms of ultrasound and MNs as it is and these mechanisms which are coupled via the two systems (i.e. MNs and ultrasound). We discuss the possible ways to achieve this combination as well as how this combination would increase the permeability. Some of the undeveloped (weaker) research areas of MNs and sonophoresis are also discussed in order to understand the true potential of combining the two technologies when they are developed further in the future. We propose several hypothetical combinations based on the possible mechanisms involved in MNs and ultrasound. Furthermore, we carry out a cluster analysis by which we determine the significance of this combinational method in comparison with some other selected combinational methods for TDD (e.g., MNs and iontophoresis). Using a time series analysis tool (ARIMA model), the current trend and the future development of combined MNs and ultrasound are also analysed. Overall, the review in this paper indicates that combining MNs and ultrasound is a promising TDD method for the future.

Relations between acoustic cavitation and skin resistance during intermediate- and high-frequency sonophoresis

Enhanced skin permeability is known to be achieved during sonophoresis due to ultrasound-induced cavitation. However, the mechanistic role of cavitation during sonophoresis has been extensively investigated only for low-frequency (LFS, <100 kHz) applications. Here, mechanisms of permeability-enhancing stable and inertial cavitation were investigated by passively monitoring subharmonic and broadband emissions arising from cavitation isolated within or external to porcine skin in vitro during intermediate- (IFS, 100-700 kHz) and high-frequency sonophoresis (HFS, >1 MHz). The electrical resistance of skin, a surrogate measure of the permeability of skin to a variety of compounds, was measured to quantify the reduction and subsequent recovery of the skin barrier during and after exposure to pulsed (1 second pulse, 20% duty cycle) 0.41 and 2.0 MHz ultrasound over a range of acoustic powers (0-21.7 W) for 30 min. During IFS, significant skin resistance reductions and acoustic emissions from cavitation were measured exclusively when cavitation was isolated outside of the skin. Time-dependent skin resistance reductions measured during IFS correlated significantly with subharmonic and broadband emission levels. During HFS, significant skin resistance reductions were accompanied by significant acoustic emissions from cavitation measured during trials that isolated cavitation activity either outside of skin or within skin. Time-dependent skin resistance reductions measured during HFS correlated significantly greater with subharmonic than with broadband emission levels. The reduction of the skin barrier due to sonophoresis was reversible in all trials; however, effects incurred during IFS recovered more slowly and persisted over a longer period of time than HFS. These results quantitatively demonstrate the significance of cavitation during sonophoresis and suggest that the mechanisms and post-treatment longevity of permeability enhancement due to IFS and HFS treatments are different.

Ultrasound mediated transdermal drug delivery

Transdermal drug delivery offers an attractive alternative to the conventional drug delivery methods of oral administration and injections. However, the stratum corneum serves as a barrier that limits the penetration of substances to the skin. Application of ultrasound (US) irradiation to the skin increases its permeability (sonophoresis) and enables the delivery of various substances into and through the skin. This review presents the main findings in the field of sonophoresis in transdermal drug delivery as well as transdermal monitoring and the mathematical models associated with this field. Particular attention is paid to the proposed enhancement mechanisms and future trends in the fields of cutaneous vaccination and gene therapy.

Exploitation of acoustic cavitation-induced microstreaming to enhance molecular transport

Ultrasound (US) exposure of soft tissues, such as the skin, has been shown to increase permeability, enhancing the passage of drug molecules via passive processes such as diffusion. However, US regimes have not been exploited to enhance active convective transport of drug molecules from a donor layer, such as a gel, into another medium. A layered tissue-mimicking material (TMM) was used as a model for a drug donor layer and underlying soft tissue to test penetration of agents in response to a range of US parameters. Influence of agent molecular mass (3-2000 kDa), US frequency (0.256/1.1 MHz) and US pressure (0-10 MPa) on transport was characterised. Agents of four different molecular sizes were embedded within the TMM with or without cavitation nuclei (CN) and US applied to achieve inertial cavitation. Post-insonation, samples were analysed to determine the concentration and penetration distance of agent transported. US exposure substantially enhanced transport. At both US frequencies, enhancement of transport was significantly higher (p < 0.05) above the cavitation threshold, and CN reduced the pressure at which cavitation, and therefore transport, was achieved. Acoustic cavitation activity and related phenomena was the predominant transport mechanism, and addition of CN significantly enhanced transport within a range of clinically applicable acoustic pressures. © 2014 Wiley Periodicals, Inc. and the American Pharmacists Association J Pharm Sci.

