Acoustic response and ambient pressure sensitivity characterization of SonoVue for noninvasive pressure estimation

Subharmonic aided pressure estimation (SHAPE) is a noninvasive pressure measurement technique based on the pressure dependent subharmonic signal from contrast microbubbles. Here, SonoVue microbubble with a sulfur hexafluoride (SF6) core, was investigated for use in SHAPE. The study uses excitations of 25-700 kPa peak negative pressure (PNP) and 3 MHz frequency over eight pressurization cycles between atmospheric pressure and overpressures, ranging from 0 to 25 kPa (0 to 186 mm Hg). The SonoVue subharmonic response was characterized into two types. Unlike other microbubbles, SonoVue showed significant subharmonic signals at low excitations (PNPs, 25-400 kPa), denoted here as type I subharmonic. It linearly decreased with increasing overpressure (-0.52 dB/kPa at 100 kPa PNP). However, over multiple pressurization-depressurization cycles, type I subharmonic changed; its value at atmospheric pressure decreased over multiple cycles, and at later cycles, it recorded an increase in amplitude with overpressure (highest, +13 dB at 50 kPa PNP and 10 kPa overpressure). The subharmonic at higher excitations (PNP > 400 kPa), denoted here as type II subharmonic, showed a consistent decrease with the ambient pressure increase with strongest sensitivity of -0.4 dB/kPa at 500 kPa PNP.

4D Printed Nerve Conduit with In Situ Neurogenic Guidance for Nerve Regeneration

Nerve repair poses a significant challenge in the field of tissue regeneration. As a bioengineered therapeutic method, nerve conduits have been developed to address damaged nerve repair. However, despite their remarkable potential, it is still challenging to encompass complex physiologically microenvironmental cues (both biophysical and biochemical factors) to synergistically regulate stem cell differentiation within the implanted nerve conduits, especially in a facile manner. In this study, a neurogenic nerve conduit with self-actuated ability has been developed by in situ immobilization of neurogenic factors onto printed architectures with aligned microgrooves. One objective was to facilitate self-entubulation, ultimately enhancing nerve repairs. Our results demonstrated that the integration of topographical and in situ biological cues could accurately mimic native microenvironments, leading to a significant improvement in neural alignment and enhanced neural differentiation within the conduit. This innovative approach offers a revolutionary method for fabricating multifunctional nerve conduits, capable of modulating neural regeneration efficiently. It has the potential to accelerate the functional recovery of injured neural tissues, providing a promising avenue for advancing nerve repair therapies.

Ambient Pressure Sensitivity of the Subharmonic Response of Coated Microbubbles: Effects of Acoustic Excitation Parameters

OBJECTIVE

The sensitivity of the acoustic response of microbubbles, specifically a strong correlation between their subharmonic response and the ambient pressure, has motivated development of a non-invasive subharmonic-aided pressure estimation (SHAPE) method. However, this correlation has previously been found to vary depending on the microbubble type, the acoustic excitation and the hydrostatic pressure range. In this study, the ambient pressure sensitivity of microbubble response was investigated.

METHODS

The fundamental, subharmonic, second harmonic and ultraharmonic responses from an in-house lipid-coated microbubble were measured for excitations with peak negative pressures (PNPs) of 50-700 kPa and frequencies of 2, 3 and 4 MHz in the ambient overpressure range 0-25 kPa (0-187 mmHg) in an in vitro setup.

RESULTS

The subharmonic response typically has three stages-occurrence, growth and saturation-with increasing excitation PNP. We find distinct decreasing and increasing variations of the subharmonic signal with overpressure that are closely related to the threshold of subharmonic generation in a lipid-shelled microbubble. Above the excitation threshold, that is, in the growth-saturation phase, subharmonic signals decreased linearly with slopes as high as -0.56 dB/kPa with ambient pressure increase; below the threshold excitation (at atmospheric pressure), increasing overpressure triggers subharmonic generation, indicating a lowering of subharmonic threshold, and therefore leads to an increase in subharmonic with overpressure, the maximum enhancement being ∼11 dB for 15 kPa overpressure at 2 MHz and 100 kPa PNP.

CONCLUSION

This study indicates the possible development of novel and improved SHAPE methodologies.

Optimizing the accuracy of viscoelastic characterization with AFM force-distance experiments in the time and frequency domains

Atomic Force Microscopy (AFM) force-distance (FD) experiments have emerged as an attractive alternative to traditional micro-rheology measurement techniques owing to their versatility of use in materials of a wide range of mechanical properties. Here, we show that the range of time dependent behaviour which can reliably be resolved from the typical method of FD inversion (fitting constitutive FD relations to FD data) is inherently restricted by the experimental parameters: sampling frequency, experiment length, and strain rate. Specifically, we demonstrate that violating these restrictions can result in errors in the values of the parameters of the complex modulus. In the case of complex materials, such as cells, whose behaviour is not specifically understood a priori, the physical sensibility of these parameters cannot be assessed and may lead to falsely attributing a physical phenomenon to an artifact of the violation of these restrictions. We use arguments from information theory to understand the nature of these inconsistencies as well as devise limits on the range of mechanical parameters which can be reliably obtained from FD experiments. The results further demonstrate that the nature of these restrictions depends on the domain (time or frequency) used in the inversion process, with the time domain being far more restrictive than the frequency domain. Finally, we demonstrate how to use these restrictions to better design FD experiments to target specific timescales of a material's behaviour through our analysis of a polydimethylsiloxane (PDMS) polymer sample.

