Biomolecular detection with a thin membrane transducer

Surface stress-induced membrane biosensor based on double-layer stable gold nanostructures for E. coli detection

The surface stress-based biosensor has been applied in fast and sensitive identification of Escherichia coli (E. coli)with significance for public health, food, and water safety. However, the stable sensitive element of flexible biosensor based on surface stress is still crucial and challengeable. Here, the authors reported surface stress-induced biosensors based on double-layer stable gold nanostructures (D-AuNS-SSMB) for E. coli O157:H7 detection. Bacterial detection demonstrates the high stability of the biosensor. The resistance change of biosensor is linear to the logarithmic value of the E. coli O157:H7 concentrations ranging from 10³ to 10⁷ CFU/mL with a limit of detection (LOD) of 43 CFU/mL. The captured signals of D-AuNS-SSMB comes from surface stress generated by antigen-antibody binding. In addition, the biosensor exhibits good stability, reproducibility and specificity in detection of E. coli O157:H7 as well. This study provides a new preparation method of stable sensitive element for the E. coli detection.

Molecular Dynamics Simulation on the Influences of Nanostructure Shape, Interfacial Adhesion Energy, and Mold Insert Material on the Demolding Process of Micro-Injection Molding

In micro-injection molding, the interaction between the polymer and the mold insert has an important effect on demolding quality of nanostructure. An all-atom molecular dynamics simulation method was performed to study the effect of nanostructure shape, interfacial adhesion energy, and mold insert material on demolding quality of nanostructures. The deformation behaviors of nanostructures were analyzed by calculating the non-bonded interaction energies, the density distributions, the radii of gyration, the potential energies, and the snapshots of the demolding stage. The nanostructure shape had a direct impact on demolding quality. When the contact areas were the same, the nanostructure shape did not affect the non-bonded interaction energy at PP-Ni interface. During the demolding process, the radii of gyration of molecular chains were greatly increased, and the overall density was decreased significantly. After assuming that the mold insert surface was coated with an anti-stick coating, the surface burrs, the necking, and the stretching of nanostructures were significantly reduced after demolding. The deformation of nanostructures in the Ni and Cu mold inserts were more serious than that of the Al₂O₃ and Si mold inserts. In general, this study would provide theoretical guidance for the design of nanostructure shape and the selection of mold insert material.

Micro-LEGO for MEMS

The recently developed transfer printing-based microassembly called micro-LEGO has been exploited to enable microelectromechanical systems (MEMS) applications which are difficult to achieve using conventional microfabrication. Micro-LEGO involves transfer printing and thermal processing of prefabricated micro/nanoscale materials to assemble structures and devices in a 3D manner without requiring any wet or vacuum processes. Therefore, it complements existing microfabrication and other micro-assembly methods. In this paper, the process components of micro-LEGO, including transfer printing with polymer stamps, material preparation and joining, are summarized. Moreover, recent progress of micro-LEGO within MEMS applications are reviewed by investigating several example devices which are partially or fully assembled via micro-LEGO.

Lab-on-a-brane for spheroid formation

Lab-On-a-Brane (LOB) represents a class of Lab-On-a-Chip (LOC) integrating flexible, highly gas permeable and biocompatible thin membranes (TMs). Here we demonstrate the potentiality of LOBs as cell biochips promoting 3D cell growth. The human cancer cells MCF-7 were cultured into standard multiwells (MWs) and into polydimethylsiloxane (PDMS) MWs, LOCs, and LOBs of different wettability. Surface treatments based on oxygen plasma and coating deposition have been performed to produce hydrophilic, hydrophobic, and oleophobic chips. By a comparison between all these chips, we observed that 3D cell aggregation is favored in LOBs, independent of substrate wettability. This may be attributed to the TM flexibility and the high oxygen/carbon dioxide permeability. Ultimately, LOBs seem to combine the advantages of LOCs as multi-well microfluidic chips to reduce operation time for cell seeding and medium refresh, with the mechanical/morphological properties of PDMS TMs. This is convenient in the perspective of applying mechanical stimuli and monitoring cell stiffness, or studying the metabolism of molecules permeable to PDMS membrane in response to external stimuli with interesting outcomes in cellular biology.

PDMS Sylgard 527-Based Freely Suspended Ultrathin Membranes Exhibiting Mechanistic Characteristics of Vascular Basement Membranes

