Osteogenic differentiation of rat bone marrow stromal cells by various intensities of low-intensity pulsed ultrasound

Bone growth and repair are under the control of biochemical and mechanical signals. Low-intensity pulsed ultrasound (LIPUS) stimulation at 30 mW/cm² is an established, widely used and FDA approved intervention for accelerating bone healing in fractures and non-unions. Although this LIPUS signal accelerates mineralization and bone regeneration, the actual intensity experienced by the cells at the target site might be lower, due to the possible attenuation caused by the overlying soft tissue. The aim of this study was to investigate whether LIPUS intensities below 30 mW/cm² are able to provoke phenotypic responses in bone cells. Rat bone marrow stromal cells were cultured under defined conditions and the effect of 2, 15, 30 mW/cm² and sham treatments were studied at early (cell activation), middle (differentiation into osteogenic cells) and late (biological mineralization) stages of osteogenic differentiation. We observed that not only 30 mW/cm² but also 2 and 15 mW/cm², modulated ERK1/2 and p38 intracellular signaling pathways as compared to the sham treatment. After 5 days with daily treatments of 2, 15 and 30 mW/cm², alkaline phosphatase activity, an early indicator of osteoblast differentiation, increased by 79%, 147% and 209%, respectively, compared to sham, indicating that various intensities of LIPUS were able to initiate osteogenic differentiation. While all LIPUS treatments showed higher mineralization, interestingly, the highest increase of 225% was observed in cells treated with 2 mW/cm². As the intensity increased to 15 and 30 mW/cm², the increase in the level of mineralization dropped to 120% and 82%. Our data show that LIPUS intensities lower than the current clinical standard have a positive effect on osteogenic differentiation of rat bone marrow stromal cells. Although Exogen™ at 30 mW/cm² continues to be effective and should be used as a clinical therapy for fracture healing, if confirmed in vivo, the increased mineralization at lower intensities might be the first step towards redefining the most effective LIPUS intensity for clinical use.     

Low-intensity pulsed ultrasound (LIPUS) and cell-to-cell communication in bone marrow stromal cells

Low-intensity pulsed ultrasound (LIPUS) is an established therapy for fracture repair and has been used widely in the clinics, but its underlying mechanism of action remains unclear. The aim of the current research was to determine the effect of LIPUS on gap junctional cell-to-cell intercellular communication in rat bone marrow stromal cells (BMSC) in vitro and to determine whether the ability of BMSCs to communicate by gap junctions would affect their response to LIPUS. Single or daily-multiple LIPUS treatment at 1.5 MHz, 30 mW/cm², for 20 min was applied to BMSC. We demonstrated that BMSC form functional gap junctions and single LIPUS treatment significantly increased the intracellular dye transfer between BMSC. In addition, activated phosphorylation of ERK1/2 and p38 by LIPUS stimulation was diminished when cells were treated with a gap junction inhibitor 18β-glycyrrhetinic acid (18β). We further demonstrated that 18β diminished the significant increase in alkaline phosphatase activity following LIPUS stimulation. These results suggest a potential role of gap junctional cell-to-cell intercellular communication on the effects of LIPUS in BMSC.     

Osteocytes exposed to far field of therapeutic ultrasound promotes osteogenic cellular activities in pre-osteoblasts through soluble factors

