Sonophoresis. II. Examination of the mechanism(s) of ultrasound-enhanced transdermal drug delivery

Current challenges in non-invasive insulin delivery systems: a comparative review

The quest to eliminate the needle from insulin delivery and to replace it with non- or less-invasive alternative routes has driven rigorous pharmaceutical research to replace the injectable forms of insulin. Recently, various approaches have been studied involving many strategies using various technologies that have shown success in delivering insulin, which are designed to overcome the inherent barriers for insulin uptake across the gastrointestinal tract, mucosal membranes and skin. This review examines some of the many attempts made to develop alternative, more convenient routes for insulin delivery to avoid existing long-term dependence on multiple subcutaneous injections and to improve the pharmacodynamic properties of insulin. In addition, this article concentrates on the successes in this new millennium in developing potential non-invasive technologies and devices, and on major new milestones in modern insulin delivery for the effective treatment of diabetes.

Dual-Channel Two-Photon Microscopy Study of Transdermal Transport in Skin Treated with Low-Frequency Ultrasound and a Chemical Enhancer

Visualization of transdermal permeant pathways is necessary to substantiate model-based conclusions drawn using permeability data. The aim of this investigation was to visualize the transdermal delivery of sulforhodamine B (SRB), a fluorescent hydrophilic permeant, and of rhodamine B hexyl ester (RBHE), a fluorescent hydrophobic permeant, using dual-channel two-photon microscopy (TPM) to better understand the transport pathways and the mechanisms of enhancement in skin treated with low-frequency ultrasound (US) and/or a chemical enhancer (sodium lauryl sulfate--SLS) relative to untreated skin (the control). The results demonstrate that (1) both SRB and RBHE penetrate beyond the stratum corneum and into the viable epidermis only in discrete regions (localized transport regions--LTRs) of US treated and of US/SLS-treated skin, (2) a chemical enhancer is required in the coupling medium during US treatment to obtain two significant levels of increased penetration of SRB and RBHE in US-treated skin relative to untreated skin, and (3) transcellular pathways are present in the LTRs of US treated and of US/SLS-treated skin for SRB and RBHE, and in SLS-treated skin for SRB. In summary, the skin is greatly perturbed in the LTRs of US treated and US/SLS-treated skin with chemical enhancers playing a significant role in US-mediated transdermal drug delivery.

Therapeutic applications of ultrasound

Therapeutic applications of ultrasound predate its use in imaging. A range of biological effects can be induced by ultrasound, depending on the exposure levels used. At low levels, beneficial, reversible cellular effects may be produced, whereas at high intensities instantaneous cell death is sought. Therapy ultrasound can therefore be broadly divided into "low power" and "high power" applications. The "low power" group includes physiotherapy, fracture repair, sonophoresis, sonoporation and gene therapy, whereas the most common use of "high power" ultrasound in medicine is probably now high intensity focused ultrasound. Therapeutic effect through the intensity spectrum is obtained by both thermal and non-thermal interaction mechanisms. At low intensities, acoustic streaming is likely to be significant, but at higher levels, heating and acoustic cavitation will predominate. While useful therapeutic effects are now being demonstrated clinically, the mechanisms by which they occur are often not well understood.

Particle-Mediated Gene Delivery and Human Skin: Ultrastructural Observations on Stratum Corneum Barrier Structures

The particle-mediated delivery systems are becoming a clinically relevant tool in dermatology and immunology. We investigated the qualitative ultrastructural morphology of skin following pressure-driven delivery of gold particles to ex vivo human breast skin, at different pressures ranging from 350 to 1,000 psi. Pressures of 800 and 1,000 psi appear to be more effective, as indicated by distribution of particles in the viable epidermis and dermis. Particle bombardment of the skin with gold beads caused microwounds that spanned the stratum corneum (SC). The SC lipids did not reseal these wounds in the SC after 24 h in organ culture. The implications of particle-mediated delivery to permeability barrier functions of the SC are discussed.

Effects of solvents on skin permeation of formoterol fumarate

Effects of various chemicals applied as penetration enhancers on the permeation of formoterol fumarate (FF) across excised rat skin were investigated. Remarkable enhancement was noted with terpenes, fatty acid esters, and higher alcohols, whereas no significant influence was observed in the case of lower alcohols, pyrrolidones, and amines. At high concentration, a cineole/N-methyl-2-pyrrolidone (NMP) mixed solvent system slightly enhanced the skin permeation of FF compared with cineole alone, and a l-menthol/NMP mixed solvent system caused significant further increase. Maximum skin permeation of FF was seen when the ratio of l-menthol/NMP was 60/40 w/w. From the present results, l-menthol/NMP and isopropyl myristate (IPM)/NMP mixed solvent systems can be considered effective for augmenting skin permeation of FF, with potential applications in transdermal delivery of the drug.

Ultrasound: Mechanical gene transfer into plant cells by sonoporation

Development of nonviral gene transfer methods would be a valuable alternative of gene therapy or transformation. Ultrasound can produce a variety of nonthermal bioeffects via acoustic cavitation. Cavitation bubbles can induce cell death or transient membrane permeabilization (sonoporation) on cells. Application of sonoporation for gene transfer into cells or tissues develops quickly in recent years. Many studies have been performed in vitro exposure systems to a variety of cell lines transfected successfully. In vivo, cavitation initiation and control are more difficult, but can be enhanced by ultrasound contrast agents (microbubbles). The use of ultrasound for nonviral gene delivery has been applied for mammalian systems, which provides a fundamental basis and strong promise for development of new gene therapy methods for clinical medicine. In this paper, ultrasound applied to plant cell transformation or gene transfer is reviewed. Recently, most researches are focused on sonication-assisted Agrobacterium-mediated transformation (SAAT) in plant cells or tissues. Microbubbles are also proposed to apply to gene transfer in plant cells and tissues.