Rayleigh limit extended: Scattering from a fluid sphere

The Reflections series takes a look back on historical articles from The Journal of the Acoustical Society of America that have had a significant impact on the science and practice of acoustics.

Material Properties, Dissolution and Time Evolution of PEGylated Lipid-Shelled Microbubbles: Effects of the Polyethylene Glycol Hydrophilic Chain Configurations

Polyethylene glycol (PEG) is often added to the lipid coating of a contrast microbubble to prevent coalescence and improve circulation. At high surface density, PEG chains are known to undergo a transition from a mushroom configuration to an extended brush configuration. We investigated the effects of PEG chain configuration on attenuation and dissolution of microbubbles by varying the molar ratio of the PEGylated lipid in the shell with three (0%, 2% and 5%) in the mushroom configuration and two (10% and 20%) in the brush configuration. We measured attenuation through the bubble suspensions and used it to obtain the characteristic rheological properties of their shells according to two interfacial rheological models. The interfacial elasticity was found to be significantly lower in the brush regime (∼0.6 N/m) than in the mushroom regime (∼1.3 N/m), but similar in value within each regime. The dissolution behavior of microbubbles under acoustic excitation inside an air-saturated medium was studied by measuring the time-dependent attenuation. Total attenuation recorded a transient increase because of growth resulting from air influx and an eventual decrease caused by dissolution. Microbubble shell composition with varying PEG concentrations had significant effects on dissolution dynamics.

Improved Sensitivity of Ultrasound-Based Subharmonic Aided Pressure Estimation Using Monodisperse Microbubbles

OBJECTIVES

Subharmonic aided pressure estimation (SHAPE) has been shown effective for noninvasively measuring hydrostatic fluid pressures in a variety of clinical applications. The objective of this study was to explore potential improvements in SHAPE sensitivity using monodisperse microbubbles.

METHODS

Populations of monodisperse microbubbles were created using a commercially available microfluidics device (Solstice Pharmaceuticals). Size distributions were assessed using a Coulter Counter and stability of the distribution following fabrication was evaluated over 24 hours. Attenuation of the microbubble populations from 1 to 10 MHz was then quantified using single element transducers to identify each formulation's resonance frequency. Frequency spectra over increasing driving amplitudes were investigated to determine the nonlinear phases of subharmonic signal generation. SHAPE sensitivity was evaluated in a hydrostatic pressure-controlled water bath using a Logiq E10 scanner (GE Healthcare).

RESULTS

Monodisperse lipid microbubble suspensions ranging from 2.4 to 5.3 μm in diameter were successfully created and they showed no discernable change in size distribution over 24 hours following activation. Calculated resonance frequencies ranged from 2.1 to 6.3 MHz and showed excellent correlation with microbubble diameter (R²  > 0.99). When investigating microbubble frequency response, subharmonic signal occurrence was shown to begin at 150 kPa peak negative pressure, grow up to 225 kPa, and saturate at approximately 250 kPa. Using the Logiq E10, monodisperse bubbles demonstrated a SHAPE sensitivity of -0.17 dB/mmHg, which was nearly twice the sensitivity of the commercial polydisperse microbubble currently being used in clinical trials.

CONCLUSIONS

Monodisperse microbubbles have the potential to greatly improve the sensitivity of SHAPE for the noninvasive measurement of hydrostatic pressures.

Modified Bovine Milk Exosomes for Doxorubicin Delivery to Triple-Negative Breast Cancer Cells

Biological nanoparticles, such as exosomes, offer an approach to drug delivery because of their innate ability to transport biomolecules. Exosomes are derived from cells and an integral component of cellular communication. However, the cellular cargo of human exosomes could negatively impact their use as a safe drug carrier. Additionally, exosomes have the intrinsic yet enigmatic, targeting characteristics of complex cellular communication. Hence, harnessing the natural transport abilities of exosomes for drug delivery requires predictably targeting these biological nanoparticles. This manuscript describes the use of two chemical modifications, incorporating a neuropilin receptor agonist peptide (iRGD) and a hypoxia-responsive lipid for targeting and release of an encapsulated drug from bovine milk exosomes to triple-negative breast cancer cells. Triple-negative breast cancer is a very aggressive and deadly form of malignancy with limited treatment options. Incorporation of both the iRGD peptide and hypoxia-responsive lipid into the lipid bilayer of bovine milk exosomes and encapsulation of the anticancer drug, doxorubicin, created the peptide targeted, hypoxia-responsive bovine milk exosomes, iDHRX. Initial studies confirmed the presence of iRGD peptide and the exosomes' ability to target the αvβ3 integrin, overexpressed on triple-negative breast cancer cells' surface. These modified exosomes were stable under normoxic conditions but fragmented in the reducing microenvironment created by 10 mM glutathione. In vitro cellular internalization studies in monolayer and three-dimensional (3D) spheroids of triple-negative breast cancer cells confirmed the cell-killing ability of iDHRX. Cell viability of 50% was reached at 10 μM iDHRX in the 3D spheroid models using four different triple-negative breast cancer cell lines. Overall, the tumor penetrating, hypoxia-responsive exosomes encapsulating doxorubicin would be effective in reducing triple-negative breast cancer cells' survival.