Low intensity pulsed ultrasound (LIPUS) was reported to accelerate the rate of fracture healing. When LIPUS is applied to fractures transcutaneously, bone tissues at different depths are exposed to different ultrasound fields. Measurement of LIPUS shows pressure variations in near field (nearby transducer); uniform profile was found beyond it (far field). Moreover, we have reported that the therapeutic effect of LIPUS is dependent on the axial distance of ultrasound beam in rat fracture model. However, the mechanisms of how different axial distances of LIPUS influence the mechanotransduction of bone cells are not understood. To understand the cellular mechanisms underlying far field LIPUS on enhanced fracture healing in rat model, the present study investigated the effect of ultrasound axial distances on (1) osteocyte, the mechanosensor, and (2) mechanotransduction between osteocyte and pre-osteoblast (bone-forming cell) through paracrine signaling. We hypothesized that far field LIPUS could enhance the osteogenic activities of osteoblasts via paracrine factors secreted from osteocytes. The objective of this study was to investigate the effect of axial distances of LIPUS on osteocytes and osteocyte–osteoblast mechanotransduction. In this study, LIPUS (plane; 2.2 cm in diameter, 1.5 MHz sine wave, I_SATA = 30 mW/cm²) was applied to osteocytes (mechanosensor) at three axial distances: 0 mm (near field), 60 mm (mid-near field) and 130 mm (far field). The conditioned medium of osteocytes (OCM) collected from these three groups were used to culture pre-osteoblasts (effector cell). In this study, (1) the direct effect of ultrasound fields on the mechanosensitivity of osteocytes; and (2) the osteogenic effect of different OCM treatments on pre-osteoblasts were assessed. The immunostaining results indicated the ultrasound beam at far field resulted in more β-catenin nuclear translocation in osteocytes than all other groups. This indicated that osteocytes could detect the acoustic differences of LIPUS at various axial distances. Furthermore, we found that the soluble factors secreted by far field LIPUS exposed osteocytes could further promote pre-osteoblasts cell migration, maturation (transition of cell proliferation into osteogenic differentiation), and matrix calcification. In summary, our results of this present study indicated that axial distance beyond near field could transmit ultrasound energy to osteocyte more efficiently. The LIPUS exposed osteocytes conveyed mechanical signals to pre-osteoblasts and regulated their osteogenic cellular activities via paracrine factors secretion. The soluble factors secreted by far field exposed osteocytes led to promotion in migration and maturation in pre-osteoblasts. This finding demonstrated the positive effects of far field LIPUS on stimulating osteocytes and promoting mechanotransduction between osteocytes and osteoblasts.     

Investigation of rat bone fracture healing using pulsed 1.5 MHz, 30 mW/cm burst ultrasound – Axial distance dependency

This study investigated the effect of LIPUS on fracture healing when fractures were exposed to ultrasound at three axial distances: z = 0 mm, 60 mm, and 130 mm. We applied LIPUS to rat fracture at these three axial distances mimicking the exposure condition of human fractures at different depths under the soft tissue. Measurement of LIPUS shows pressure variations in near field (nearby transducer); uniform profile was found beyond it (far field). We asked whether different positions of the fracture within the ultrasound field cause inconsistent biological effect during the healing process. Closed femoral fractured Sprague–Dawley rats were randomized into control, near-field (0 mm), mid-near field (60 mm) or far-field (130 mm) groups. Daily LIPUS treatment (plane, but apodized source, see details in the text; 2.2 cm in diameter; 1.5 MHz sine waves repeating at 1 kHz PRF; spatial average temporal average intensity, I_SATA = 30 mW/cm²) was given to fracture site at the three axial distances. Weekly radiographs and endpoint microCT, histomorphometry, and mechanical tests were performed. The results showed that the 130 mm group had the highest tissue mineral density; and significantly higher mechanical properties than control at week 4. The 60 mm and 0 mm groups had significantly higher (i.e. p < 0.05) woven bone percentage than control group in radiological, microCT and histomorphometry measurements. In general, LIPUS at far field augmented callus mineralization and mechanical properties; while near field and mid-near field enhanced woven bone formation. Our results indicated the therapeutic effect of LIPUS is dependent on the axial distance of the ultrasound beam. Therefore, the depth of fracture under the soft tissue affects the biological effect of LIPUS. Clinicians have to be aware of the fracture depth when LIPUS is applied transcutaneously.     