Transdermal iontophoresis: combination strategies to improve transdermal iontophoretic drug delivery

For several decades, there has been interest in using the skin as a port of entry into the body for the systemic delivery of therapeutic agents. However, the upper layer of the skin, the stratum corneum, poses a barrier to the entry of many therapeutic entities. Given a compound, passive delivery rate is often dependent on two major physicochemical properties: the partition coefficient and solubility. The use of chemical enhancers and modifications of the thermodynamic activity of the applied drug are two frequently employed strategies to improve transdermal permeation. Chemical enhancers are known to enhance drug permeation by several mechanisms which include disrupting the organized intercellular lipid structure of the stratum corneum , 'fluidizing' the stratum corneum lipids , altering cellular proteins, and in some cases, extracting intercellular lipids . However, the resulting increase in drug permeation using these techniques is rather modest especially for hydrophilic drugs. A number of other physical approaches such as iontophoresis, sonophoresis, ultrasound and the use of microneedles are now being studied to improve permeation of hydrophilic as well as lipophilic drugs. This article presents an overview of the use of iontophoresis alone and in conjunction with other approaches such as chemical enhancement, electroporation, sonophoresis, and use of microneedles and ion-exchange materials.

Emerging strategies for the transdermal delivery of peptide and protein drugs

Transdermal delivery has been at the forefront of research addressing the development of non-invasive methods for the systemic administration of peptide and protein therapeutics generated by the biotechnology revolution. Numerous approaches have been suggested for overcoming the skin's formidable barrier function; whereas certain strategies simply act on the drug formulation or transiently increase the skin permeability, others are designed to bypass or even remove the outermost skin layer. This article reviews the technologies currently under investigation, ranging from those in their early-stage of development, such as laser-assisted delivery to others, where feasibility has already been demonstrated, such as microneedle systems, and finally more mature techniques that have already led to commercialisation (e.g., velocity-based technologies). The principles, mechanisms involved, potential applications, limitations and safety considerations are discussed for each approach, and the most advanced devices in each field are described.

Experimental demonstration of the existence of highly permeable localized transport regions in low‐frequency sonophoresis

Recent advances in low-frequency sonophoresis have focused on the existence of hypothesized localized transport regions (LTRs). However, there has been no actual experimental demonstration that the hypothesized LTRs are, in fact, localized regions of high permeability. Through a series of low-frequency sonophoresis experiments conducted with full-thickness pig skin, in the presence of the surfactant sodium lauryl sulfate (SLS), in which we have separately measured the transport of calcein through the LTRs, which have areas ranging from 10 to 40 mm(2), and the surrounding regions of the skin (the non-LTRs) by means of a novel masking technique, we demonstrate that the calcein permeability through the LTRs is approximately 80-fold higher than the calcein permeability through the non-LTRs, suggesting that the LTRs are structurally perturbed to a greater extent than the non-LTRs from the exposure to the ultrasound/SLS system. In addition, we propose basic models to predict the total skin transdermal permeability from the transdermal permeabilities of the LTRs and the non-LTRs, and then compare the predictions to the experimental data obtained from the masking experiments. We also demonstrate that both the LTRs and the non-LTRs exhibit significant decreases in skin electrical resistivity relative to untreated skin ( approximately 5000-fold and approximately 170-fold, respectively), suggesting the existence of two levels of significant skin structural perturbation due to ultrasound exposure in the presence of SLS. Finally, an analysis of the porosity/tortuosity ratio (epsilon/tau) values suggests that trans-cellular transdermal transport pathways are present within the highly permeable, and highly structurally perturbed, LTRs.

Ultrasonic drug delivery--a general review

Ultrasound has an ever-increasing role in the delivery of therapeutic agents, including genetic material, protein and chemotherapeutic agents. Cavitating gas bodies, such as microbubbles, are the mediators through which the energy of relatively non-interactive pressure waves is concentrated to produce forces that permeabilise cell membranes and disrupt the vesicles that carry drugs. Thus, the presence of microbubbles enormously enhances ultrasonic delivery of genetic material, proteins and smaller chemical agents. Numerous reports show that the most efficient delivery of genetic material occurs in the presence of cavitating microbubbles. Attaching the DNA directly to the microbubbles, or to gas-containing liposomes, enhances gene uptake even further. Ultrasonic-enhanced gene delivery has been studied in various tissues, including cardiac, vascular, skeletal muscle, tumour and even fetal tissue. Ultrasonic-assisted delivery of proteins has found most application in transdermal transport of insulin. Cavitation events reversibly disrupt the structure of the stratus corneum to allow transport of these large molecules. Other hormones and small proteins could also be delivered transdermally. Small chemotherapeutic molecules are delivered in research settings from micelles and liposomes exposed to ultrasound. Cavitation appears to play two roles: it disrupts the structure of the carrier vesicle and releases the drug; and makes cell membranes and capillaries more permeable to drugs. There remains a need to better understand the physics of cavitation of microbubbles and the impact that such cavitation has on cells and drug-carrying vesicles.

Ultrasound and transdermal drug delivery

Transdermal drug delivery offers an attractive alternative to the conventional drug delivery methods of oral administration and injection. However, the stratum corneum acts as a barrier that limits the penetration of substances through the skin. Application of ultrasound 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, namely transdermal drug delivery and transdermal monitoring. Particular attention is paid to proposed enhancement mechanisms and future trends in the field of cutaneous vaccination and gene delivery.