Enhanced Aeromonas salmonicida bacterin uptake and side effects caused by low frequency sonophoresis in rainbow trout (Oncorhynchus mykiss)

Low frequency sonophoresis (LFS) has been recognized as one of the most advanced technologies in transdermal delivery of substances, due to the modification of the stratum corneum lipid bilayer, in focal skin applications in mammals. Based on these findings, LFS has been suggested as a potential technology to be used for enhancement in immersion fish vaccination. In contrast to mammals where LFS is applied to discrete regions of the skin, in fish the whole individual needs to be exposed for practical purposes. The current study evaluated the impact of LFS at 37 kHz on the uptake of an Aeromonas salmonicida bacterin and side effects of the treatment in rainbow trout. Quantitative real time PCR (qPCR) and immunohistochemistry were used to examine the bacterin uptake into skin and gill tissue. Side effects were assessed by behavioural examination, histology and blood serum analysis. The sonication intensity of 171 mW/cm² was enough for increasing skin permeability, but caused heavy erratic swimming and gill haemorrhages. Sonication intensities as low as 105 mW/cm² did not modify skin permeability and enhanced the bacterin uptake into the gill tissue by factor 15 compared to conventional immersion. Following sonication, the gill permeability for the bacterin decreased after 20 min and 120 min by factor 3 and 2, respectively. However, during sonication, erratic swimming of the fish raised some concerns. Further reduction of the sonication intensity to 57 mW/cm² did not induce erratic swimming, and the bacterin uptake into the gill tissue was still increased by factor 3. In addition, a decreasing albumin-globulin ratio in the serum of the rainbow trout within 40 min revealed that LFS leads to an inflammatory response. Consequently, based on both increased bacterin uptake and the inflammatory response, low intensity LFS has the potential to enhance vaccine immunity without significant side effects.

Sonophoresis in transdermal drug deliverys

Transdermal drug delivery (TDD) has several significant advantages compared to oral drug delivery, including elimination of pain and sustained drug release. However, the use of TDD is limited by low skin permeability due to the stratum corneum (SC), the outermost layer of the skin. Sonophoresis is a technique that temporarily increases skin permeability such that various medications can be delivered noninvasively. For the past several decades, various studies of sonophoresis in TDD have been performed focusing on parameter optimization, delivery mechanism, transport pathway, or delivery of several drug categories including hydrophilic and high molecular weight compounds. Based on these various studies, several possible mechanisms of sonophoresis have been suggested. For example, cavitation is believed to be the predominant mechanism responsible for drug delivery in sonophoresis. This review presents details of various studies on sonophoresis including the latest trends, delivery of various therapeutic drugs, sonophoresis pathways and mechanisms, and outlook of future studies.

Quantification of quantum dot murine skin penetration with UVR barrier impairment

Ultraviolet radiation (UVR) skin exposure is a common exogenous insult that can alter skin barrier and immune functions. With the growing presence of nanoparticles (NPs) in consumer goods and technological applications the potential for NPs to contact UVR-exposed skin is increasing. Therefore it is important to understand the effect of UVR on NP skin penetration and the potential for systemic translocation. Previous studies qualitatively showed that UVR skin exposure can increase the penetration of NPs below the stratum corneum. In this work, an in vivo mouse model was used to quantitatively examine the skin penetration of carboxylated (CdSe/ZnS, core/shell) quantum dots (QDs) through intact and UVR barrier-disrupted murine skin by organ Cd mass analysis. Transepidermal water loss was used to measure the magnitude of the skin barrier defect as a function of UVR dose and time post-UVR exposure. QDs were applied to mice 3-4 days post-UVR exposure at the peak of the skin barrier disruption. Our results reveal unexpected trends that suggest these negative-charged QDs can penetrate barrier intact skin and that penetration and systemic transport depends on the QD application time post-UVR exposure. The effect of UVR on skin-resident dendritic cells and their role in the systemic translocation of these QDs are described. Our results suggest that NP skin penetration and translocation may depend on the specific barrier insult and the inflammatory status of the skin.