Optimization of un-tethered, low voltage, 20-100kHz flexural transducers for biomedical ultrasonics applications

This paper describes optimization of un-tethered, low voltage, 20-100 kHz flexural transducers for biomedical ultrasonics applications. The goal of this work was to design a fully wearable, low weight (<100 g), battery operated, piezoelectric ultrasound applicator providing maximum output pressure amplitude at the minimum excitation voltage.Such implementation of ultrasound applicators that can operate at the excitation voltages on the order of only 10-25 V is needed in view of the emerging evidence that spatial-peak temporal-peak ultrasound intensity (I_SPTP) on the order of 100 mW/cm² delivered at frequencies below 100 kHz can have beneficial therapeutic effects. The beneficial therapeutic applications include wound management of chronic ulcers and non-invasive transdermal delivery of insulin and liposome encapsulated drugs.The early prototypes of the 20 and 100 kHz applicators were optimized using the maximum electrical power transfer theorem, which required a punctilious analysis of the complex impedance of the piezoelectric disks mounted in appropriately shaped metal housings.In the implementation tested, the optimized ultrasound transducer applicators were driven by portable, customized electronics, which controlled the excitation voltage amplitude and facilitated operation in continuous wave (CW) or pulsed mode with adjustable (10-90%) duty cycle. The driver unit was powered by remotely located rechargeable lithium (Li) polymer batteries. This was done to further minimize the weight of the applicator unit making it wearable. With DC voltage of approximately 15 V the prototypes were capable of delivering pressure amplitudes of about 55 kPa or 100 mW/cm² (I_SPTP). This level of acoustic output was chosen as it is considered safe and side effects free, even at prolonged exposure.     

All-optical continuous-tunable image switch based on nonlinear photoinduced anisotropy in bacteriorhodopsin film

Using nonlinear photoinduced anisotropy in bacteriorhodopsin (bR) film, we presented and demonstrated an image switch in which the output can be tuned continuously by the intensity of a pumping beam. A laser with wavelength 532 nm was used as the pumping beam, and a He-Ne laser at wavelength 632.8 nm was used as the probe beam. Without pumping light, a little of polarized probe beam can transmit the crossed polarizers and the output is very low. With the presence of pumping light, owing to photoinduced anisotropy in the bR film, a portion of the probe beam transmits the crossed polarizers, depending on the intensity of the pumping beam. For the low-intensity probe beam (0.44 mW/cm²), the output is dependent on a wide range of pumping beam (2-30 mW/cm²). On the contrary, for the high-intensity probe beam (0.80 mW/cm²), the output is dependent on a narrow range of pumping beam (2-7 mW/cm²).     

Deposition and electrostatic removal of gaseous organic contaminants on substrate surfaces

The adhesion behavior of di-n-butyl phthalate (DBP) onto different substrates (quartz, glass, and silicon) used as wafer surfaces was studied by using an in situ UV spectrophotometric technique. The results from the closed cell experiments revealed that greatest extent of DBP adhesion occurred on the quartz chip (0.154 μg cm⁻²), followed in the order by the glass (0.054 μg cm⁻²) and silicon (0.039 μg cm⁻²). By means of the in situ spectrophotometric observation, application of an electrical field at 290 V cm⁻¹ in the cell proved to be effective in inducing charging of DBP aerosols, which were consequently attracted towards the electrodes. This method can be applied to wafer storage and transport equipments to prevent wafer contamination from material outgassing representative by DBP.     

Fabrication and characterization of nanocrystalline n-CdO/p-Si as a solar cell

A nanocrystalline CdO/Si solar cell was fabricated via deposition of a CdO thin film on p-type silicon substrate with approximately 370 nm thickness using solid–vapor deposition for Cd powder at 1274 K with argon and oxygen flow. Scanning electron microscopy revealed that the product was a Cadmium oxide nanocrystalline. X-ray diffraction and energy dispersive X-ray analysis were used to characterize the structural properties of the solar cell. The nanocrystalline thin film had a grain size of 38 nm. The solar cell yielded a minimum effective reflectance that exhibited excellent light-trapping at wavelengths ranging from 400 to 1000 nm. Photoluminescence spectroscopy was conducted to investigate the optical properties. The direct band gap energy of the nanocrystalline CdO thin film was 2.46 eV. CdO/Si solar cell photovoltaic properties were examined under 100 mW/cm² solar radiation. The cell showed an open circuit voltage (V_oc) of 457 mV, a short-circuit current density (J_sc) of 18.5 mA/cm², a fill factor (FF) of 0.652, and a conversion efficiency (η) of 5.51%.     