Enhancement of antigen uptake and antibody production in goldfish (Carassius auratus) following bath immunization and ultrasound treatment

Ultrasound irradiation and hyperosmotic treatments were compared as facilitators of antigen (BSA) penetration through the skin by bath vaccination and as enhancers of the antibody response in goldfish. The kinetics of BSA penetration and accumulation into the skin, and via it to the blood, and the consequent specific stimulation of the humoral immune response, were studied. The main findings are: (1). ultrasonic treatment is more effective than hyperosmotic treatment in enhancing both antigen transport through the skin and antibody production; (2). the requirements for high antigen concentrations, which are needed for simple bath immersion, could be reduced five times in presonicated fish; and (3). anesthesia, which significantly reduced gill uptake following hyperosmotic treatment, had no effect on skin uptake. The importance of these finding for mass vaccination of adult fish and larvae is discussed.

Low-frequency sonophoresis: a review

Application of ultrasound enhances skin permeability to a variety of molecules (sonophoresis). The enhancement induced by ultrasound is particularly significant at low-frequencies (f<100 kHz, low-frequency sonophoresis). This review summarizes mechanisms and applications of low-frequency sonophoresis. In vitro, in vivo, as well as clinical studies demonstrating the effect of low-frequency ultrasound on transdermal drug delivery and glucose extraction are summarized. Mechanistic insights gained through a number of investigations are also reviewed. Finally, reports on the synergistic effect of low-frequency ultrasound with other enhancers including chemicals and iontophoresis are summarized.

Skin permeability enhancement by low frequency sonophoresis: lipid extraction and transport pathways

The objective of this study was to shed light on the mechanism(s) by which low-frequency ultrasound (20 KHz) enhances the permeability of the skin. The physical effects on the barrier and the transport pathway, in particular, were examined. The amount of lipid removed from the intercellular domains of the stratum corneum following sonophoresis was determined by infrared spectroscopy. Transport of the fluorescent probes nile red and calcein, under the influence of ultrasound, was evaluated by laser-scanning confocal microscopy. The results were compared with the appropriate passive control data and with data obtained from experiments in which the skin was exposed simply to the thermal effects induced by ultrasound treatment. A significant fraction ( approximately 30%) of the intercellular lipids of the stratum corneum, which are principally responsible for skin barrier function, were removed during the application of low-frequency sonophoresis. Although the confocal images from the nile red experiments were not particularly informative, ultrasound clearly and significantly (again, relative to the corresponding controls) facilitated transport of the hydrophilic calcein via discrete permeabilized regions, whereas other areas of the barrier were apparently unaffected. Lipid removal from the stratum corneum is implicated as a factor contributing the observed permeation enhancement effects of low-frequency ultrasound. However, microscopic observations imply that sonophoresis induces localized (aqueous?) permeation pathways at discrete sites.

Ultrasound-enhanced transdermal transport

The enhancement of transdermal transport by ultrasound is reviewed. After a brief discussion of the physics of ultrasound and its medical applications, the effects of high- and low-frequency ultrasound on the transport of substances across the skin are examined. The impact of low-frequency sonophoresis appears to be much more important, with significant increases in transport into and from the skin following its application. Although the mechanism of action remains incompletely defined, cavitation and thermal processes are strongly implicated.

Optimisation of treatment by applying programmable rate-controlled drug delivery technology

A number of programmable rate-controlled drug delivery technologies have been developed during the last two decades with the aim of regulating the rate of drug delivery, sustaining the duration of therapeutic action and/or targeting the delivery of drug to a specific tissue. As a result, several therapeutically beneficial outcomes can be achieved, such as: (i) controlled delivery of a therapeutic dose at a desirable rate of delivery; (ii) maintenance of drug concentrations within an optimal therapeutic range for prolonged duration of treatment; (iii) maximisation of efficacy-dose relationship; (iv) reduction of adverse effects; (v) minimisation of the need for frequent dose intake; and (vi) enhancement of patient compliance. The treatment of illness can thus be optimised. To gain a better understanding of how to optimise the treatment of illnesses by applying programmable rate-controlled drug delivery technologies, this article reviews the scientific concepts and technical principles behind the development of various programmable rate-controlled drug delivery systems that have been marketed or are under active development. Finally, the roles of these technologies in optimising therapeutic outcomes in nine therapeutic areas are discussed.

An investigation of the role of cavitation in low-frequency ultrasound-mediated transdermal drug transport

PURPOSE

Low-frequency ultrasound (20 kHz) has been shown to increase the skin permeability to drugs, a phenomenon referred to as low-frequency sonophoresis (LFS). Many previous studies of sonophoresis have proposed that ultrasound-induced cavitation plays the central role in enhancing transdermal drug transport. In this study, we sought to definitively test the role of cavitation during LFS, as well as to identify the critical type(s) and site(s) of cavitation that are responsible for skin permeabilization during LFS.

METHODS

Pig full-thickness skin was treated by 20 kHz ultrasound and the effect of LFS on the skin permeability was monitored by measuring the increase in the skin electrical conductance. A high pressure LFS cell was constructed to completely suppress cavitation during LFS. An acoustic method, as well as chemical and physical dosimetry techniques, was utilized to monitor the cavitation activities during LFS.