Acoustic Droplet Vaporization of Perfluorocarbon Droplets in 3D-Printable Gelatin Methacrylate Scaffolds

Scientists face a significant challenge in creating effective biomimetic constructs in tissue engineering with sustained and controlled delivery of growth factors. Recently, the addition of phase-shift droplets inside the scaffolds is being explored for temporal and spatial control of biologic delivery through vaporization using external ultrasound stimulation. Here, we explore acoustic droplet vaporization (ADV) in gelatin methacrylate (GelMA), a popular hydrogel used for tissue engineering applications because of its biocompatibility, tunable mechanical properties and rapid reproducibility. We embedded phase-shift perfluorocarbon droplets within the GelMA resin before crosslinking and characterized ADV and inertial cavitation (IC) thresholds of the embedded droplets. We were successful in vaporizing two different perfluorocarbon---perfluoropentane (PFP) and perfluorohexane (PFH)--cores at 2.25- and 5-MHz frequencies and inside hydrogels with varying mechanical properties. The ADV and IC thresholds for PFP droplets in GelMA scaffolds increased with frequency and in stiffer scaffolds. The PFH droplets exhibited ADV and IC activity only at 5 MHz for the range of excitations below 3MPa investigated here and at threshold values higher than those of PFP droplets. The results provide a proof of concept for the possible use of ADV in hydrogel scaffolds for tissue engineering.

Inhibition of Human Breast Cancer Cell Proliferation by Low-Intensity Ultrasound Stimulation

OBJECTIVES

Cancer is characterized by uncontrolled cell proliferation, which makes novel therapies highly desired. In this study, the effects of near-field low-intensity pulsed ultrasound (LIPUS) stimulation on T47D human breast cancer cell and healthy immortalized MCF-12A breast epithelial cell proliferation were investigated in monolayer cultures.

METHODS

A customized ultrasound (US) exposure setup was used for the variation of key US parameters: intensity, excitation duration, and duty cycle. Cell proliferation was quantified by 5-bromo-2'-deoxyuridine and alamarBlue assays after LIPUS excitation.

RESULTS

At a 20% duty cycle and 10-minute excitation period, we varied LIPUS intensity from to 100 mW/cm² (spatial-average temporal-average) to find a gradual decrease in T47D cell proliferation, the decrease being strongest at 100 mW/cm² . In contrast, healthy MCF-12A breast cells showed an increase in proliferation when exposed to the same conditions. Above a 60% duty cycle, T47D cell proliferation decreased drastically. Effects of continuous wave US stimulation were further explored by varying the intensity and excitation period.

CONCLUSIONS

These experiments concluded that, irrespective of the waveform (pulsed or continuous), LIPUS stimulation could inhibit the proliferation of T47D breast cancer cells, whereas the same behavior was not observed in healthy cells. The study demonstrates the beneficial bioeffects of LIPUS on breast cancer cells and offers the possibility of developing novel US-mediated cancer therapy.

Chlorophyll(a)/Carbon Quantum Dot Bio-Nanocomposite Activated Nano-Structured Silicon as an Efficient Photocathode for Photoelectrochemical Water Splitting

Solar-driven water splitting is considered as a futuristic sustainable way to generate hydrogen and chemical storage of solar energy. Further, considering the technological competence, silicon is one of the potential materials for developing large-scale and cost-effective photocathodes (PCs), but it lacks efficacy and stability. Here, we show that chlorophyll(a)/carbon quantum dots (Chl/CQDs) bio-nanocomposite (b-NC)-decorated Si-nanowires (SiNWs) as PC can surpass the reported efficiency for photoelectrochemical (PEC) hydrogen generation along with stability. The optimized heterojunction (Chl/CQDs_SiNW) significantly enhances broad-band solar absorption and protects Si surface from corrosion. Further, the appropriate band alignment enforces efficient photogenerated charge separation and possesses directional exciton transport property via the Förster resonance energy transfer (FRET) mechanism. This synergic effect demonstrates an ∼18 times increase in photocurrent density (26.36 mA/cm²) compared to pristine SiNW PC at 1.07 V vs reversible hydrogen electrode (RHE). The efficiency reaches ∼7.86%, which is comparably the highest reported for hybrid Si-based photocathodes. Hydrogen evaluation rate was measured to be ∼113 μmol/h at 0.8 V vs RHE under 1 sun illumination. With Si-process line compatibility, this new finding opens a new direction toward the development of Si-based efficient and stable PCs at a large scale for commercial applications.