The effect of etching time of porous silicon on solar cell performance

Porous silicon (PS) layers based on crystalline silicon (c-Si) n-type wafers with (1 0 0) orientation were prepared using electrochemical etching process at different etching times. The optimal etching time for fabricating the PS layers is 20 min. Nanopores were produced on the PS layer with an average diameter of 5.7 nm. These increased the porosity to 91%. The reduction in the average crystallite size was confirmed by an increase in the broadening of the FWHM as estimated from XRD measurements. The photoluminescence (PL) peaks intensities increased with increasing porosity and showed a greater blue shift in luminescence. Stronger Raman spectral intensity was observed, which shifted and broadened to a lower wave numbers of 514.5 cm⁻¹ as a function of etching time. The lowest effective reflectance of the PS layers was obtained at 20 min etching time. The PS exhibited excellent light-trapping at wavelengths ranging from 400 to 1000 nm. The fabrication of the solar cells based on the PS anti-reflection coating (ARC) layers achieved its highest efficiency at 15.50% at 20 min etching time. The I-V characteristics were studied under 100 mW/cm² illumination conditions.     

Influence of low-intensity pulsed ultrasound on osteogenic tissue regeneration in a periodontal injury model: X-ray image alterations assessed by micro-computed tomography

Objective

This study was conducted to evaluate, with micro-computed tomography, the influence of low-intensity pulsed ultrasound on wound-healing in periodontal tissues.

Methods

Periodontal disease with Class II furcation involvement was surgically produced at the bilateral mandibular premolars in 8 adult male beagle dogs. Twenty-four teeth were randomly assigned among 4 groups (G): G1, periodontal flap surgery; G2, periodontal flap surgery + low-intensity pulsed ultrasound (LIPUS); G3, guided tissue regeneration (GTR) surgery; G4, GTR surgery plus LIPUS. The affected area in the experimental group was exposed to LIPUS. At 6 and 8 weeks, the X-ray images of regenerated teeth were referred to micro-CT scanning for 3-D measurement.

Results

Bone volume (BV), bone surface (BS), and number of trabeculae (Tb) in G2 and G4 were higher than in G1 and G3 (p < 0.05). BV, BS, and Tb.N of the GTR + LIPUS group were higher than in the GTR group. BV, BS, and Tb.N of the LIPUS group were higher than in the periodontal flap surgery group.

Conclusion

LIPUS irradiation increased the number, volume, and area of new alveolar bone trabeculae. LIPUS has the potential to promote the repair of periodontal tissue, and may work effectively if combined with GTR.     

Investigation of the effect of power ultrasound on the nucleation of water during freezing of agar gel samples in tubing vials

Nucleation, as an important stage of freezing process, can be induced by the irradiation of power ultrasound. In this study, the effect of irradiation temperature (−2 °C, −3 °C, −4 °C and −5 °C), irradiation duration (0 s, 1 s, 3 s, 5 s, 10 s or 15 s) and ultrasound intensity (0.07 W cm⁻², 0.14 W cm⁻², 0.25 W cm⁻², 0.35 W cm⁻² and 0.42 W cm⁻²) on the dynamic nucleation of ice in agar gel samples was studied. The samples were frozen in an ethylene glycol-water mixture (−20 °C) in an ultrasonic bath system after putting them into tubing vials. Results indicated that ultrasound irradiation is able to initiate nucleation at different supercooled temperatures (from −5 °C to −2 °C) in agar gel if optimum intensity and duration of ultrasound were chosen. Evaluation of the effect of 0.25 W cm⁻² ultrasound intensity and different durations of ultrasound application on agar gels showed that 1 s was not long enough to induce nucleation, 3 s induced the nucleation repeatedly but longer irradiation durations resulted in the generation of heat and therefore nucleation was postponed. Investigation of the effect of ultrasound intensity revealed that higher intensities of ultrasound were effective when a shorter period of irradiation was used, while lower intensities only resulted in nucleation when a longer irradiation time was applied. In addition to this, higher intensities were not effective at longer irradiation times due to the heat generated in the samples by the heating effect of ultrasound. In conclusion, the use of ultrasound as a means to control the crystallization process offers promising application in freezing of solid foods, however, optimum conditions should be selected.     