RESULTS

The study using the high-pressure LFS cell showed definitively that ultrasound-induced cavitation is the key mechanism via which LFS permeabilizes the skin. By selectively suppressing cavitation outside the skin using a high-viscosity coupling medium, we further demonstrated that cavitation occurring outside the skin is responsible for the skin permeabilization effect, while internal cavitation (cavitation inside the skin) was not detected using the acoustic measurement method under the ultrasound conditions examined. Acoustic measurement of the two types of cavitation activities (transient vs. stable) indicates that transient cavitation plays the major role in LFS-induced skin permeabilization. Through quantification of the transient cavitation activity at two specific locations of the LFS system, including comparing the dependence of these cavitation activities on ultrasound intensity with that of the skin permeabilization effect, we demonstrated that transient cavitation occurring on, or in the vicinity of, the skin membrane is the central mechanism that is responsible for the observed enhancement of skin permeability by LFS.

CONCLUSIONS

LFS-induced skin permeabilization results primarily from the direct mechanical impact of gas bubbles collapsing on the skin surface (resulting in microjets and shock waves).

Ultrasound-mediated transdermal transport of insulin in vitro through human skin using novel transducer designs

Recent studies have shown that ultrasound (US)-mediated transdermal drug delivery offers a promising potential for noninvasive drug administration. The purpose of this study was to improve low-frequency (20 kHz) US methods for enhancing the transport of insulin in vitro across human skin. The feasibility of using US produced by small, lightweight novel transducers was explored for enhancing the transport of insulin across skin. Previous investigators have used US devices such as large, heavy sonicators or commercially obtained transducers for this type of research. The experiments carried out in this study used two low-profile novel US transducer arrays, the stack and standard array, for improved transport of insulin. The stack array generated a spatial peak temporal peak intensity (I(SPTP)) of 15.4 +/- 0.6 mW/cm(2) and the standard array had an I(SPTP) of 173.7 +/- 1.2 mW/cm(2). Spectrophotometeric absorption techniques were used for determining insulin transport in vitro across human skin. Compared with passive transmission (4.1 +/- 0.5 U) over an exposure period of 1 h, the standard array facilitated over a sevenfold increase in the noninvasive transdermal transport of Humulin R insulin (45.9 +/- 12.9 U). Using Humalog insulin with the standard array, there was a fourfold increase in the US-facilitated transmission over that in the control. These promising results indicate that low-frequency US can be used in a practical device for enhanced transport across the stratum corneum.

The potential of metabolic interventions to enhance transdermal drug delivery

The stratum corneum is a complex tissue that is metabolically active, and undergoes dynamic structural modifications due to the presence of several self-regulating enzymatic systems. A large number of defensive (protective) functions are embodied in this tissue, each with its own structural and biochemical basis. Moreover, the stratum corneum is responsive to external perturbations to the permeability barrier, upregulating a variety of metabolic processes aimed at restoring normal barrier function. Traditional drug delivery methods, which are of limited effectiveness, view the stratum corneum as a static, but semipermeable membrane. In contrast, newer metabolically based methods, which can be deployed alone, or in conjunction with standard methods, have been shown to expand the spectrum of drugs that can be delivered transdermally in hairless mouse epidermis. Yet, while these new approaches hold great promise, if equally effective in human skin, they pose new questions about the risks of a highly permeabilized stratum corneum.

Drug delivery in pluronic micelles: effect of high-frequency ultrasound on drug release from micelles and intracellular uptake

The effect of high-frequency ultrasound on doxorubicin (DOX) release from Pluronic micelles and intracellular DOX uptake was studied for promyelocytic leukemia HL-60 cells, ovarian carcinoma drug-sensitive and multidrug-resistant (MDR) cells (A2780 and A2780/ADR, respectively), and breast cancer MCF-7 cells. Cavitation events initiated by high-frequency ultrasound were recorded by radical trapping. The onset of transient cavitation and DOX release from micelles were observed at much higher power densities than at low-frequency ultrasound (20-100 kHz). Even a short (15-30 s) exposure to high-frequency ultrasound significantly enhanced the intracellular DOX uptake from PBS, RPMI 1640, and Pluronic micelles. The mechanisms of the observed effects are discussed.

Sonicated transdermal drug transport

The following review explores the promise shown by sonicated transdermal drug transport as a novel drug delivery system in great detail. It elucidates the advantages of transdermal drug transport (TDT) over the currently prevalent modes of drug administration and then goes on to explain why despite these obvious advantages TDT is so sparingly used. This discussion includes the problems posed by the impregnable barrier--our skin, or more precisely the stratum corneum (SC), and how sonicated TDT breaches this barrier. A succinct definition of sonophoresis is included along with a description of the experimental setup and a discussion of the results. The mechanism of sonophoresis with particular emphasis on the role of cavitation (both inside and outside the skin), thermal effects, convective transport, and mechanical stresses is also included. The paper also includes a discussion on the variation of sonophoretic enhancement from drug to drug along with a recent mathematical model explaining this. The paper concludes with a section detailing possible applications of sonicated TDT in the near future.

Transdermal delivery of poly-l-lysine by sonomacroporation

A feasibility study of using high-amplitude ultrasound (US) to deliver large molecules transdermally was undertaken. US (20 kHz) of intensity in the range between 2 to 50 W/cm(2) was used to increase the permeability of skin in vitro to large size molecules. For example, when 20-kHz, 5% duty cycle US at the spatial average and pulse-average intensity I(SAPA) = 19 W/cm(2) was applied for 10 min and the distance between the US source and the surface of a skin specimen was 2 mm, the skin permeability was calculated to be 0.5 +/- 0.2 cm/h and 8.5 +/- 4.2 cm/h, respectively, for poly l-lysine-fluorescein isothiocyanate (FITC) (51 kDa) and octa-1-lysine-FITC (2.5 kDa). Without application of US, the skin permeability of the above-mentioned molecules would be essentially zero. A transdermal flux enhancement occurring during the process reported here was much higher than that due to sonophoresis (I(SAPA) < 2 W/cm(2)) as reported in the literature. For comparison, for example, the skin permeability for delivering erythropoeitin (48 kDa) and insulin (6 kDa) reached 9.8 x 10(-6) and 3.3 x10(-3) cm/h, respectively, by using sonophoresis for 1 h US exposure. Experimental results from transdermal flux kinetics, and confocal microscopic cross-sectional and optical images, suggested that the formation of pores in the stratum corneum, whose size varies with skin samples, may be in the range of 1 to 100 microm. The confocal images also suggest the formation of microm-size pathways in epidermis during US exposure.

