Gang-Yu Liu

Knowledge of AFM tip and lipid bilayer stack interactions is essential for designing and developing nano- to mesoscale lipid constructs. In this study, a model was developed to reveal the complex molecular mechanism governing tip–lipid interactions. This complexity in the interaction forces during penetration leading to a disturbance of lipid bilayers is explored using coarse-grained MARTINI molecular dynamics simulations for a multibilayer 1-palmitoyl-2-oleoylphosphatidylcholine (POPC) lipid system and an AFM tip apex. Force–distance profiles acquired during AFM tip penetration displayed a sawtooth pattern with a periodicity consistent with the bilayer thickness. This computation provides mechanistic insights into this process: a layer-by-layer penetration with distinct features in force–distance traces to specific molecular events such as climbing, adhesion, and compression. The outcomes benefit development of engineered lipid-based materials used in drug and vaccine delivery and fabrication of lipid-based nanodevices and biosensors.

The concept and feasibility of producing liposomes by rehydrating engineered lipid nanoconstructs are demonstrated in this study. Nanoconstructs of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) were produced using a microfluidic delivery probe integrated with an atomic force microscope. The subsequent rehydration of these POPC constructs led to the formation of liposomes, most of which remained adhered to the surface. The size (e.g., diameter) of the liposomes could be tuned by varying the lateral dimension of the lipid constructs. Hierarchical liposomal structures, such as pentagons containing five liposomes at the corners, could also be designed and produced by depositing lipid constructs to designated locations on the surfaces, followed by rehydration. This new means allows for regulating liposomal sizes, distributions, and compositions. The outcomes benefit applications of liposomes as delivery vehicles, sensors, and building blocks in biomaterials design. The ability to produce hierarchical liposomal structures benefits numerous applications such as proto-cell development, multiplexed bio-composite materials, and the engineering of local bio-environments.

Whereas single crystals of organic compounds that respond to heat or light have been reported and studied in detail, studies on crystalline organic compounds that elicit an extreme mechanical response upon cooling to very low temperatures are relatively rare in the chemical literature. A tetrafluoro(aryl)sulfanylated bicyclopentane synthesized in our laboratory was discovered to exhibit such behavior; i.e., the crystals jumped and forcefully disintegrated upon cooling below ∼193 K. Accordingly, the origin of this low-temperature thermosalient effect was investigated through NMR, SC-XRD, PXRD, microscopy, DSC, Raman, and Brillouin experiments. To our surprise, NMR, SC-XRD, PXRD, and DSC experiments suggest the phenomenon can neither be attributed solely to a chemical transformation nor a phase transition of the entire material. Rather, XRD, Raman, and Brillouin experiments provide evidence that built-up strain released from the crystal upon self-destruction may be associated with crystal microstructure or a phase transition that occurs in another material (i.e., an impurity) in the crystal. This study demonstrates that molecular structural changes in organic material microstructure or impurity phases (which may not necessarily be visible by X-ray diffraction) can have a significant impact on the behavior of the bulk crystalline material. Thus, the role of microstructure may be considered more heavily in future mechanistic studies on mechanically responsive crystals.

Alginate hydrogels are frequently used in 3D bioprinting and tissue repair and regeneration. Establishing the structure-property-performance correlation of these materials would benefit significantly from high-resolution structural characterization in aqueous environments from the molecular level to continuum. This study overcomes technical challenges and enables high-resolution atomic force microscopy (AFM) imaging of hydrated alginate hydrogels in aqueous media. By combining a new sample preparation protocol with extremely gentle tapping mode AFM imaging, we characterized the morphology and regional mechanical properties of the hydrated alginate. Upon cross-linking, basic units of these hydrogel materials consist of egg-box dimers, which assemble into long fibrils. These fibrils congregate and pile up, forming a sponge-like structure, whose pore size and distribution depend on the cross-linking conditions. At the exterior, surface tension impacts the piling of fibrils, leading to stripe-like features. These structural features contribute to local, regional, and macroscopic mechanics. The outcome provides new insights into its structural characteristics from nanometers to tens of micrometers, i.e., at the dimensions pertaining to biomaterial and hydrogel-cell interactions. Collectively, the results advance our knowledge of the structure and mechanics from the nanometer to continuum, facilitating advanced applications in hydrogel biomaterials.

The constructs of lipid molecules follow self-assembly, driven by intermolecular interactions, forming stacking of lipid bilayer films. Achieving designed geometry at nano- to micro-levels with packing deviating from the near-equilibrium structure is difficult to achieve due to the strong tendency of lipid molecules to self-assemble. Using ultrasmall (sn-glycero-3-phosphocholine (POPC). Taking advantage of the high spatial precision and the minute size of the delivery probe in our combined atomic force microscopy and microfluidic delivery, the transient shape of each liquid droplet is regulated. In doing so, the final geometry of the POPC assemblies has been regulated to the designed geometry with nanometer precision. The results extend the concept of controlled assembly of molecules to amphiphilic systems. The outcomes exhibit high potential in lipid-based biomaterial science and biodevice engineering.

Whereas single crystals of organic compounds that respond to heat ("thermosalient") or light ("photosalient") have been reported and studied in detail, crystalline organic compounds that elicit an extreme mechanical response at cryogenic temperatures ("cryosalient") are relatively rare in the chemical literature. During an attempt to measure an X-ray crystal structure of a tetrafluoro(aryl)sulfanylated bicyclopentane synthesized in our laboratory, we discovered that the crystals jump and forcefully disintegrate upon cooling below ~193 K. Accordingly, we investigated the origin of this cryogenic mechanical response (CMR) through NMR, SC-XRD, DSC, Raman, Brillouin, and microscopy experiments. To our surprise, NMR, SC-XRD, and DSC experiments suggest the phenomenon can neither be attributed to a chemical transformation nor a crystalline-crystalline or crystalline-amorphous phase transition. Rather, microscopy, Raman, and Brillouin experiments provide evidence that the built-up strain released from the crystal upon self-destruction may be associated with a phase transition that occurs in amorphous microstructure within the crystal. This study demonstrates that molecular structural changes within organic material microstructure – which may not necessarily be visible by SC-XRD – can have a significant impact on the thermodynamic behavior of the bulk crystal-line material. Thus, the role of microstructure may be considered more heavily in future mechanistic studies on mechanically responsive crystals.

Atomic force microscopy (AFM) in conjunction with microfluidic delivery was utilized to produce three-dimensional (3D) lipid structures following a custom design. While AFM is well-known for its spatial precision in imaging and 2D nanolithography, the development of AFM-based nanotechnology into 3D nanoprinting requires overcoming the technical challenges of controlling material delivery and interlayer registry. This work demonstrates the concept of 3D nanoprinting of amphiphilic molecules such as 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC). Various formulations of POPC solutions were tested to achieve point, line, and layer-by-layer material delivery. The produced structures include nanometer-thick disks, long linear spherical caps, stacking grids, and organizational chiral architectures. The POPC molecules formed stacking bilayers in these constructions, as revealed by high-resolution structural characterizations. The 3D printing reached nanometer spatial precision over a range of 0.5 mm. The outcomes reveal the promising potential of our designed technology and methodology in the production of 3D structures from nanometer to continuum, opening opportunities in biomaterial sciences and engineering, such as in the production of 3D nanodevices, chiral nanosensors, and scaffolds for tissue engineering and regeneration.