Preparation of chemically deposited thin films of CdS/PbS solar cell

The present paper reports the preparation of a solar cell which has a cross-sectional scheme: ITO/CdS/PbS, containing a commercially transparent conductive ITO; chemically deposited n-type CdS (340 nm) and absorbed layer of p-type PbS (1400 nm). The structural and optical properties of the constituent films are presented. X-ray diffraction showed that all of the thin films are polycrystalline. Using scanning electron microscopy, the present study revealed that the films have uniform surface morphology over the substrate. The solar cell was characterized by determining the open circuit voltage, short-circuit current density, and J–V under 40 mW/cm² solar radiation. The efficiency of the solar cells was 1.35%, which is much higher (0.041, 0.5 and 0.1–0.4%) and slightly smaller (1.65%) than some solar cells reported in the literature.     

Effect of citric acid on photoelectrochemical properties of tungsten trioxide films prepared by the polymeric precursor method

Effect of citric acid (CA) on microstructure and photoelectrochemical properties of WO₃ films prepared by the polymeric precursor method was investigated. The obtained materials were characterized by means of X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), and high-resolution transmission electron microscopy (HRTEM). The results showed that samples prepared with adding different amounts of citric acid had a pure phase of cubic. The addition of citric acid could significantly increase the particle size and change the surface of WO₃ films. The photoelectrochemical measurements were performed using a standard three-electrode system cell. The films prepared from mass ratios of CA/PEG (R = 0, 0.2, 0.4, 0.6 and 1) showed 1.0, 1.4, 1.7, 2.1 and 0.9 mA cm⁻² at 1.2 V under illumination with a 500 W xenon lamp (I₀ = 100 mW/cm²), respectively.     

In vivo imaging of blood flow in the mouse Achilles tendon using high-frequency ultrasound

Objective

Achilles tendinitis is a common clinical problem with many treatment modalities, including physical therapy, exercise and therapeutic ultrasound. However, evaluating the effects of current therapeutic modalities and studying the therapeutic mechanism(s) in vivo remains problematic. In this study, we attempted to observe the morphology and microcirculation changes in mouse Achilles tendons between pre- and post-treatment using high-frequency (25 MHz) ultrasound imaging. A secondary aim was to assess the potential of high-frequency ultrasound in exploring therapeutic mechanisms in small-animal models in vivo.

Methods

A collagenase-induced mouse model of Achilles tendinitis was adopted, and 5 min treatment of continuous-mode low-frequency (45 kHz) ultrasound with 47 mW/cm² maximum intensity and 16.3 cm² effective beam radiating area was applied. The B-mode images showed no focal hypoechoic regions in normal Achilles tendons either pre- or post-treatment. The Doppler power energy and blood flow rate were measured within the peritendinous space of the Achilles tendon.

Conclusion

An increase in the microcirculation was observed soon after the low-frequency ultrasound treatment, which was due to immediate induction of vascular dilatation. The results suggest that applying high-frequency Doppler imaging to small-animal models will be an invaluable aid in explorations of the therapeutic mechanism(s). Our future work includes using imaging to assess microcirculation changes in tendinitis between before and after treatment over a long time period, which is expected to yield useful physiological data for future human studies.     