Phonophoresis: efficiency, mechanisms and skin tolerance

Phonophoresis or sonophoresis is the use of ultrasound to increase percutaneous absorption of a drug. The technique has been widely used in sports medicine since the sixties. Controlled studies in humans in vivo have demonstrated absence or mild effects of the technique with the parameters currently used (frequency 1-3 MHz, intensity 1-2 W/cm(2), duration 5-10 mins, continuous or pulse mode). However, it was demonstrated in 1995 that administration of macromolecules with conserved biological activity was feasible in animals in vivo using low frequency ultrasound. This led to new research into this method of transdermal administration. The aim of this review is to present the main findings published with low frequency and high frequency ultrasound over the last ten years, and to discuss the respective roles of thermal, cavitational and non-cavitational effects on the reduction of the skin barrier. Particular attention is paid to the biological effects on living skin which might be of importance for tolerance and practical use in humans.

Effect of sonication parameters on transdermal delivery of insulin to hairless rats

Application of low-frequency ultrasound has been shown to enhance transdermal drug transport of large molecules such as insulin. In this study, we investigated the dependence of ultrasound-induced transdermal delivery of insulin on ultrasound parameters. Insulin was delivered in vivo to hairless rats using 20 kHz ultrasound applied over a range of ultrasound intensity, application time and pulse length. Change in blood glucose levels of the animals was monitored to assess insulin transport. The results showed a threshold below which no detectable changes in blood glucose level was observed for each ultrasound parameter. Moreover, our findings indicated that sonophoretic enhancement is dependent on energy dose and length of ultrasound pulse that is consistent with a cavitation-based mechanism. The more significant effect of lowering glycemia was obtained with application of less than 15 min ultrasound and was similar to subcutaneous injection of 0.5 U of insulin. Pretreatment of hairless rat skin with ultrasound followed by application of insulin resulted in no significant modification in blood glucose level, indicating that transdermal transport of insulin mainly occurred during sonication. Sonophoresis may therefore potentially be applied for non-invasive and painless delivery of insulin in the treatment of insulin-dependent diabetes.

Dependence of low-frequency sonophoresis on ultrasound parameters; distance of the horn and intensity

Sonophoresis at a frequency of 20 kHz has been shown to enhance transdermal drug delivery, a phenomenon referred to as low-frequency sonophoresis. This study provides an investigation of the dependence of low-frequency sonophoresis on various ultrasound parameters, including the distance of the horn from the skin, intensity, and frequency. We performed in vitro experiments with full thickness pig skin to measure enhancements of skin conductivity and drug permeability. Ultrasound was applied to pretreat the skin using a sonicator operating at a frequency of either 20 or 40 kHz. We also measured pitting of aluminum foil to measure cavitation, which is the principal mechanism of low-frequency sonophoresis. The skin conductivity enhancement was found to be inversely proportional to the distance of the horn from the skin. As the intensity increased, skin conductivity enhancement also increased up to a certain threshold, and then dropped off. The intensities (I(max)) at which maximum enhancement occur are about 14 W/cm2 for 20 kHz and 17 W/cm2 for 40 kHz. These findings may be useful in optimizing low-frequency sonophoresis. Overall, the dependence of transport on ultrasound parameters is similar to that of aluminum foil pitting. These results support the role of cavitation in low-frequency sonophoresis.

Porous resins as a cavitation enhancer for low-frequency sonophoresis

The application of low-frequency ultrasound enhances drug transport through the skin, a phenomenon referred to as low-frequency sonophoresis. This enhancement is mediated through cavitation, the formation and collapse of gaseous bubbles. We hypothesized that the efficacy of low-frequency sonophoresis can be significantly enhanced by provision of nuclei for cavitation. In this study, we used two porous resins, Diaion HP20 and Diaion HP2MG (2MG), as cavitation nuclei. We measured the effect of these resins on cavitation using pitting of aluminum foil. 2MG showed a higher efficacy in enhancing cavitation compared with Diaion HP20. 2MG was also effective in enhancing transdermal mannitol transport. These results confirm that the addition of cavitation nuclei such as porous resins further increases the effect of low-frequency ultrasound on skin permeability.

Investigations of the role of cavitation in low-frequency sonophoresis using acoustic spectroscopy

Application of low-frequency ultrasound significantly enhances skin permeability. The enhancement of skin permeability is mediated by cavitation, oscillation, and collapse of gaseous cavities. In this article, we report detailed investigations of the occurrence of cavitation during low-frequency sonophoresis. Cavitation was monitored by recording pressure amplitudes of subharmonic emission and broadband noise at four different ultrasound frequencies in the range of 20-100 kHz and at various intensities in the range of 0-2.6 W/cm(2). Enhancement of skin conductivity, in the presence of sodium lauryl sulfate (SLS), was also measured under the same ultrasound conditions. Enhancement of skin conductivity correlated well with the amplitude of broadband noise, which suggests the role of transient cavitation in low-frequency sonophoresis. No correlation was found between the subharmonic pressure amplitude and conductivity enhancement.