In the past, significant effort has been made to develop ultrathin membranes exhibiting physiologically relevant mechanical properties, such as thickness and elasticity of native basement membranes. However, most of these fabricated membranes have a relatively high elastic modulus, ∼MPa-GPa, relevant only to retinal and epithelial basement membranes. Vascular basement membranes exhibiting relatively low elastic modulus, ∼kPa, on the contrary, have seldom been mimicked. Membranes demonstrating high compliance, with moduli ranging in ∼kPa along with sub-microscale thicknesses have rarely been reported, and would be ideal to mimic vascular basement membranes in vitro. To address this, we fabricate ultrathin membranes demonstrating the mechanistic features exhibited by their vascular biological counterparts. Salient features of the fabricated ultrathin membranes include free suspension, physiologically relevant thickness ∼sub-micrometers, relatively low modulus ∼kPa, and sufficiently large culture area ∼20 mm². To fabricate such ultrathin membranes, undiluted PDMS Sylgard 527 was utilized as opposed to the conventional diluted polymer-solvent mixture approach. In addition, the necessity to have a sacrificial layer for releasing membranes from the underlying substrates was also eliminated in our approach. The novelty of our work lies in achieving the distinct combination of membranes having thickness in sub-micrometers and the associated elasticity in kilopascal using undiluted polymer, which past approaches with dilution have not been able to accomplish. The ultrathin membranes with average thickness of 972 nm (thick) and 570 nm (thin) were estimated to have an elastic modulus of 45 and 214 kPa, respectively. Contact angle measurements revealed the ultrathin membranes exhibited hybrophobic characteristics in unpeeled state and transformed to hydrophilic behavior when freely suspended. Human umbilical vein endothelial cells were cultured on the polymeric ultrathin membranes, and the temporal cell response to change in local compliance of the membranes was studied by evaluating the cell spread area, density, percentage area coverage, and spread rate. After 24 h, single cells, pairs, and group of three to four cells were noticed on highly compliant thick membranes, having average thickness of 972 nm and modulus of 45 kPa. On the contrary, the cell monolayer was noted on the glass slide acting as a control. For the thin membranes featuring average thickness of 570 nm and modulus of 214 kPa, the cells tend to exhibit response similar to that on control with initiation of monolayer formation. Our results indicate, the local compliance, in turn, the membrane thickness governs the cell behavior and this can have vital implications during disease initiation and progression, wound healing, and cancer cell metastasis.

DNA aptamer release from the DNA-SWNT hybrid by protein recognition

Here we show the formation of the complex between a DNA aptamer and a single-walled carbon nanotube (SWNT) and its reaction with its target protein. The aptamer, which is specifically bound with thrombin, the target protein in this study, easily wraps and disperses the SWNT by noncovalent π-π stacking.

Surface stress-based biosensors

Surface stress-based biosensors, as one kind of label-free biosensors, have attracted lots of attention in the process of information gathering and measurement for the biological, chemical and medical application with the development of technology and society. This kind of biosensors offers many advantages such as short response time (less than milliseconds) and a typical sensitivity at nanogram, picoliter, femtojoule and attomolar level. Furthermore, it simplifies sample preparation and testing procedures. In this work, progress made towards the use of surface stress-based biosensors for achieving better performance is critically reviewed, including our recent achievement, the optimally circular membrane-based biosensors and biosensor array. The further scientific and technological challenges in this field are also summarized. Critical remark and future steps towards the ultimate surface stress-based biosensors are addressed.

Large-scale, ultrapliable, and free-standing nanomembranes

The creation and characterization of large-area ultrathin highly pliable free-standing PDMS membranes and their application to the study of cellular epithelia is described. The ultra-thin membranes permitted the straight forward calculation of cell monolayer moduli, derived from measured stress-strain curves. These measurements allowed the unprecedented detection of cellular-level injury in the epithelia caused by the rupture of cell-cell tight junctions in response to stretching.

Transfer of CVD-grown monolayer graphene onto arbitrary substrates

Reproducible dry and wet transfer techniques were developed to improve the transfer of large-area monolayer graphene grown on copper foils by chemical vapor deposition (CVD). The techniques reported here allow transfer onto three different classes of substrates: substrates covered with shallow depressions, perforated substrates, and flat substrates. A novel dry transfer technique was used to make graphene-sealed microchambers without trapping liquid inside. The dry transfer technique utilizes a polydimethylsiloxane frame that attaches to the poly(methyl methacrylate) spun over the graphene film, and the monolayer graphene was transferred onto shallow depressions with 300 nm depth. The improved wet transfer onto perforated substrates with 2.7 μm diameter holes yields 98% coverage of holes covered with continuous films, allowing the ready use of Raman spectroscopy and transmission electron microscopy to study the intrinsic properties of CVD-grown monolayer graphene. Additionally, monolayer graphene transferred onto flat substrates has fewer cracks and tears, as well as lower sheet resistance than previous transfer techniques. Monolayer graphene films transferred onto glass had a sheet resistance of ∼980 Ω/sq and a transmittance of 97.6%. These transfer techniques open up possibilities for the fabrication of various graphene devices with unique configurations and enhanced performance.

Capacitive microsystems for biological sensing

The growing interest in personalized medicine leads to the need for fast, cheap and portable devices that reveal the genetic profile easily and accurately. To this direction, several ideas to avoid the classical methods of diagnosis and treatment through miniaturized and label-free systems have emerged. Capacitive biosensors address these requirements and thus have the perspective to be used in advanced diagnostic devices that promise early detection of potential fatal conditions. The operation principles, as well as the design and fabrication of several capacitive microsystems for the detection of biomolecular interactions are presented in this review. These systems are micro-membranes based on surface stress changes, interdigitated micro-electrodes and electrode-solution interfaces. Their applications extend to DNA hybridization, protein-ligand binding, antigen-antibody binding, etc. Finally, the limitations and prospects of capacitive microsystems in biological applications are discussed.

Detection of DNA mutations using a capacitive micro-membrane array

The detection of DNA hybridization using capacitive readout and a biosensor array of ultrathin Si membranes is presented. The biosensor exploits the ability of the ultrathin membranes to deflect upon surface stress variations caused by biological interactions. Probe DNA molecules are immobilized on the membrane surface and the surface stress variations during hybridization with their complementary strands force the membrane to deflect and effectively change the capacitance between the flexible membrane and the fixed substrate. The sensor array comprises 256 such sensing sites thus allowing the concurrent sensing of multiple DNA mutations. The biosensor and its performance for the detection of complementary DNA strands are demonstrated using beta-thalassemia oligonucleotides. The experimental results show that the presented sensors are able to detect DNA hybridization and to discriminate single nucleotide mismatches.