Sigrid Berg

Ultrasound-enhanced drug delivery in prostate cancer xenografts by nanoparticles stabilizing microbubbles.

The delivery of nanoparticles to solid tumors is often ineffective due to the lack of specificity towards tumor tissue, limited transportation of the nanoparticles across the vascular wall and poor penetration through the extracellular matrix of the tumor. Ultrasound is a promising tool that can potentially improve several of the transportation steps, and the interaction between sound waves and microbubbles generates biological effects that can be beneficial for the successful delivery of nanocarriers and their contents. In this study, a novel platform consisting of nanoparticle-stabilized microbubbles has been investigated for its potential for ultrasound-enhanced delivery to tumor xenografts. Confocal laser scanning microscopy was used to study the supply of nanoparticles from the vasculature and to evaluate the effect of different ultrasound parameters at a microscopic level. The results demonstrated that although the delivery is heterogeneous within tumors, there is a significant improvement in the delivery and the microscopic distribution of both nanoparticles and a released model drug when the nanoparticles are combined with microbubbles and ultrasound. The mechanisms that underlie the improved delivery are discussed.

Nanoparticle delivery to the brain--By focused ultrasound and self-assembled nanoparticle-stabilized microbubbles.

The blood-brain barrier (BBB) constitutes a significant obstacle for the delivery of drugs into the central nervous system (CNS). Nanoparticles have been able to partly overcome this obstacle and can thus improve drug delivery across the BBB. Furthermore, focused ultrasound in combination with gas filled microbubbles has opened the BBB in a temporospatial manner in animal models, thus facilitating drug delivery across the BBB. In the current study we combine these two approaches in our quest to develop a novel, generic method for drug delivery across the BBB and into the CNS. Nanoparticles were synthesized using the polymer poly(butyl cyanoacrylate) (PBCA), and such nanoparticles have been reported to cross the BBB to some extent. Together with proteins, these nanoparticles self-assemble into microbubbles. Using these novel microbubbles in combination with focused ultrasound, we successfully and safely opened the BBB transiently in healthy rats. Furthermore, we also demonstrated that the nanoparticles could cross the BBB and deliver a model drug into the CNS.

Nanoparticle-stabilized microbubbles for multimodal imaging and drug delivery

Microbubbles (MBs) are routinely used as contrast agents for ultrasound imaging. The use of ultrasound in combination with MBs has also attracted attention as a method to enhance drug delivery. We have developed a technology platform incorporating multiple functionalities, including imaging and therapy in a single system consisting of MBs stabilized by polyethylene glycol (PEG)-coated polymeric nanoparticles (NPs). The NPs, containing lipophilic drugs and/or contrast agents, are composed of the widely used poly(butyl cyanoacrylate) (PBCA) polymer and prepared in a single step. MBs stabilized by these NPs are subsequently prepared by self-assembly of NPs at the MB air-liquid interface. Here we show that these MBs can act as contrast agents for conventional ultrasound imaging. Successful encapsulation of iron oxide NPs inside the PBCA NPs is demonstrated, potentially enabling the NP-MBs to be used as magnetic resonance imaging (MRI) and/or molecular ultrasound imaging contrast agents. By precise tuning of the applied ultrasound pulse, the MBs burst and the NPs constituting the shell are released. This could result in increased local deposit of NPs into target tissue, providing improved therapy and imaging contrast compared with freely distributed NPs.

5F-5 Reducing Fluid Coupled Crosstalk Between Membranes in CMUT Arrays by Introducing a Lossy Top Layer

Capacitive micromachined ultrasound transducers (CMUTs) promise high transducer performance for several ultrasound applications. Likely the most promising opportunities will be found in applications which require arrays with a large numbers of individual elements for precise beam steering and focusing such as in ultrasound imaging. In off-axis beam steering neighbor elements operate at different phase. This leads to deformation gradients along the surface of the array which cause local tangential forces acting upon the adjacent medium. In the case of an adjacent lossless fluid this results in local high-Q resonances which have significant detrimental impact on the transducer array performance in off-axis operation. It is therefore of paramount importance to control and reduce the excitation of such resonances to an acceptable level through the design. The present paper presents one approach to this. Simulations show that by introducing a soft intermediate surface layer which is only a few per cent of its longitudinal wavelength in thickness, and which has high shear deformation losses - of the order of 0.4 in loss tangent, quite acceptable results may be obtained without adding more than 0.5-1 dB in transmit losses. Although not readily commercially available, it may be possible to develop adequate polymer materials for this purpose. It is shown also that such a layer may be mechanically protected by an additional stiff layer without significant degradation of the ultrasound transducer performance

Co-optimization of CMUT and receive amplifiers to suppress effects of neighbor coupling between CMUT elements

