Lang Xia

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.

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.

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.

Acoustic characterization of polymer-encapsulated microbubbles with different shell-thickness-to-radius ratios using in vitro attenuation and scattering: Comparison between different rheological models

Acoustic behaviors of five different polymer (polylactide) encapsulated microbubbles—PB127 (Point Biomedical), PH37, PH43, PH44, PH45 (Phillips Healthcare) with different shell-thickness-to-radius ratios (STRR) of 3.5, 30, 40, 65, and 100 nm/μm have been characterized. In vitro attenuation data were used to determine the interfacial rheological properties of their shells. Use of different models—Newtonian, viscoelastic, strain-softening, Marmottant, and Church—resulted in similar rheological properties. The shell elasticity and shell viscosity were found to increase with increasing shell thickness as expected. The nonlinear scattered response from these microbubbles were measured. Experimentally measured scattered subharmonic response were compared with the model predictions.

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.

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.

A new expression for the second-order acoustical nonlinearity parameter B/A for a suspension of free or encapsulated bubbles

The presence of bubbles in the liquid introduces dispersion, increased attenuation and nonlinearities in the medium (bubbly liquid). The second-order acoustical parameter B/A describes the nonlinearity of the bubbly liquid. The nonlinear parameter B/A is an important property for characterizing different media and materials, and also important for ultrasound imaging. It can be derived from the equation of state of a fluid using the Taylor expansion to the second order. B/A has been studied extensively in industrial, chemical and biological fluids since Beyer proposed this thermodynamic technique. However, certain important aspect for this criterion remains unexplored specifically for bubbly liquid. Here, we develop a new formula based on the thermodynamic method which correlates both attenuation and phase velocity of ultrasound waves in liquids containing free or encapsulated microbubbles. These quantities can be measured directly using a broadband technique. The formula offered here is relatively simple ...

In vitro acoustic characterization of echogenic polymersomes with PLA-PEG and PLLA-PEG shells

Echogenic liposomes (ELIPs), lipid bilayer-coated vesicles, have been widely studied as an acoustically triggerable drug delivery agent or an ultrasound contrast agent. Instead of liposomes, polymersomes, amphiphilic vesicles, offer additional stability and chemical flexibility. Here, we report the acoustic behaviors of echogenic polymersomes made of block copolymers PLA-PEG and PLLA-PEG, which are stereo-isomers. Polymersomes were excited with three different frequencies, 2.25 MHz, 5 MHz and 10 MH, and their scattered responses were measured. Both PLA-PEG and PLLA-PEG shell polymersomes produce strong acoustic responses as high as 50 dB in the fundamental component, thus demonstrating their potential as contrast agents. Significant subharmonic as well as second harmonic responses were observed at excitation frequencies of 2.25 MHz and 5 MHz. The gas dissolved in the suspension was found to be essential for the echogenicity of polymersomes.

Interpreting attenuation at different excitation amplitudes to estimate strain-dependent interfacial rheological properties of monodisperse contrast 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. This issue is investigated by examining ultrasound attenuation through a monodisperse lipid coated microbubble suspension measured at four different acoustic excitation amplitudes. We use the attenuation data 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 N/m to 0.18 N/m) and viscosity (2.4×10−8 Ns/m to 1.52×10−8 Ns/m) both decrease with increasing pressure. Numerically...