Synergistic effect of low-frequency ultrasound and surfactants on skin permeability

Low-frequency ultrasound (20 kHz) and surfactants have been individually shown to enhance transdermal drug transport. In this study, we investigated the synergistic effect of ultrasound and surfactants on transdermal drug delivery. Surfactants with different head group chemistries including anionic, cationic, and nonionic with varying tail lengths (8-16-carbon atoms) were studied. We found that surfactants possessing anionic and cationic head groups were more potent than those possessing nonionic head groups in increasing skin conductivity in the presence of ultrasound. Furthermore, for surfactants possessing the same head group, those with a 14-carbon tail length were found to be most effective in enhancing skin permeability. The data presented in this report show that ultrasound and surfactants synergistically enhance skin permeability. Two mechanisms are shown to play a role in this synergistic effect. First, ultrasound enhances surfactant delivery (enhanced delivery) into the skin and, second, ultrasound disperses surfactant (enhanced dispersion) within the skin. In general, surfactants that are potent enhancers by themselves are potent enhancers in the presence of ultrasound as well. We performed imaging experiments to assess the effect of ultrasound on delivery of a model permeant, sulforhodamine B, into the skin. These experiments show that ultrasound enhances surfactant delivery and dispersion in the skin.

Ultrasound-assisted insulin delivery and noninvasive glucose sensing

Peptides and proteins are emerging as an increasingly important class of drugs as they become more readily available through improvement in recombinant DNA technology and chemical synthesis techniques. The application of peptides and proteins as clinically useful drugs is, however, seriously hampered owing to the substantial delivery problems requiring frequent injections. Considerable effort has been directed therefore to developing painless and convenient methods for delivery of peptides and proteins. In diabetes, in addition to the need of insulin injections there is a need to develop painless and convenient methods to measure blood glucose. This review describes approaches based on the application of ultrasound for noninvasive and painless transdermal glucose sensing and delivery of insulin.

Comparison of the effect of ultrasound and of chemical enhancers on transdermal permeation of caffeine and morphine through hairless mouse skin in vitro

The effect of ultrasound (US) on permeation of two model drugs, caffeine (CAF) and morphine (MOR), through hairless mouse skin in vitro was compared with that of three chemical enhancers. Low-frequency (40 KHz), low-power (<0.5 W/cm(2)) US was used; the effect of high-frequency US (1.5-3.0 MHz) was also evaluated in the case of CAF. The chemical enhancers, tested in combination with propylene glycol (PG), were benzalkonium chloride (BAC) oleyl alcohol (OA) and alpha-terpineol (TER). The high-frequency US enhancement of CAF transdermal flux was not statistically significant, while low frequency produced a small but significant increase of the enhancement factor. The effect of US on CAF permeation, however, was lower than that produced by chemical enhancers, in particular OA. The effect of low-frequency US on permeation of MOR was significantly greater (about 10-fold) when compared, on the same frequency and intensity basis, with the effect on CAF. The most active chemical enhancer for MOR, OA, had practically the same effect as low-frequency US. Sonicated skin, although showing slight histological changes, recovered its original low permeability characteristics after turning off sonication. Within the tested system, chemical enhancement appears to offer some advantages over low-frequency US.

Transdermal delivery of heparin and low-molecular weight heparin using low-frequency ultrasound

PURPOSE. Heparin and low-molecular weight heparin (LMWH) are the most commonly used anticoagulants and are administered by intravenous or subcutaneous injections. However, injections of heparin have the potential risk of bleeding complications and the requirement of close monitoring in some cases. We hypothesized that transdermal delivery of heparin may provide an attractive alternative to injections. However, the dose of transdermally delivered heparin is limited by low skin permeability.

METHODS

We increased skin permeability to heparin and LMWH using low-frequency (20 kHz) ultrasound. Biologic activity of transdermally delivered heparin was measured by using activated clotting time assays and by using anti-Xa (aXa) activity. Structural integrity of heparin was also assessed by using gel electrophoresis.

RESULTS

Low-frequency ultrasound increased permeability of pigskin in vitro and rat skin in vivo and allowed delivery of biologically active doses of heparin and low-molecular weight heparin transdermally. A prolonged contact of transdermally delivered heparin with pigskin was found to reduce the biologic activity of heparin, although no such deactivation was observed during short exposures. Transdermally delivered LMWH resulted in sustained aXa levels in the blood. This result was in strong contrast to subcutaneous or intravenous injections of LMWH, which resulted in only temporary elevations of aXa level.

CONCLUSIONS

Transdermal delivery of low-molecular weight heparin is a potential alternative to injections.

Theoretical description of transdermal transport of hydrophilic permeants: application to low-frequency sonophoresis

Application of ultrasound enhances transdermal transport of drugs (sonophoresis). The enhancement may result from enhanced diffusion due to ultrasound-induced skin alteration and/or from forced convection. To understand the relative roles played by these two mechanisms in low-frequency sonophoresis (LFS, 20 kHz), a theory describing the transdermal transport of hydrophilic permeants in both the absence and the presence of ultrasound was developed using fundamental equations of membrane transport, hindered-transport theory, and electrochemistry principles. With mannitol as the model permeant, the role of convection in LFS was evaluated experimentally with two commonly used in vitro skin models- human cadaver heat-stripped skin (HSS) and pig full-thickness skin (FTS). Our results suggest that convection plays an important role during LFS of HSS, whereas its effect is negligible when FTS is utilized. The theory developed was utilized to characterize the transport pathways of hydrophilic permeants during both passive diffusion and LFS with mannitol and sucrose as two probe molecules. Our results show that the porous pathway theory can adequately describe the transdermal transport of hydrophilic permeants in both the presence and the absence of ultrasound. Ultrasound alters the skin porous pathways by two mechanisms: (1) enlarging the skin effective pore radii, or (2) creating more pores and/or making the pores less tortuous. During passive diffusion, both HSS and FTS exhibit the same skin effective pore radii (r = 28 +/- 13 A). In contrast, during LFS, r within HSS is greatly enlarged (r > 125 A), whereas r within FTS does not change significantly (23 +/- 10 A). The observed different roles of convection during LFS across HSS and FTS can be attributed to the different degrees of structural alteration that these two types of skin undergo during LFS.