Engineering approaches to transdermal drug delivery: a tribute to contributions of prof. Robert Langer

Transdermal drug delivery continues to provide an advantageous route of drug administration over injections. While the number of drugs delivered by passive transdermal patches has increased over the years, no macromolecule is currently delivered by the transdermal route. Substantial research efforts have been dedicated by a large number of researchers representing varied disciplines including biology, chemistry, pharmaceutics and engineering to understand, model and overcome the skin's barrier properties. This article focuses on engineering contributions to the field of transdermal drug delivery. The article pays tribute to Prof. Robert Langer, who pioneered the engineering approach towards transdermal drug delivery. Over a period spanning nearly 25 years since his first publication in the field of transdermal drug delivery, Bob Langer has deeply impacted the field by quantitative analysis and innovative engineering. At the same time, he has inspired several generations of engineers by collaborations and mentorship. His scientific insights, innovative technologies, translational efforts and dedicated mentorship have transformed the field.

Devices for overcoming biological barriers: the use of physical forces to disrupt the barriers

Overcoming biological barriers including skin, mucosal membranes, blood brain barrier as well as cell and nuclear membrane constitutes a key hurdle in the field of drug delivery. While these barriers serve the natural protective function in the body, they limit delivery of drugs into the body. A variety of methods have been developed to overcome these barriers including formulations, targeting peptides and device-based technologies. This review focuses on the use of physical methods including acoustic devices, electric devices, high-pressure devices, microneedles and optical devices for disrupting various barriers in the body including skin and other membranes. A summary of the working principles of these devices and their ability to enhance drug delivery is presented.

Intelligent drug-delivery devices based on micro- and nano-technologies

This review focuses on the current drug-delivery modalities in R&D, as well as commercially available. Intelligent drug-delivery systems are described as novel technological innovations and clinical approaches to improve conventional treatments. These systems differ in methodology of therapeutic administration, intricacy, materials and patient compliance to address numerous clinical conditions that require various pharmacological therapies. These systems have been primarily described as active and passive microelectrical mechanical system devices, injectors and nanoparticle-based therapies, optimized to tailor specific pharmacokinetic profiles. The most critical considerations for the design of these intelligent delivery systems include the controlled release, target specificity, on-demand dosage adjustment, mass transfer and stability of the pharmacological agents. Drug-delivery systems continue to be developed and enhanced to provide better and more sophisticated treatments, promising an improvement in quality of life and extension of life expectancy.

The impact of UVB exposure and differentiation state of primary keratinocytes on their interaction with quantum dots

In this study we utilised an in vitro model system to gain insight into the potential cellular interactions that quantum dot (QD) nanoparticles may experience while transiting the viable skin epidermis, and we consider the effects of UVB exposure. UVB skin exposure is known to induce a skin barrier defect that facilitates QD stratum corneum penetration. Primary human keratinocytes were cultured in low and high calcium to induce basal and differentiated phenotypes, respectively. Results suggest that differentiation state plays a role in keratinocyte response to UVB exposure and exposure to negatively charged CdSe/ZnS core/shell QD. QD cell uptake increased with QD dose but association with differentiated cells was significantly lower than the basal keratinocyte phenotype. Differentiated keratinocytes were also less sensitive to the cytotoxic effects of UVB exposure. We did not observe an effect of UVB preexposure on QD cytotoxicity level despite the fact that fluorescent microscopy and flow cytometry data suggest that UVB may slightly increase QD uptake in the basal cell phenotype. The implications of these findings for assessing potential risk of human skin exposure are discussed.

Interaction of inorganic nanoparticles with the skin barrier: current status and critical review

Integration of nanotechnology with biology leads to various advantages in applied pharmaceutical and medical sciences. In that regard, the behavior of nanoparticles (NPs) in relation to the skin, an important biological barrier, has been the target of several recent studies. Yet the potential ability of NPs to penetrate into the underlying viable tissue lies at the center of debate. This review briefly highlights the current applications of inorganic NPs, then discusses the current status of their skin penetration in view of the vast variation among the experimental setups in use. Determinants of particle penetration, adopted approaches for enhanced penetration, the underlying mechanism, as well as qualitative and quantitative analysis of NPs present in the skin are also within the scope of this review article. We emphasize analyzing the data generated from experiments on human skin, the "gold standard" for assessment of in vitro skin penetration. Based on this, we include some recommendations for future research.

FROM THE CLINICAL EDITOR

Transdermal application of inorganic nanoparticle-based medications is of growing interest in nanomedicine research. This critical review discusses the knowns and the unknowns of this field, providing insightful recommendations for future research.