Echogenic Exosomes as ultrasound contrast agents

Exosomes are naturally secreted extracellular bilayer vesicles (diameter 40-130 nm), which have recently been found to play a critical role in cell-to-cell communication and biomolecule delivery. Their unique characteristics-stability, permeability, biocompatibility and low immunogenicity-have made them a prime candidate for use in delivering cancer therapeutics and other natural products. Here we present the first ever report of echogenic exosomes, which combine the benefits of the acoustic responsiveness of traditional microbubbles with the non-immunogenic and small-size morphology of exosomes. Microbubbles, although effective as ultrasound contrast agents, are restricted to intravascular usage due to their large size. In the current study, we have rendered bovine milk-derived exosomes echogenic by freeze drying them in the presence of mannitol. Ultrasound imaging and direct measurement of linear and nonlinear scattered responses were used to investigate the echogenicity and stability of the prepared exosomes. A commercial scanner registered enhancement (28.9% at 40 MHz) in the brightness of ultrasound images in presence of echogenic exosomes at 5 mg/mL. The exosomes also showed significant linear and nonlinear scattered responses-11 dB enhancement in fundamental, 8.5 dB in subharmonic and 3.5 dB in second harmonic all at 40 μg/mL concentration. Echogenic exosomes injected into the tail vein of mice and the synovial fluid of rats resulted in significantly higher brightness-as much as 300%-of the ultrasound images, showing their promise in a variety of in vivo applications. The echogenic exosomes, with their large-scale extractability from bovine milk, lack of toxicity and minimal immunogenic response, successfully served as ultrasound contrast agents in this study and offer an exciting possibility to act as an effective ultrasound responsive drug delivery system.

Surface Plasmon-Enhanced Carbon Dot-Embellished Multifaceted Si(111) Nanoheterostructure for Photoelectrochemical Water Splitting

Because of the excellent electronic properties, Si is a well-established semiconducting material for PV technology. However, slow kinetics and a fast corroding nature make Si inefficient for the hydrogen evolution reaction (HER) in photoelectrochemical (PEC) applications. Herein, we demonstrate a multifacet Si nanowire (SiNW) decorated with surface plasmon-enhanced carbon quantum dots (AuCQDs) as efficient, stable, economical, and scalable photocathodes (PCs) for HER. The PEC performance of SiNW_AuCQDs has more than a fourfold efficiency enhancement than the pristine SiNW, which we have attributed to the combined effect of enhanced solar absorption and efficient carrier transport. The optimized PC SiNW_AuCQDs results in the highest photocurrent ∼1.7 mA/cm², an applied bias photon-to-current conversion efficiency of ∼0.8%, and H₂ gas evolution rate of ∼182.93 μmol·h^-1. Furthermore, these SiNW_AuCQDs PCs provide extraordinary stability under continuous operating conditions with 1 sun illumination (100 mW/cm²). The process-line compatible fabrication process of these PCs will open a new direction at the wafer-level designing of a heterostructure for large-scale solar-fuel conversion.

Mechanisms of spontaneous chain formation and subsequent microstructural evolution in shear-driven strongly confined drop monolayers

It was experimentally demonstrated by Migler and his collaborators [Phys. Rev. Lett., 2001, 86, 1023; Langmuir, 2003, 19, 8667] that a strongly confined drop monolayer sheared between two parallel plates can spontaneously develop a flow-oriented drop-chain morphology. Here we show that the formation of the chain-like microstructure is driven by far-field Hele-Shaw quadrupolar interactions between drops, and that drop spacing within chains is controlled by the effective drop repulsion associated with the existence of confinement-induced reversing streamlines, i.e., the swapping trajectory effect. Using direct numerical simulations and an accurate quasi-2D model that incorporates quadrupolar and swapping-trajectory contributions, we analyze microstructural evolution in a monodisperse drop monolayer. Consistent with experimental observations, we find that drop spacing within individual chains is usually uniform. Further analysis shows that at low area fractions all chains have the same spacing, but at higher area fractions there is a large spacing variation from chain to chain. These findings are explained in terms of uncompressed and compressed chains. At low area fractions most chains are uncompressed (spacing equals lst, which is the stable separation of an isolated pair). At higher area fractions compressed chains (with tighter spacing) are formed in a process of chain zipping along y-shaped structural defects. We also discuss the relevance of our findings to other shear-driven systems, such as suspensions of spheres in non-Newtonian fluids.

Exosomes as Drug Carriers for Cancer Therapy

Exosomes, biological extracellular vesicles, have recently begun to find use in targeted drug delivery in solid tumor research. Ranging from 30-120 nm in size, exosomes are secreted from cells and isolated from bodily fluids. Exosomes provide a unique material platform due to their characteristics, including physical properties such as stability, biocompatibility, permeability, low toxicity, and low immunogenicity-all critical to the success of any nanoparticle drug delivery system. In addition to traditional chemotherapeutics, natural products and RNA have been encapsulated for the treatment of breast, pancreatic, lung, prostate cancers, and glioblastoma. This review discusses current research on exosomes for drug delivery to solid tumors.

Effects of droplet size and perfluorocarbon boiling point on the frequency dependence of acoustic vaporization threshold

Phase shift liquid perfluorocarbon (PFC) droplets vaporizable by ultrasound into echogenic microbubble above a threshold pressure, termed acoustic droplet vaporization (ADV), are used for therapeutic and diagnostic applications. This study systematically investigated the effect of excitation frequency (2.25, 10, and 15 MHz) on the ADV and inertial cavitation (IC) thresholds of lipid-coated PFC droplets of three different liquid cores-perfluoropentane (PFP), perfluorohexane (PFH), and perfluorooctyl bromide (PFOB)-and of two different sizes-average diameters smaller than 3 μm and larger than 10 μm-in a tubeless setup. This study found that the ADV threshold increases with frequency for the lowest boiling point liquid, PFP, for both large and small size droplets. For higher boiling point liquids, PFH and PFOB, this study did not detect vaporization for small size droplets at the excitation levels (maximum 4 MPa peak negative) studied here. The large PFOB droplets experienced ADV only at the highest excitation frequency 15 MHz. For large PFH droplets, ADV threshold decreases with frequency that could possibly be due to the superharmonic focusing being a significant effect at larger sizes and the higher excitation pressures. ADV thresholds at all the frequencies studied here occurred at lower rarefactional pressures than IC thresholds indicating that phase transition precedes inertial cavitation.