Postoperative management of functionally restrictive muscular adherence, a corollary to surgical tenolysis: a case report

After a surgical release of adhered nongliding tendons, early active mobilization is encouraged to prevent the reformation of unfavorable adhesions that would limit functional tendon excursion. These restricting adhesions can also occur in non-synovial regions, such as within the flexor mass in the forearm. A "myolysis," or release of muscle fibers from tethering adhesions, can be performed surgically to restore the muscle's gliding and lengthening properties. Postoperative management consists of treatment techniques that include low-load prolonged stress, differential tendon gliding, and active-resistive exercises, all of which are effective in restoring and maximizing a patient's active and passive range of motion to allow optimal mobility and performance. This case study demonstrates the successful management of a patient following a surgical myolysis, utilizing treatment techniques conceptually derived from postoperative tenolysis rehabilitation.

Synergistic effect of enhancers for transdermal drug delivery

Transdermal drug delivery offers a non-invasive route of drug administration, although its applications are limited by low skin permeability. Various enhancers including iontophoresis, chemicals, ultrasound, and electroporation have been shown to enhance transdermal drug transport. Although all these methods have been individually shown to enhance transdermal drug transport, their combinations have often been found to enhance transdermal transport more effectively than each of them alone. This paper summarizes literature studies on these combinations with respect to their efficacy and mechanisms.

Transdermal drug delivery: overcoming the skin's barrier function

The skin represents an extraordinary evolutionary feat. Not only does it physically encapsulate the organism and provide a multifunctional interface between us and our surroundings, but it is perpetually engaged in the assembly of a highly efficient homeostatic barrier to the outward loss of water(1). In so doing, it furnishes a membrane that is equally adept at limiting molecular transport both from and into the body. Overcoming this barrier function then, for the purpose of transdermal drug delivery, has been a necessarily challenging task for the pharmaceutical scientist, and one that boasts significant progress.

Ultrasound-facilitated transport of silver chloride (AgCl) particles in fish skin

Electron-dense nano-particles in aqueous suspension were administered by immersion into the epidermis of fish using ultrasound in the therapeutic range. Enhanced permeability of the tissues to the particles was achieved by acoustic cavitation, which induced a controlled level of necrosis in the outer cell layers, and by non-cavitational exposures, which widened intercellular spaces of non-necrosed tissue in deeper regions of the epidermis. Both particle concentration and penetration depth were quantified using transmission electron microscopy. While cavitation-induced perforation was necessary for particles to penetrate into the tissues, non-cavitational exposures during immersions increased the particle flux towards the skin surface, as well as the diffusion rate of the particles within the epidermis and their depth of penetration. The technique described above may potentially be applied for non-stressful, mass-administration of substances into aquatic animals, as well as the relatively new field of ultrasound-facilitated delivery in moist epithelial tissues in humans.

Synergistic effect of low-frequency ultrasound and sodium lauryl sulfate on transdermal transport

Application of low-frequency ultrasound has been shown to enhance transdermal transport of drugs (low-frequency sonophoresis). In this paper, we show that the efficacy of low-frequency ultrasound in enhancing transdermal transport can be further increased by its combination with sodium lauryl sulfate (SLS), a well-known surfactant. The dependence of the ultrasound-SLS-mediated transport on ultrasound parameters, including intensity, net exposure time, and duty cycle, is discussed. The transdermal transport enhancement is proportional to ultrasound intensity as well as to exposure time, and is independent of duty cycle as long as the net exposure time is the same. The synergistic effect of SLS and ultrasound on transdermal transport increases linearly with SLS concentration. The enhancement is also proportional to the ultrasound energy density beyond a threshold value, which suggests that a certain minimum amount of energy density is required before noticeable changes in skin permeability occur. A similar dependence of the transdermal transport enhancement on energy density is observed in the case of the enhancement induced by ultrasound alone. Although the threshold energy density value in the presence of SLS is about 10 times lower than that in the case of ultrasound alone, the relationship between enhancement and energy density in the presence and in the absence of SLS is otherwise similar. Possible mechanisms for the synergistic effect of ultrasound and SLS are also discussed.

Ultrasound-induced intercellular space widening in fish epidermis

Transmission electron microscopy was employed to determine the effects of therapeutic ultrasound (US) (I(sata) < or =2.2 W cm(-2), 3 MHz), sonicated at different angles and durations, on the external epithelia of fish skin. Sonication at 1.7 W cm(-2) (90 s), where the ultrasonic beam was perpendicular to the skin surface, produced minor intercellular space widening (ICSW), as well as the disruption of desmosomes connecting between the cells. Increasing the intensity to 2.2 W cm(-2) increased ICSW, the extent of which was positively correlated to the duration of exposure (30 to 90 s). Perpendicular sonication produced ICSW, almost exclusively between cells of the two outermost cell layers, parallel to the skin surface. Sonicating at 45 degrees (2.2 W cm(-2), 90 s) produced ICSW in deeper cell layers in the tissues, in which the spaces were at seemingly random orientations. Mucous cells and macrophages were also found to be damaged, as were apoptotic epidermal cells. The suggested mechanism for ICSW is the formation of transverse (shear) waves at the interface between the aquatic medium and the skin surface. The waves, which are damped out within a few cell layers, give rise to shear stresses that, in turn, cause strains that act to separate between cells and damage some of the relatively weaker cells.