Capacitive micromachined ultrasound transducers (CMUTs) promise high transducer performance for several ultrasound applications. When using the CMUT array for medical applications where a focused ultrasound image with a 90 degree image sector is needed, we need a large number of individual elements. In off-axis beam steering, neighbor elements operate at different phase. This leads to unwanted acoustic effects caused by the interaction with the fluid medium outside the array. We see high-Q resonances close to the center frequency of the array at off-axis angles, which we want to reduce. We propose to use Transimpedance Amplifiers (TIAs) and Charge Sampling Amplifiers (CSAs) where we can easily adjust the input impedance, which opens up the possibility to design amplifiers that are optimized for an ultrasound system with CMUTs. Simulations show that a low impedance path results in suppression of the effects of resonances for both CSAs and TIAs and that co-optimization is important since the frequency of the CMUT array affects the Q-factor of the unwanted resonances. Even though we introduce an impedance mismatch the noise figure is still at an acceptable level. We present simulations in water, blood plasma with estimated data, and olive oil and show that the viscosity of the medium greatly influences the presence of resonances. This indicates that effects that might be present in human tissue may be much reduced in olive oil or other vegetable oils.

Backing requirements for CMUT arrays on silicon

Capacitive Micromachined Ultrasonic Transducer (CMUT) have been subject to research by several research groups during the last two decades. Despite many potential advantages over traditional piezoelectric ultrasound transducers, the CMUT technology has not yet made a proper commercial breakthrough. One issue which we believe need more investigation is the control of the acoustic crosstalk, both through the silicon substrate and through the adjacent fluid medium.The work presented in this thesis concerns the modeling of immersed CMUT arrays and the investigation of two different acoustic crosstalk effects which may harm the transducer response. The CMUT array is modeled with an analytical model describing the motion of the single acoustically isolated CMUT cell as a combination of free acoustic vibration modes. Several CMUT cells may be connected to form larger elements, and the vibration modes of adjacent CMUTs are coupled through the fluid outside, by an acoustic impedance matrix. In addition, the model may also include the motion of the silicon substrate and the in uence from the source impedance of the electronics.The first crosstalk effect which we focus on in this work, is the generation of surface acoustic waves (SAWs) along the surface of the silicon substrate supporting the CMUT array. If the SAWs are not damped in any way, they may couple to waves in the fluid at certain steering angles, and cause a drop in transmission eciency at these angles. In the presented simulation we show that the silicon substrate must be limited in thickness in order for the backing material to be able to damp the SAWs. If the CMUT array is mounted on top of a stack of silicon substrates containing transmit and receive circuitry, the thickness and composition of both the bonding materials and the silicon substrates must be taken into account. We have compared three different bonding techniques, surface liquid interdi usion (SLID) bonding, anisotropic conductive adhesive (ACA) and direct fusion bonding, and we show that fusion bonding is the technique which is best suited for high frequency CMUT arrays with several IC chips underneath. The penetration depth of the SAWs is frequency dependent, and we show that a high frequency CMUT array with center frequency around 30 MHz, stacked on top of three circuit layers, should have a total silicon thickness below 100 m. If the acoustic backing material instead is placed between the CMUT array and the rst circuit chip, other bonding techniques than fusion bonding may also be used, and the thicknesses of the IC chips are no longer of importance. However, the transmission of the electric signal through or around the backing layer might be a challenge.The other crosstalk effect which has been investigated in this thesis, is the inter-element coupling at the CMUT- fluid interface. We refer to this effect as dispersive guided modes, and show that the excitation of local resonances in the interface region may affect the overall transmission from the array at frequencies well within the 100 % bandwidth of the transducer. These waves are not damped by the acoustic backing material. Many CMUT designers choose to have larger distances between CMUT cells in neighbor elements than between CMUTs within an element. We denote this as double periodicity. We have compared the e ect of the dispersive guided modes in arrays with the same distance between all the CMUT cells, regardless if they are in the same or in adjacent elements (denoted as single periodicity), and arrays with double periodicity. Simulations show that the response from arrays with double periodicity is affected by the crosstalk even at broadside radiation, whereas the effect becomes apparent at o -axis beam steering for arrays with single periodicity.Through simulations, we have shown that it is possible to mechanically damp the dispersive guided modes substantially by introducing a lossy coating material of a few micrometer thickness. The PDMS material RTV516 from GE Silicones seems to have material properties which are well suited for such damping. We have also shown that introducing a low electric source impedance in the transmit electronics may reduce the e ect of the local CMUT- uid resonances on the detected signal.

