Frequency-Dependent Ultrasonic Stimulation of Poly(N-isopropylacrylamide) Microgels in Water

As a novel stimulus, we use high-frequency ultrasonic waves to provide the required energy for breaking hydrogen bonds between Poly(N-isopropylacrylamide) (PNIPAM) and water molecules while the solution temperature is maintained below the volume phase transition temperature (VPTT = 32 °C). Ultrasonic waves propagate through the solution and their energy will be absorbed due to the liquid viscosity. The absorbed energy partially leads to the generation of a streaming flow and the rest will be spent to break the hydrogen bonds. Therefore, the microgels collapse and become insoluble in water and agglomerate, resulting in solution turbidity. We use turbidity to quantify the ultrasound energy absorption and show that the acousto-response of PNIPAM microgels is a temporal phenomenon that depends on the duration of the actuation. Increasing the solution concentration leads to a faster turbidity evolution. Furthermore, an increase in ultrasound frequency leads to an increase in the breakage of more hydrogen bonds within a certain time and thus faster turbidity evolution. This is due to the increase in ultrasound energy absorption by liquids at higher frequencies

Aufbau eines in eine Hohlnadel integrierten Kraftsensors

Die Brachytherapie ist ein minimal-invasiver chirurgischer Eingriff zur Behandlung von Prostatakrebs. Dabei werden hohle Nadeln durch die Haut (Perineum) zwischen Genital und Anus des Mannes eingeführt, um radioaktive Metallstifte (Seeds) in das Krebsgewebe einzusetzen und dieses gezielt zu zerstören. Aufgrund fehlenden haptischen Feedbacks und mangelnder Erfahrung kommt es häufiger zu Komplikationen, wie Inkontinenz, Impotenz oder Erektionsstörungen. Um neue Ansätze für automatisierte Prozesse oder Umsetzungen mit verbessertem haptischen Feedback für den Chirurgen zu ermöglichen, wird im Rahmen dieser Arbeit ein 1-DOFKraftsensor entwickelt, der die Kräfte unmittelbar an der Nadelspitze misst. Diese Methode minimiert den Störeinfluss durch Reibungskräfte, die entlang des Nadelschaftes entstehen. Der zu entwickelnde Sensor soll in einer Testumgebung der TU Hamburg-Harburg genutzt und durch einen lichtleiterbasierten Kraftsensor auf Glasfaserbasis ergänzt werden. Die Eigenschaften und Abmessungen handelsüblicher Nadeln werden untersucht, um klinisch realitätsnahe Randbedingungen zu formulieren. Ihre Durchmesser betragen 1,27mm sowie 1,47mm und stellen hohe Anforderungen an die Integrierbarkeit eines Messelementes. Daher verkörpern am Institut für Elektromechanische Konstruktionen (EMK) entwickelte Silizium-Dehnungsmesstreifen das Kernelement dieser Aufgabe. Sie nehmen eine Fläche von 0,5 x 0,5mm2 ein, messen Dehnungen im Bereich von 10�7mm{mm ¤ � ¤ 1,7 � 10�3mm{mm und ihre intrinsischen vier Widerstandsgebiete sind innerhalb dieser Abmessungen zu einer Vollbrücke verschaltet. In Folge einer Literaturrecherche und Untersuchung bestehender Systeme, werden Anforderungen formuliert und Konzepte zur Lösungsfindung abgeleitet. Es werden zwei Varianten verschiedener Nadelarten näher untersucht. Um eine finale Auswahl zu treffen, werden beide Konzepte als CAD-Modelle entworfen und ihr Dehnungsverhalten in Abhängigkeit der Krafteinleitungsrichtung und -intensität analysiert und bewertet. Das resultierende Gesamtkonzept sieht einen separaten Entwurf der Nadelspitze als Verformungskörper vor. Er wird in Form von zwei Halbschalen mit einem Durchmesser von 1,2mm und einer Länge von 10mm unter Anwendung eines speziellen Fertigungsverfahrens am Institut EMK hergestellt. Die Hälften sind so modifiziert, dass sowohl das esselement als auch dessen Verdrahtung sowie eine Glasfaser dort Platz finden bzw. letztere diesen passieren kann. Sie besitzen miniaturisierte Absätze, um eine passgenaue Vereinigung beider Teile zu gewährleisten. Durch den modularen Aufbau des Verformungskörpers und der separaten Ankopplung an ein dünnwandiges Kapillarrohr können Nadeln in beliebiger Länge realisiert werden, wobei die Kosten für die mikromechanische Fertigung des Verformungskörpers unabhängig vom Nadelaufbau ist

Ultra-Low-Voltage Capacitive Micromachined Ultrasonic Transducers with Increased Output Pressure Due to Piston-Structured Plates