Determination of threshold energy dose for ultrasound-induced transdermal drug transport

Low-frequency (20 kHz) ultrasound has been shown to enhance transdermal transport of drugs, a phenomenon referred to as sonophoresis. In this paper, we report the threshold energy dose for ultrasound-induced transdermal drug transport. The threshold was determined by in vitro measurements of the dependence of sonophoretic enhancement on ultrasound parameters, including intensity, duty cycle, and exposure time. While the enhancement varies linearly with ultrasound intensity and exposure times, it is independent of the duty cycle in the range of parameters studied. The enhancement is also directly proportional to the ultrasound energy density once the threshold value is crossed. For full thickness pig skin, the threshold value is about 222 J/cm(2). The overall dependence of transport enhancement on ultrasound parameters is similar to that of cavitation measured in a model system, pitting of aluminum foil. Specifically, the extent of pitting is proportional to ultrasound intensity and exposure time and is independent of duty cycle. Furthermore, the extent of pitting is also proportional to the ultrasound energy density. The similarity between the parametric dependence of transport enhancement and cavitation is consistent with previous findings that cavitation plays the dominant role in sonophoresis.

Effect of low frequency ultrasound on the in vitro percutaneous absorption of clobetasol 17-propionate

The effect of low frequency ultrasound (20 kHz) on the permeation of clobetasol 17-propionate (CP) through skin (sonophoresis) was studied. The ultrasound was applied at either continuous or discontinuous modes and at different intensities. The results showed that low frequency ultrasound significantly enhanced the permeability of CP across hairless mouse skin in vitro. Delivering the same amount of ultrasonic energy in different modes of application markedly influenced the flux and skin residual of CP. The on/off discontinuous ultrasound had greater enhancement on CP permeation than the continuous ultrasound. The results of skin histopathology and permeation experiment using various membranes demonstrate that both disordering of stratum corneum and convective flow resulted from the cavitation effect were responsible for sonophoretic enhancement of CP. The permeation of CP through hair follicles and sweat ducts was susceptible to the application of ultrasound.

Topical drug delivery in humans with a single photomechanical wave

PURPOSE

Assess the feasibility of in vivo topical drug delivery in humans with a single photomechanical wave.

METHODS

Photomechanical waves were generated with a 23 nsec Q-switched ruby laser. In vivo fluorescence spectroscopy was used as an elegant non-invasive assay of transport of 5-aminolevulinic acid into the skin following the application of a single photomechanical wave.

RESULTS

The barrier function of the human stratum corneum in vivo may be modulated by a single (110 nsec) photomechanical compression wave without adversely affecting the viability and structure of the epidermis and dermis. Furthermore, the stratum corneum barrier always recovers within minutes following a photomechanical wave. The application of the photomechanical wave did not cause any pain. The dose delivered across the stratum corneum depends on the peak pressure and has a threshold at approximately 350 bar. A 30% increase in peak pressure, produced a 680% increase in the amount delivered.

CONCLUSIONS

Photomechanical waves may have important implications for transcutaneous drug delivery.

Ultrasound-induced cavitation damage to external epithelia of fish skin

Transmission electron microscopy was used to show the effects of therapeutic ultrasound (< or = 1.0 W/cm2, 1 MHz) on the external epithelia of fish skin. Exposures of up to 90 s produced damage to 5 to 6 of the outermost layers. Negligible temperature elevations and lack of damage observed when using degassed water indicated that the effects were due to cavitation. The minimal intensity was determined for inducing cellular damage, where the extent and depth of damage to the tissues was correlated to the exposure duration. The results may be interpreted as a damage front, advancing slowly from the outer cells inward, presumably in association with the slow replacement of the perforated cell contents with the surrounding water. This study illustrates that a controlled level of microdamage may be induced to the outer layers of the tissues.

Therapeutic modalities in hand surgery

Therapeutic modalities are useful adjuncts in the rehabilitation of many patients commonly seen by hand surgeons. Therapeutic heat, cold, electrical stimulation, and laser and magnetic field treatments are evaluated for their respective mechanisms of action, indications, contraindications, and clinical results. The majority of therapeutic modalities have been extensively investigated and relevant basic science and randomized well-controlled clinical studies addressing the efficacy of therapeutic modalities are emphasized.

Mechanistic studies of molecular transdermal transport due to skin electroporation

The application of electrical high voltage pulses has been shown to greatly enhance the transdermal transport of water-soluble compounds. The resistance of the skins most important barrier, the stratum corneum, drops within less than 1 µs by orders of magnitude. This effect is attributed to electroporation, a nonthermic phenomena known to occur in phospholipid double layers. The striking difference between the stratum corneum lipid layers and the usually investigated phospholipid systems is the phase transition temperature. While lipid layers used for electroporation experiments are in liquid crystal phase above the phase transition temperature, the stratum corneum lipids (phase transition at approximately 70 degrees C) form a rigid quasi-crystalline membrane at room temperature.After the electrical stimulus a recovery of the passive flux was found making high voltage pulsing a suitable tool for controlling transdermal drug delivery. By ordinary light microscopy no dramatic changes in skin structure were found supporting the thesis of electroporation. However the microstructure shows clearly persistent structural changes. Recently the involvement of Joule heating due to the electric stimulus was shown as an important factor for skin permeabilization and molecular transport.