A physical mechanism to explain the delivery of chemical penetration enhancers into skin during transdermal sonophoresis - Insight into the observed synergism

The synergism between low-frequency sonophoresis (LFS) and chemical penetration enhancers (CPEs), especially surfactants, in transdermal enhancement has been investigated extensively since this phenomenon was first observed over a decade ago. In spite of the identifying that the origin of this synergism is the increased penetration and subsequent dispersion of CPEs in the skin in response to LFS treatment, to date, no mechanism has been directly proposed to explain how LFS induces the observed increased transport of CPEs. In this study, we propose a plausible physical mechanism by which the transport of all CPEs is expected to have significantly increased flux into the localized-transport regions (LTRs) of LFS-treated skin. Specifically, the collapse of acoustic cavitation microjets within LTRs induces a convective flux. In addition, because amphiphilic molecules preferentially adsorb onto the gas/water interface of cavitation bubbles, amphiphiles have an additional adsorptive flux. In this sense, the cavitation bubbles effectively act as carriers for amphiphilic molecules, delivering surfactants directly into the skin when they collapse at the skin surface as cavitation microjets. The flux equations derived for CPE delivery into the LTRs and non-LTRs during LFS treatment, compared to that for untreated skin, explain why the transport of all CPEs, and to an even greater extent amphiphilic CPEs, is increased during LFS treatment. The flux model is tested with a non-amphiphilic CPE (propylene glycol) and both nonionic and ionic amphiphilic CPEs (octyl glucoside and sodium lauryl sulfate, respectively), by measuring the flux of each CPE into untreated skin and the LTRs and non-LTRs of LFS-treated skin. The resulting data shows very good agreement with the proposed flux model.

Novel methods and devices to enhance transdermal drug delivery: the importance of laser radiation in transdermal drug delivery

Skin permeation-enhancement technology is a rapidly developing field, which could significantly increase the number of drugs suitable for transdermal delivery. In this review, we highlight recent advances in both ‘passive’ and ‘active’ transdermal drug-delivery technologies, as well as in the laser ablation method. This paper concludes with a brief forward-looking perspective discussing what can be expected as laser technology continues to develop in the coming years.

Mechanism and determinants of nanoparticle penetration through human skin

The ability of nanoparticles to penetrate the stratum corneum was the focus of several studies. Yet, there are controversial issues available for particle penetration due to different experimental setups. Meanwhile, there is little known about the mechanism and determinants of their penetration. In this paper the penetration of four model gold nanoparticles of diameter 6 and 15 nm, differing in surface polarity and the nature of the vehicle, through human skin was studied using multiphoton microscopy. This is in an attempt to profoundly investigate the parameters governing particle penetration through human skin. Our results imply that nanoparticles at this size range permeate the stratum corneum in a similar manner to drug molecules, mainly through the intercellular pathways. However, due to their particulate nature, permeation is also dependent on the complex microstructure of the stratum corneum with its tortuous aqueous and lipidic channels, as shown from our experiments performed using skin of different grades of barrier integrity. The vehicle (toluene-versus-water) had a minimal effect on skin penetration of gold nanoparticles. Other considerations in setting up a penetration experiment for nanoparticles were also studied. The results obtained are important for designing a new transdermal carrier and for a basic understanding of skin-nanoparticle interaction.

Nanoparticles for transcutaneous vaccination

The living epidermis and dermis are rich in antigen presenting cells (APCs). Their activation can elicit a strong humoral and cellular immune response as well as mucosal immunity. Therefore, the skin is a very attractive site for vaccination, and an intradermal application of antigen may be much more effective than a subcutaneous or intramuscular injection. However, the stratum corneum (SC) is a most effective barrier against the invasion of topically applied vaccines. Products which have reached the stage of clinical testing, avoid this problem by injecting the nano‐vaccine intradermally or by employing a barrier disrupting method and applying the vaccine to a relatively large skin area. Needle‐free vaccination is desirable from a number of aspects: ease of application, improved patient acceptance and less risk of infection among them. Nanocarriers can be designed in a way that they can overcome the SC. Also incorporation into nanocarriers protects instable antigen from degradation, improves uptake and processing by APCs, and facilitates endosomal escape and nuclear delivery of DNA vaccines. In addition, sustained release systems may build a depot in the tissue gradually releasing antigen which may avoid booster doses. Therefore, nanoformulations of vaccines for transcutaneous immunization are currently a very dynamic field of research. Among the huge variety of nanocarrier systems that are investigated hopes lie on ultra‐flexible liposomes, superfine rigid nanoparticles and nanocarriers, which are taken up by hair follicles. The potential and pitfalls associated with these three classes of carriers will be discussed.