Enhanced Osteogenic Differentiation of Human Mesenchymal Stem Cells Using Microbubbles and Low Intensity Pulsed Ultrasound on 3D Printed Scaffolds

Lipid-coated microbubbles, clinically approved as contrast enhancing agents for ultrasound imaging, are investigated for the first time for their possible applications in bone tissue engineering. Effects of microbubbles (average diameter 1.1 µm) coated by a mixture of lipids (1,2-dipalmitoyl-sn-glycero-3-phosphocholine, 1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine-N-[methoxy(polyethylene glycol)-2000], and 1,2-dipalmitoyl-3-trimethylmmonium-propane) in the presence of low intensity pulsed ultrasound (LIPUS) on human mesenchymal stem cells seeded on 3D printed poly(lactic acid) porous scaffolds are investigated. LIPUS stimulation (30 mW cm^-2 , 1.5 MHz, 20% duty cycle) for 3 min a day with 0.5% v/v microbubbles results in a significant increase in proliferation (up to 19.3%) when compared to control after 1, 3, and 5 d. A 3-week osteogenic differentiation study shows a significant increase in total protein content (up to 27.5%), calcium deposition (up to 4.3%), and alkaline phosphatase activity (up to 43.1%) initiated by LIPUS with and without the presence of microbubbles. The microbubbles are found to remain stable during exposure, and their sustained oscillations demonstrably help focus the LIPUS energy toward enhanced cellular response. Integrating LIPUS and microbubbles promises to be a novel and effective strategy for bone tissue engineering and regeneration therapies.

Nucleus-Targeted, Echogenic Polymersomes for Delivering a Cancer Stemness Inhibitor to Pancreatic Cancer Cells

Chemotherapeutic agents for treating cancers show considerable side effects, toxicity, and drug resistance. To mitigate the problems, we designed nucleus-targeted, echogenic, stimuli-responsive polymeric vesicles (polymersomes) to transport and subsequently release the encapsulated anticancer drugs within the nuclei of pancreatic cancer cells. We synthesized an alkyne-dexamethasone derivative and conjugated it to N₃-polyethylene glycol (PEG)-polylactic acid (PLA) copolymer employing the Cu²⁺ catalyzed "Click" reaction. We prepared polymersomes from the dexamethasone-PEG-PLA conjugate along with a synthesized stimuli-responsive polymer PEG-S-S-PLA. The dexamethasone group dilates the nuclear pore complexes and transports the vesicles to the nuclei. We designed the polymersomes to release the encapsulated drugs in the presence of a high concentration of reducing agents in the nuclei of pancreatic cancer cells. We observed that the nucleus-targeted, stimuli-responsive polymersomes released 70% of encapsulated contents in the nucleus-mimicking environment in 80 min. We encapsulated the cancer stemness inhibitor BBI608 in the vesicles and observed that the BBI608 encapsulated polymersomes reduced the viability of the BxPC3 cells to 43% in three-dimensional spheroid cultures. The polymersomes were prepared following a special protocol so that they scatter ultrasound, allowing imaging by a medical ultrasound scanner. Therefore, these echogenic, targeted, stimuli-responsive, and drug-encapsulated polymersomes have the potential for trackable, targeted carrier of chemotherapeutic drugs to cancer cell nuclei.

Stereolithographic 4D Bioprinting of Multiresponsive Architectures for Neural Engineering

4D printing represents one of the most advanced fabrication techniques for prospective applications in tissue engineering, biomedical devices, and soft robotics, among others. In this study, a novel multiresponsive architecture is developed through stereolithography-based 4D printing, where a universal concept of stress-induced shape transformation is applied to achieve the 4D reprogramming. The light-induced graded internal stress followed by a subsequent solvent-induced relaxation, driving an autonomous and reversible change of the programmed configuration after printing, is employed and investigated in depth and details. Moreover, the fabricated construct possesses shape memory property, offering a characteristic of multiple shape change. Using this novel multiple responsive 4D technique, a proof-of-concept smart nerve guidance conduit is demonstrated on a graphene hybrid 4D construct providing outstanding multifunctional characteristics for nerve regeneration including physical guidance, chemical cues, dynamic self-entubulation, and seamless integration. By employing this fabrication technique, creating multiresponsive smart architectures, as well as demonstrating application potential, this work paves the way for truly initiation of 4D printing in various high-value research fields.

Tissue-Penetrating, Hypoxia-Responsive Echogenic Polymersomes For Drug Delivery To Solid Tumors

Hypoxia in solid tumors facilitates the progression of the disease, develops resistance to chemo and radiotherapy, and contributes to relapse. Due to the lack of tumor penetration, most of the reported drug carriers are unable to reach the hypoxic niches of the solid tumors. We have developed tissue-penetrating, hypoxia-responsive echogenic polymersomes to deliver anticancer drugs to solid tumors. The polymersomes are composed of a hypoxia-responsive azobenzene conjugated and a tissue penetrating peptide functionalized polylactic acid-polyethylene glycol polymer. The drug-encapsulated, hypoxia-responsive polymersomes substantially decreased the viability of pancreatic cancer cells in spheroidal cultures. Under normoxic conditions, polymersomes were echogenic at diagnostic ultrasound frequencies but lose the echogenicity under hypoxia. In-vivo imaging studies with xenograft mouse model further confirmed the ability of the polymersomes to target, penetrate, and deliver the encapsulated contents in hypoxic pancreatic tumor tissues.