Impact of thermal effects induced by ultrasound on viability of rat C6 glioma cells

In order to have consistent and repeatable effects of sonodynamic therapy (SDT) on various cancer cells or tissue lesions we should be able to control a delivered ultrasound energy and thermal effects induced. The objective of this study was to investigate viability of rat C6 glioma cells in vitro depending on the intensity of ultrasound in the region of cells and to determine the exposure time inducing temperature rise above 43 °C, which is known to be toxic for cells. For measurements a planar piezoelectric transducer with a diameter of 20 mm and a resonance frequency of 1.06 MHz was used. The transducer generated tone bursts with 94 μs duration, 0.4 duty-cycle and initial intensity I_SATA (spatial averaged, temporal averaged) varied from 0.33 W/cm² to 8 W/cm² (average acoustic power varied from 1 W to 24 W). The rat C6 glioma cells were cultured on a bottom of wells in 12-well plates, incubated for 24 h and then exposed to ultrasound with measured acoustic properties, inducing or causing no thermal effects leading to cell death. Cell viability rate was determined by MTT assay (a standard colorimetric assay for assessing cell viability) as the ratio of the optical densities of the group treated by ultrasound to the control group. Structural cellular changes and apoptosis estimation were observed under a microscope. Quantitative analysis of the obtained results allowed to determine the maximal exposure time that does not lead to the thermal effects above 43 °C in the region of cells for each initial intensity of the tone bursts used as well as the threshold intensity causing cell death after 3 min exposure to ultrasound due to thermal effects. The averaged threshold intensity was found to be about 5.7 W/cm².     

Biological effects of combined ultrasound and cisplatin treatment on ovarian carcinoma cells

The effects of low-power ultrasound, the anti-cancer drug cisplatin, and their combined application were studied in two lines of human ovarian carcinoma cells, A2780 and A2780cis. Four modes of treatment were used: exposure to ultrasonic field, application of cisplatin, exposure to ultrasound followed by cisplatin, and presence of cisplatin followed by exposure to application ultrasound. Ultrasound was used at intensities of 0.5 W/cm² and 1.0 W/cm² for 10 min, cisplatin was applied at concentrations of 1 μM and 6 μM per cell suspension treated in A2780 and cisplatin-resistant A2780cis cells, respectively. The results of each experimental treatment were assessed by the resultant cell viability related to the viability of control cells, using a standard MTT test. It was shown that a combined effect of ultrasound and cisplatin was more effective than that of ultrasound or cisplatin alone. It also appeared that the order of application played a role, with the cisplatin-ultrasound treatment lowering cell viability more than the ultrasound-cisplatin treatment. It can be assumed that the exposure of cells to a low-power ultrasonic field has an immediate effect on the structure of cell surfaces and, consequently, on entry of cisplatin into the cell.The study also included observations on changes in the cell cycle associated with the treatments used in both cell lines and their evaluation by flow cytometry.     

Ultrasound therapy modulates osteocalcin expression during bone repair in rats

The aim of this study was to measure the temporal pattern of the protein expression of RUNX2, RANKL, OPG, and osteocalcin after ultrasound therapy during the process of bone healing by immunohistochemistry. The animals were randomly distributed into two groups: control or ultrasound-treated group. A non-critical size bone defects were surgically created at the upper third of the tibia. The treatments started 24 h post-surgery, and they are performed for 3, 6, and 12 sessions, with an interval of 48 h. A low-intensity pulsed ultrasound (1.5 MHz, 1:4 duty cycle, intensity SATA 30 mW/cm², 20 min/session, stationary mode application) was used. On days 7, 13, and 25 post-injury, rats were killed individually by carbon dioxide asphyxia. The tibias were removed for analysis. The histopathological analysis pointed out no remarkable differences between groups for all periods evaluated. However, immunohistochemical data revealed that ultrasound therapy produced an up-regulation of osteocalcin at day 7th and 13th post-surgery. Taken together, our results indicate that ultrasound therapy modulates osteocalcin expression during bone repair in rats as depicted by differential immunopression at the initial and intermediate stages of recovery.     