Acoustic backing in 3-D integration of CMUT with front-end electronics

Capacitive micromachined ultrasonic transducers (CMUTs) have shown promising qualities for medical imaging. However, there are still some problems to be investigated, and some challenges to overcome. Acoustic backing is necessary to prevent SAWs excited in the surface of the silicon substrate from affecting the transmit pattern from the array. In addition, echoes resulting from bulk waves in the substrate must be removed. There is growing interest in integrating electronic circuits to do some of the beamforming directly below the transducer array. This may be easier to achieve for CMUTs than for traditional piezoelectric transducers. We will present simulations showing that the thickness of the silicon substrate and thicknesses and acoustic properties of the bonding material must be considered, especially when designing high-frequency transducers. Through simulations, we compare the acoustic properties of 3-D stacks bonded with three different bonding techniques; solid-liquid interdiffusion (SLID) bonding, direct fusion bonding, and anisotropic conductive adhesives (ACA). We look at a CMUT array with a center frequency of 30 MHz and three silicon wafers underneath, having a total silicon thickness of 100 μm. We find that fusion bonding is most beneficial if we want to prevent surface waves from damaging the array response, but SLID and ACA are also promising if bonding layer thicknesses can be reduced.

Challenges with acoustic backing of CMUT arrays on silicon with integrated electronics

Capacitive micromachined ultrasonic transducers (CMUTs) have in the last decade shown promising qualities for medical imaging. But there are some acoustical challenges that have to be overcome before the performance of a CMUT array with integrated electronics is satisfactory. Acoustic backing is necessary to avoid surface acoustic waves (SAW) excited in the surface of the silicon substrate to affect the transmit pattern from the array. For the backing to be able to damp the SAW, the silicon wafer stack has to be thin enough for some of the acoustic energy to reach the backing. We will present simulations showing that the thickness of the silicon substrate and thicknesses and acoustic properties of the bonding material have to be considered, especially when designing high frequency transducers. Through simulations we compare the acoustic properties of 3D stacks bonded with three different bonding techniques; anisotropic conductive adhesives (ACA), Solid-Liquid Interdiffusion (SLID) bonding and direct fusion bonding. We look at a CMUT array with a center frequency of 30 MHz and three silicon wafers underneath. We find that fusion bonding is most beneficial if we want to prevent surface waves from damaging the array response, but SLID and ACA are also promising if bonding layer thicknesses can be reduced.

Ultrasound Improves the Delivery and Therapeutic Effect of Nanoparticle-Stabilized Microbubbles in Breast Cancer Xenografts

Compared with conventional chemotherapy, encapsulation of drugs in nanoparticles can improve efficacy and reduce toxicity. However, delivery of nanoparticles is often insufficient and heterogeneous because of various biological barriers and uneven tumor perfusion. We investigated a unique multifunctional drug delivery system consisting of microbubbles stabilized by polymeric nanoparticles (NPMBs), enabling ultrasound-mediated drug delivery. The aim was to examine mechanisms of ultrasound-mediated delivery and to determine if increased tumor uptake had a therapeutic benefit. Cellular uptake and toxicity, circulation and biodistribution were characterized. After intravenous injection of NPMBs into mice, tumors were treated with ultrasound of various pressures and pulse lengths, and distribution of nanoparticles was imaged on tumor sections. No effects of low pressures were observed, whereas complete bubble destruction at higher pressures improved tumor uptake 2.3 times, without tissue damage. An enhanced therapeutic effect was illustrated in a promising proof-of-concept study, in which all tumors exhibited regression into complete remission.

Measurements of CMUT neighbour coupling resonances in fluids of different viscosities

In medical imaging it is in most cases necessary to steer the beam to form an ultrasound image. This is done by adding phase shifts between neighbour elements. When using CMUT arrays for this kind of imaging, neighbour coupling between elements through the fluid might give resonances at certain frequencies. We will present measurement results and simulations of input admittance of a linear array of CMUT elements, where neighbour elements are excited 180 degrees out of phase. An excitation phase difference of 180 degrees will not occur in a real imaging situation, the phase difference will in most cases be substantially smaller. Simulations for phase differences that are realistic in an imaging situation are also shown. Measurements have been performed in air, rapeseed oil and kerosene (lamp oil). The measurements show resonances in rapeseed oil between 14 MHz and 16 MHz, and between 15.5 MHz and 16.5 MHz in kerosene depending on the DC bias applied. This corresponds to phase velocities between 700 and 825 m/s. In air the resonances occur between 30.5 MHz and 36 MHz. In air and kerosene the simulations show somewhat higher Q-values than the measurements. By adding case independent losses to the CMUTs, it is possible to obtain good match to all measured Q-values. Comparison of measurements and simulations show that the model is well suited to describe the measurements performed. The results presented indicate that the CMUT model can be used to simulate more realistic neighbour element phase shifts. Such simulations show that resonances will disturb the transmitted ultrasonic wave at frequencies close to the centre frequency at large steering angles.