Acoustic vaporization threshold of lipid-coated perfluoropentane droplets

Phase shift droplets vaporizable by acoustic stimulation offer the advantages of producing microbubbles as contrast agents in situ as well as higher stability and the possibility of achieving smaller sizes. Here, the acoustic droplet vaporization (ADV) threshold of a suspension of droplets with a perfluoropentane (PFP) core (diameter 400-3000 nm) is acoustically measured as a function of the excitation frequency in a tubeless setup at room temperature. The changes in scattered responses-fundamental, sub-, and second harmonic-are investigated, a quantitative criterion is used to determine the ADV phenomenon, and findings are discussed. The average threshold obtained using three different scattered components increases with frequency-1.05 ± 0.28 MPa at 2.25 MHz, 1.89 ± 0.57 MPa at 5 MHz, and 2.34 ± 0.014 MPa at 10 MHz. The scattered response from vaporized droplets was also found to qualitatively match with that from an independently prepared lipid-coated microbubble suspension in magnitude as well as trends above the determined ADV threshold value.

Acoustic Characterization of Echogenic Polymersomes Prepared From Amphiphilic Block Copolymers

Polymersomes are a class of artificial vesicles prepared from amphiphilic polymers. Like lipid vesicles (liposomes), they too can encapsulate hydrophilic and hydrophobic drug molecules in the aqueous core and the hydrophobic bilayer respectively, but are more stable than liposomes. Although echogenic liposomes have been widely investigated for simultaneous ultrasound imaging and controlled drug delivery, the potential of the polymersomes remains unexplored. We prepared two different echogenic polymersomes from the amphiphilic copolymers polyethylene glycol-poly-DL-lactic acid (PEG-PLA) and polyethylene glycol-poly-L-lactic acid (PEG-PLLA), incorporating multiple freeze-dry cycles in the synthesis protocol to ensure their echogenicity. We investigated acoustic behavior with potential applications in biomedical imaging. We characterized the polymeric vesicles acoustically with three different excitation frequencies of 2.25, 5 and 10 MHz at 500 kPa. The polymersomes exhibited strong echogenicity at all three excitation frequencies (about 50- and 25-dB enhancements in fundamental and subharmonic, respectively, at 5-MHz excitation from 20 µg/mL polymers in solution). Unlike echogenic liposomes, they emitted strong subharmonic responses. The scattering results indicated their potential as contrast agents, which was also confirmed by clinical ultrasound imaging.

4D printing of polymeric materials for tissue and organ regeneration

Four dimensional (4D) printing is an emerging technology with great capacity for fabricating complex, stimuli-responsive 3D structures, providing great potential for tissue and organ engineering applications. Although the 4D concept was first highlighted in 2013, extensive research has rapidly developed, along with more-in-depth understanding and assertions regarding the definition of 4D. In this review, we begin by establishing the criteria of 4D printing, followed by an extensive summary of state-of-the-art technological advances in the field. Both transformation-preprogrammed 4D printing and 4D printing of shape memory polymers are intensively surveyed. Afterwards we will explore and discuss the applications of 4D printing in tissue and organ regeneration, such as developing synthetic tissues and implantable scaffolds, as well as future perspectives and conclusions.

Role of freeze-drying in the presence of mannitol on the echogenicity of echogenic liposomes

Echogenic liposomes (ELIPs) are an excellent candidate for ultrasound activated therapeutics and imaging. Although multiple experiments have established their echogenicity, the underlying mechanism has remained unknown. However, freeze-drying in the presence of mannitol during ELIP preparation has proved critical to ensuring echogenicity. Here, the role of this key component in the preparation protocol was investigated by measuring scattering from freshly prepared freeze-dried aqueous solution of mannitol-and a number of other excipients commonly used in lyophilization-directly dispersed in water without any lipids in the experiment. Mannitol, meso-erythritol, glycine, and glucose that form a highly porous crystalline phase upon freeze-drying generated bubbles resulting in strong echoes during their dissolution. On the other hand, sucrose, trehalose, and xylitol, which become glassy while freeze-dried, did not. Freeze-dried mannitol and other crystalline substances, if thawed before being introduced into the scattering volume, did not produce echogenicity, as they lost their crystallinity in the thawed state. The echogenicity disappeared in a degassed environment. Higher amounts of sugar in the original aqueous solution before freeze-drying resulted in higher echogenicity because of the stronger supersaturation and crystallinity. The bubbles created by the freeze-dried mannitol in the ELIP formulation play a critical role in making ELIPs echogenic.