Low intensity pulsed ultrasound for fracture healing: a review of the clinical evidence and the associated biological mechanism of action

Low intensity pulsed ultrasound is used in the clinical treatment of fractures and other osseous defects. Level I clinical studies demonstrate the ability of a specific ultrasound signal (1.5 MHz ultrasound pulsed at 1 kHz, 20% duty cycle, 30 mW/cm² intensity (SATA)) to accelerate the healing time in fresh tibia, radius and scaphoid fractures by up to 40%. Additionally, the same ultrasound signal has been shown to be effective at resolving all types of nonunions of all ages, following a wide range of fracture types and primary fracture management techniques.Recently, significant efforts have resulted in a more comprehensive understanding of the biological mechanism of action that produces the documented clinical outcomes. Low intensity pulsed ultrasound has been demonstrated to accelerate in vivo all stages of the fracture repair process (inflammation, soft callus formation, hard callus formation). In particular, accelerated mineralisation has been demonstrated in vitro with increases in osteocalcin, alkaline phosphatase, VEGF and MMP-13 expression. Integrins, a family of mechanoreceptors present on a wide range of cells involved in the fracture healing process, have been shown to be activated by the ultrasound signal. Downstream of the integrin activation, focal adhesions occur on the surface of cells with the activation of multiple signalling pathways, including the ERK, NF-κβ, and PI3 kinase pathways. These pathways have been directly linked to the production of COX-2 and prostaglandin, which are key to the processes of mineralisation and endochondral ossification in fracture healing.     

Ultrasound generation with high power and coil only EMAT concepts

Electro-magnetic acoustic transducers (EMATs) are intended as non-contact and non-destructive ultrasound transducers for metallic material. The transmitted intensities from EMATS are modest, particularly at notable lift off distances. Some time ago a concept for a “coil only EMAT” was presented, without static magnetic field. In this contribution, such compact “coil only EMATs” with effective areas of 1–5 cm² were driven to excessive power levels at MHz frequencies, using pulsed power technologies. RF induction currents of 10 kA and tens of Megawatts are applied. With increasing power the electroacoustic conversion efficiency also increases. The total effect is of second order or quadratic, therefore non-linear and progressive, and yields strong ultrasound signals up to kW/cm² at MHz frequencies in the metal. Even at considerable lift off distances (cm) the ultrasound can be readily detected. Test materials are aluminum, ferromagnetic steel and stainless steel (non-ferromagnetic). Thereby, most metal types are represented. The technique is compared experimentally with other non-contact methods: laser pulse induced ultrasound and spark induced ultrasound, both damaging to the test object’s surface. At small lift off distances, the intensity from this EMAT concept clearly outperforms the laser pulses or heavy spark impacts.     

Silicon donor and Stokes terahertz lasers

Two types of lasers based on hydrogen-like impurity-related transitions in bulk silicon operate at frequencies between 1 and 7 THz (wavelength range of 50-230 μm). These lasers operate under mid-infrared optical pumping of n-doped silicon crystals at low temperatures (<30 K). Dipole-allowed optical transitions between particular excited states of group-V substitutional donors are utilized in the first type of terahertz silicon lasers. These lasers have a gain ∼1-3 cm⁻¹ above the laser thresholds (>1 kW cm⁻²) and provide 10 ps-1 μs pulses with a few mW output power on discrete lines. Raman-type Stokes stimulated emission in the range 4.6-5.8 THz has been observed from silicon crystals doped by antimony and phosphorus donors when optically excited by radiation from a tunable infrared free electron laser. The scattering occurs on the 1s(E)→1s(A₁) donor electronic transition accompanied by an emission of the intervalley transverse acoustic g-phonon. The Stokes lasing has a peak power of a few tenths of a mW and a pulse width of a few ns. The Raman optical gain is about 7.4 cm GW⁻¹ and the optical threshold intensity is ∼100 kW cm⁻².