Gold nanoparticle penetration and reduced metabolism in human skin by toluene

PURPOSE

To measure penetration and metabolic effects of ion-stabilized, polar, 15 nm gold nanoparticles in aqueous solution (AuNP-Aq) and sterically stabilized, non-polar, 6 nm gold nanoparticles in toluene (AuNP-TOL) on excised human skin.

METHODS

Gold nanoparticles were characterized with dynamic light scattering and transmission electron microscopy (TEM). Skin penetration studies were done on frozen or fresh excised skin using static Franz diffusion cells. Viable treated skin was assessed by dermoscopy, reflectance confocal microscopy (RCM), multiphoton tomography (MPT) with fluorescence lifetime imaging microscopy (FLIM), and TEM.

RESULTS

Dermoscopy and RCM showed large aggregates in the furrows of AuNP-Aq-treated skin. Treatment of thawed and viable skin only showed enhanced permeability to nanoparticles in the AuNP-TOL group with MPT and FLIM imaging to stratum spinosum of epidermis. TEM analysis revealed gold nanoparticles within AuNP-treated stratum corneum. FLIM analysis of NAD(P)H showed a significant decrease in total NAD(P)H in all toluene-treated groups.

CONCLUSIONS

Gold nanoparticles, 15 nm, in aqueous solution aggregated on the skin surface. Toluene treatment eliminated skin metabolism; skin treated with toluene/gold nanoparticles (6 nm) for 24 h, but not at 4 h, showed increased nanoparticle permeability. These results are of value to nanotoxicology.

Ultrasound-mediated transdermal drug delivery: mechanisms, scope, and emerging trends

The use of ultrasound for the delivery of drugs to, or through, the skin is commonly known as sonophoresis or phonophoresis. The use of therapeutic and high frequencies of ultrasound (≥0.7MHz) for sonophoresis (HFS) dates back to as early as the 1950s, while low-frequency sonophoresis (LFS, 20-100kHz) has only been investigated significantly during the past two decades. Although HFS and LFS are similar because they both utilize ultrasound to increase the skin penetration of permeants, the mechanisms associated with each physical enhancer are different. Specifically, the location of cavitation and the extent to which each process can increase skin permeability are quite dissimilar. Although the applications of both technologies are different, they each have strengths that could allow them to improve current methods of local, regional, and systemic drug delivery. In this review, we will discuss the mechanisms associated with both HFS and LFS, specifically concentrating on the key mechanistic differences between these two skin treatment methods. Background on the relevant physics associated with ultrasound transmitted through aqueous media will also be discussed, along with implications of these phenomena on sonophoresis. Finally, a thorough review of the literature is included, dating back to the first published reports of sonophoresis, including a discussion of emerging trends in the field.

Application of hydrotropy to transdermal formulations: hydrotropic solubilization of polyol fatty acid monoesters in water and enhancement effect on skin permeation of 5-FU

OBJECTIVES

A hydrotropic formulation containing a percutaneous enhancer was developed for the transdermal formulation of a water-soluble drug and the solubilizing mechanisms of a percutaneous enhancer in water by a hydrotropic agent were investigated. The enhancement effect was also compared with the hydrotropic formulation and the other formulations using ethanol, propylene glycol or mixed micelles.

METHODS

Sodium salicylate (SA) and sodium benzoate (BA) were selected as hydrotropic agents, and polyol fatty acid ester (POFE) and 5-fluorouracil (5-FU) were selected as a percutaneous enhancer and a water-soluble drug, respectively. Near-infrared (NIR) spectrophotometric and ¹H NMR spectroscopic studies were carried out to investigate the solubilizing mechanisms. The mean particle size in the hydrotropic formulation was measured. The in-vitro skin permeation of 5-FU and the accumulation in the skin of propylene glycol monocaprylate (PGMC), one of the monoesters of POFE, from the hydrotropic formulation or the other formulations were investigated by using Franz-type diffusion cell.

KEY FINDINGS

The presence of SA and BA had a visible effect on the O-H stretching band of water in the NIR region. The surface tension of SA and BA aqueous solutions was found to decrease with an increase in SA or BA concentration. Although SA interacted with PGMC in the presence of water, it did not interact with PGMC in the absence of water. Mean particle size in a solution consisting of 5% (v/v) PGMC and 30% SA aqueous solution was approximately 14 nm. ¹H NMR spectroscopic studies indicated that the hydrotropic salts formed aggregates with which PGMC interacted from the outside. The hydrotropic formulation prepared in this study enhanced skin permeation of 5-FU when compared with the other formulations.