Single In x Ga1-x As nanowire/p-Si heterojunction based nano-rectifier diode

Nanoscale power supply units will be indispensable for fabricating next generation smart nanoelectronic integrated circuits. Fabrication of nanoscale rectifier circuits on a Si platform is required for integrating nanoelectronic devices with on-chip power supply units. In the present study, a nanorectifier diode based on a single standalone In _x Ga_1-x As nanowire/p-Si (111) heterojunction fabricated by metal organic chemical vapor deposition technique has been studied. The nanoheterojunction diodes have shown good rectification and fast switching characteristics. The rectification characteristics of the nanoheterojunction have been demonstrated by different standard waveforms of sinusoidal, square, sawtooth and triangular for two different frequencies of 1 and 0.1 Hz. Reverse recovery time of around 150 ms has been observed in all wave response. A half wave rectifier circuit with a simple capacitor filter has been assembled with this nanoheterojunction diode which provides 12% output efficiency. The transport of carriers through the heterojunction is investigated. The interface states density of the nanoheterojunction has also been determined. Occurrence of output waveforms incommensurate with the input is attributed to higher series resistance of the diode which is further explained considering the dimension of p-side and n-side of the junction. The sudden change of ideality factor after 1.7 V bias is attributed to recombination through interface states in space charge region. Low interface states density as well as high rectification ratio makes this heterojunction diode a promising candidate for future nanoscale electronics.

Lipid Coated Microbubbles and Low Intensity Pulsed Ultrasound Enhance Chondrogenesis of Human Mesenchymal Stem Cells in 3D Printed Scaffolds

Lipid-coated microbubbles are used to enhance ultrasound imaging and drug delivery. Here we apply these microbubbles along with low intensity pulsed ultrasound (LIPUS) for the first time to enhance proliferation and chondrogenic differentiation of human mesenchymal stem cells (hMSCs) in a 3D printed poly-(ethylene glycol)-diacrylate (PEG-DA) hydrogel scaffold. The hMSC proliferation increased up to 40% after 5 days of culture in the presence of 0.5% (v/v) microbubbles and LIPUS in contrast to 18% with LIPUS alone. We systematically varied the acoustic excitation parameters-excitation intensity, frequency and duty cycle-to find 30 mW/cm2, 1.5 MHz and 20% duty cycle to be optimal for hMSC proliferation. A 3-week chondrogenic differentiation results demonstrated that combining LIPUS with microbubbles enhanced glycosaminoglycan (GAG) production by 17% (5% with LIPUS alone), and type II collagen production by 78% (44% by LIPUS alone). Therefore, integrating LIPUS and microbubbles appears to be a promising strategy for enhanced hMSC growth and chondrogenic differentiation, which are critical components for cartilage regeneration. The results offer possibilities of novel applications of microbubbles, already clinically approved for contrast enhanced ultrasound imaging, in tissue engineering.

Improved Human Bone Marrow Mesenchymal Stem Cell Osteogenesis in 3D Bioprinted Tissue Scaffolds with Low Intensity Pulsed Ultrasound Stimulation

3D printing and ultrasound techniques are showing great promise in the evolution of human musculoskeletal tissue repair and regeneration medicine. The uniqueness of the present study was to combine low intensity pulsed ultrasound (LIPUS) and advanced 3D printing techniques to synergistically improve growth and osteogenic differentiation of human mesenchymal stem cells (MSC). Specifically, polyethylene glycol diacrylate bioinks containing cell adhesive Arginine-Glycine-Aspartic acid-Serene (RGDS) peptide and/or nanocrystalline hydroxyapatite (nHA) were used to fabricate 3D scaffolds with different geometric patterns via novel table-top stereolithography 3D printer. The resultant scaffolds provide a highly porous and interconnected 3D environment to support cell proliferation. Scaffolds with small square pores were determined to be the optimal geometric pattern for MSC attachment and growth. The optimal LIPUS working parameters were determined to be 1.5 MHz, 20% duty cycle with 150 mW/cm(2) intensity. Results demonstrated that RGDS peptide and nHA containing 3D printed scaffolds under LIPUS treatment can greatly promote MSC proliferation, alkaline phosphatase activity, calcium deposition and total protein content. These results illustrate the effectiveness of the combination of LIPUS and biomimetic 3D printing scaffolds as a valuable combinatorial tool for improved MSC function, thus make them promising for future clinical and various regenerative medicine application.

Interfacial Rheological Properties of Contrast Microbubble Targestar P as a Function of Ambient Pressure

In this Technical Note, we determine the interfacial rheological parameters of the encapsulation of the contrast agent Targestar P using ultrasound attenuation. The characteristic parameters are obtained according to two interfacial rheological models. The properties-surface dilatational elasticity (0.09 ± 0.01 N/m) and surface dilatational viscosity (8 ± 0.1E-9 N·s/m)-are found to be of similar magnitude for both models. Contrast microbubbles experience different ambient pressure in different organs. We also measure these parameters as functions of ambient pressure using attenuation measured at different overpressures (0, 100 and 200 mm Hg). For each value of ambient hydrostatic pressure, we determine the rheological properties, accounting for changes in the size distribution caused by the pressure change. We discuss different models of size distribution change under overpressure: pure adiabatic compression or gas exchange with surrounding medium. The dilatational surface elasticity and viscosity are found to increase with increasing ambient pressure.