Capacitive micromachined ultrasonic transducers (CMUTs) represent an accepted technology for ultrasonic transducers, while high bias voltage requirements and limited output pressure still need to be addressed. In this paper, we present a design for ultra-low-voltage operation with enhanced output pressure. Low voltages allow for good integrability and mobile applications, whereas higher output pressures improve the penetration depth and signal-to-noise ratio. The CMUT introduced has an ultra-thin gap (120 nm), small plate thickness (800 nm), and is supported by a non-flexural piston, stiffening the topside for improved average displacement, and thus higher output pressure. Three designs for low MHz operation are simulated and fabricated for comparison: bare plate, plate with small piston (34% plate coverage), and big piston (57%). The impact of the piston on the plate mechanics in terms of resonance and pull-in voltage are simulated with finite element method (FEM). Simulations are in good agreement with laser Doppler vibrometer and LCR-meter measurements. Further, the sound pressure output is characterized in immersion with a hydrophone. Pull-in voltages range from only 7.4 V to 25.0 V. Measurements in immersion with a pulse at 80% of the pull-in voltage present surface output pressures from 44.7 kPa to 502.1 kPa at 3.3 MHz to 4.2 MHz with a fractional bandwidth of up to 135%. This leads to an improvement in transmit sensitivity in pulsed (non-harmonic) driving from 7.8 kPa/V up to 24.8 kPa/V

Frequency-dependent Ultrasonic Stimulation of PNIPAM Microgels in Water

As a novel stimulus, we used high-frequency ultrasonic waves to provide the required energy for breaking hydrogen bonds between Poly(N-isopropylacrylamide) (PNIPAM) and water molecules while the solution temperature maintains below the volume phase transition temperature (VPTT=$$32^\circ C$$). Ultrasonic waves propagate through the solution and their energy will be absorbed due to the liquid viscosity. The absorbed energy partially leads to the generation of a streaming flow and the rest will be spent to break the hydrogen bonds. Therefore, the microgels collapse and become insoluble in the water and agglomerate, resulting in turbidity. We used turbidity to quantify the ultrasound energy absorption and showed that the acousto-response of PNIPAM microgels is a temporal phenomenon that depends on the duration of the actuation. Increasing the solution concentration leads to a faster hydrogen bond breakage and turbidity evolution. Furthermore, the frequency of imposed waves is important and affects the stimulation kinetics of PNIPAM microgels. Increasing the frequency of actuation increases the speed of hydrogen bond breakage and thus turbidity evolution. This is due to the increase in ultrasound energy absorption by liquids at higher frequencies

Frequency-Dependent Ultrasonic Stimulation of Poly(N-isopropylacrylamide) Microgels in Water

As a novel stimulus, we use high-frequency ultrasonic waves to provide the required energy for breaking hydrogen bonds between Poly(N-isopropylacrylamide) (PNIPAM) and water molecules while the solution temperature is maintained below the volume phase transition temperature (VPTT = 32 °C). Ultrasonic waves propagate through the solution and their energy will be absorbed due to the liquid viscosity. The absorbed energy partially leads to the generation of a streaming flow and the rest will be spent to break the hydrogen bonds. Therefore, the microgels collapse and become insoluble in water and agglomerate, resulting in solution turbidity. We use turbidity to quantify the ultrasound energy absorption and show that the acousto-response of PNIPAM microgels is a temporal phenomenon that depends on the duration of the actuation. Increasing the solution concentration leads to a faster turbidity evolution. Furthermore, an increase in ultrasound frequency leads to an increase in the breakage of more hydrogen bonds within a certain time and thus faster turbidity evolution. This is due to the increase in ultrasound energy absorption by liquids at higher frequencies

Ultrasound‐Induced Adsorption of Acousto‐Responsive Microgels at Water–Oil Interface

Abstract Ultrasonic mixing is a well‐established method to disperse and mix substances. However, the effects of ultrasound on dispersed soft particles as well as on their adsorption kinetics at interfaces remain unexplored. Ultrasound not only accelerates the movement of particles via acoustic streaming, but recent research indicates that it can also manipulate the interaction of soft particles with the surrounding liquid. In this study, it evaluates the adsorption kinetics of microgel at the water‐oil interface under the influence of ultrasound. It quantifies how acoustic streaming accelerates the reduction of interfacial tension. It uses high‐frequency and low‐amplitude ultrasound, which has no destructive effects. Furthermore, it discusses the ultrasound‐induced shrinking and thus interfacial rearrangement of the microgels, which plays a secondary but non‐negligible role on interfacial tension reduction. It shows that the decrease in interfacial tension due to the acoustic streaming is stronger for microgels with higher cross‐linker density. Moreover, it shows that ultrasound can induce a reversible decrease in interfacial tension due to the shrinkage of microgels at the interface. The presented results may lead to a better understanding in any field where ultrasonic waves meet soft particles, e.g., controlled destabilization in foams and emulsions or systems for drug release