CONCLUSIONS

SA and BA solubilized monoesters of POFE in water, and SA interacted with PGMC in water. The hydrotropic formulation prepared in this study significantly enhanced skin permeation of 5-FU compared with the other formulations. The results suggest that a hydrotropic formulation containing PGMC may be a useful transdermal formulation for water-soluble drugs.

Near-IR fluorescence and reflectance confocal microscopy for imaging of quantum dots in mammalian skin

Understanding the skin penetration of nanoparticles (NPs) is an important concern due to the increasing presence of NPs in consumer products, including cosmetics. Technical challenges have slowed progress in evaluating skin barrier and NP factors that contribute to skin penetration risk. To limit sampling error and other problems associated with histological processing, many researchers are implementing whole tissue confocal or multiphoton microscopies. This work introduces a fluorescence and reflectance confocal microscopy system that utilizes near-IR excitation and emission to detect near-IR lead sulfide quantum dots (QDs) through ex vivo human epidermis. We provide a detailed prediction and experimental analysis of QD detection sensitivity and demonstrate detection of QD skin penetration in a barrier disrupted model. The unique properties of near-IR lead-based QDs will enable future studies that examine the impact of further barrier-disrupting agents on skin penetration of QDs and elucidate mechanistic insight into QD tissue interactions at the cellular level.

Low-frequency sonophoresis: application to the transdermal delivery of macromolecules and hydrophilic drugs

IMPORTANCE OF THE FIELD

Transdermal delivery of macromolecules provides an attractive alternative route of drug administration when compared to oral delivery and hypodermic injection because of its ability to bypass the harsh gastrointestinal tract and deliver therapeutics non-invasively. However, the barrier properties of the skin only allow small, hydrophobic permeants to traverse the skin passively, greatly limiting the number of molecules that can be delivered via this route. The use of low-frequency ultrasound for the transdermal delivery of drugs, referred to as low-frequency sonophoresis (LFS), has been shown to increase skin permeability to a wide range of therapeutic compounds, including both hydrophilic molecules and macromolecules. Recent research has demonstrated the feasibility of delivering proteins, hormones, vaccines, liposomes and other nanoparticles through LFS-treated skin. In vivo studies have also established that LFS can act as a physical immunization adjuvant. LFS technology is already clinically available for use with topical anesthetics, with other technologies currently under investigation.

AREAS COVERED IN THIS REVIEW

This review provides an overview of mechanisms associated with LFS-mediated transdermal delivery, followed by an in-depth discussion of the current applications of LFS technology for the delivery of hydrophilic drugs and macromolecules, including its use in clinical applications.

WHAT THE READER WILL GAIN

The reader will gain an insight into the field of LFS-mediated transdermal drug delivery, including how the use of this technology can improve on more traditional drug delivery methods.

TAKE HOME MESSAGE

Ultrasound technology has the potential to impact many more transdermal delivery platforms in the future due to its unique ability to enhance skin permeability in a controlled manner.

Development of an optimised application protocol for sonophoretic transdermal delivery of a model hydrophilic drug

It has now been known for over a decade that low frequency ultrasound can be used to effectively enhance transdermal drug penetration - an approach termed sonophoresis. Mechanistically, acoustic cavitation results in the creation of defects in the stratum corneum that allow accelerated absorption of topically applied molecules. The aim of this study was to develop an optimised sonophoresis protocol for studying transdermal drug delivery in vitro. To this end, caffeine was selected as a model hydrophilic drug while porcine skin was used as a model barrier. Following acoustic validation, 20kHz ultrasound was applied for different durations (range: 5 s to 10 min) using three different modes (10%, 33% or 100% duty cycles) and two distinct sonication procedures (either before or concurrent with drug deposition). Each ultrasonic protocol was assessed in terms of its heating and caffeine flux-enhancing effects. It was found that the best regimen was a concurrent 5 min, pulsed (10% duty cycle) beam of SATA intensity 0.37 W/cm². A key insight was that in the case of pulsed beams of 10% duty cycle, sonication concurrent with drug deposition was superior to sonication prior to drug deposition and potential mechanisms for this are discussed.