Hydrodynamic interactions between pairs of capsules and drops in a simple shear: Effects of viscosity ratio and heterogeneous collision

Hydrodynamic interactions between a pair of capsules in simple shear are numerically investigated using a front-tracking finite difference method. The membrane of the capsule is modeled using different hyperelastic constitutive relations. We also compare the pair interactions between drops to those between capsules. An increased viscosity ratio leads to a reduced net cross-stream separation between capsules as well as drops after collision. At low viscosity ratios, for the same capillary number drop-pairs show higher cross-stream separation than those for capsule-pairs, while substantially large viscosity ratios result in almost the same value for both cases. We investigate pair-collisions between two heterogeneous capsules C(1) and C(2) with two different capillary numbers. The maximum deformation of C(1) was seen to increase with increasing stiffness (decreasing capillary number) of C(2), even though the stiffness of C(1) was kept fixed. The findings are similar for a drop-pair, however, with a smaller maximum deformation for the same combinations of capillary numbers. The final cross-stream drift of the trajectory of C(1) decreases with the increasing stiffness of C(2), but the relative trajectory between the capsules remains unchanged. The maximum deformation and the cross-stream drift of the trajectory of C(1) are shown to approximately vary with power-law functions of the ratio of the capillary numbers of C(1) and C(2). An analytical explanation of the dependence on the two capillary numbers is offered. Different membrane constitutive laws result in similar deformation and drift in trajectory.

Interpreting attenuation at different excitation amplitudes to estimate strain-dependent interfacial rheological properties of lipid-coated monodisperse microbubbles

Broadband attenuation of ultrasound measured at different excitation pressures being different raises a serious theoretical concern, because the underlying assumption of linear and independent propagation of different frequency components nominally requires attenuation to be independent of excitation. Here, this issue is investigated by examining ultrasound attenuation through a monodisperse lipid-coated microbubble suspension measured at four different acoustic excitation amplitudes. The attenuation data are used to determine interfacial rheological properties (surface tension, surface dilatational elasticity, and surface dilatational viscosity) of the encapsulation according to three different models. Although different models result in similar rheological properties, attenuation measured at different excitation levels (4-110 kPa) leads to different values for them; the dilatation elasticity (0.56 to 0.18 N/m) and viscosity (2.4 × 10(-8) to 1.52 × 10(-8) Ns/m) both decrease with increasing pressure. Numerically simulating the scattered response, nonlinear energy transfer between frequencies are shown to be negligible, thereby demonstrating the linearity in propagation and validating the attenuation analysis. There is a second concern to the characterization arising from shell properties being dependent on excitation amplitude, which is not a proper constitutive variable. It is resolved by arriving at a strain-dependent rheology for the encapsulation. The limitations of the underlying analysis are discussed.

Effects of ambient hydrostatic pressure on the material properties of the encapsulation of an ultrasound contrast microbubble

Ultrasound contrast microbubbles experience widely varying ambient blood pressure in different organs, which can also change due to diseases. Pressure change can alter the material properties of the encapsulation of these microbubbles. Here the characteristic rheological parameters of contrast agent Definity are determined by varying the ambient pressure (in a physiologically relevant range 0-200 mm Hg). Four different interfacial rheological models are used to characterize the microbubbles. Effects of gas diffusion under excess ambient pressure are investigated in detail accounting for size decrease of contrast microbubbles. Definity contrast agent show a change in their interfacial dilatational viscosity (3.6 × 10(-8) Ns/m at 0 mm Hg to 4.45 × 10(-8) Ns/m at 200 mm Hg) and interfacial dilatational elasticity (0.86 N/m at 0 mm Hg to 1.06 N/m at 200 mm Hg) with ambient pressure increase. The increase results from material consolidation, similar to such enhancement in bulk properties under pressure. The model that accounts for enhancement in material properties with increasing ambient pressure matches with experimentally measured subharmonic response as a function of ambient pressure, while assuming constant material parameters does not.

Synthesis of graphene oxide–silver nanocomposite with photochemically grown silver nanoparticles to use as a channel material in thin film field effect transistors

The transport of charge carriers in a graphene oxide–silver nanoparticle composite is found to be controlled by nanoparticle dose.

Leakage current characteristics in MOCVD grown InAs quantum dot embedded GaAs metal-oxide-semiconductor capacitor

Carrier transport vis-a-vis leakage current in GaAs MOS capacitors with various structures; quantum dot embedded devices show the lowest leakage.

pH-triggered echogenicity and contents release from liposomes

Liposomes are representative lipid nanoparticles widely used for delivering anticancer drugs, DNA fragments, or siRNA to cancer cells. Upon targeting, various internal and external triggers have been used to increase the rate for contents release from the liposomes. Among the internal triggers, decreased pH within the cellular lysosomes has been successfully used to enhance the rate for releasing contents. However, imparting pH sensitivity to liposomes requires the synthesis of specialized lipids with structures that are substantially modified at a reduced pH. Herein, we report an alternative strategy to render liposomes pH sensitive by encapsulating a precursor which generates gas bubbles in situ in response to acidic pH. The disturbance created by the escaping gas bubbles leads to the rapid release of the encapsulated contents from the liposomes. Atomic force microscopic studies indicate that the liposomal structure is destroyed at a reduced pH. The gas bubbles also render the liposomes echogenic, allowing ultrasound imaging. To demonstrate the applicability of this strategy, we have successfully targeted doxorubicin-encapsulated liposomes to the pancreatic ductal carcinoma cells that overexpress the folate receptor on the surface. In response to the decreased pH in the lysosomes, the encapsulated anticancer drug is efficiently released. Contents released from these liposomes are further enhanced by the application of continuous wave ultrasound (1 MHz), resulting in substantially reduced viability for the pancreatic cancer cells (14%).