Younjin Min

Recent advances in the surface forces apparatus (SFA) technique

The surface forces apparatus (SFA) has been used for many years to measure the physical forces between surfaces, such as van der Waals (including Casimir) and electrostatic forces in vapors and liquids, adhesion and capillary forces, forces due to surface and liquid structure (e.g. solvation and hydration forces), polymer, steric and hydrophobic interactions, bio-specific interactions as well as friction and lubrication forces. Here we describe recent developments in the SFA technique, specifically the SFA 2000, its simplicity of operation and its extension into new areas of measurement of both static and dynamic forces as well as both normal and lateral (shear and friction) forces. The main reason for the greater simplicity of the SFA 2000 is that it operates on one central simple-cantilever spring to generate both coarse and fine motions over a total range of seven orders of magnitude (from millimeters to angstroms). In addition, the SFA 2000 is more spacious and modulated so that new attachments and extra parts can easily be fitted for performing more extended types of experiments (e.g. extended strain friction experiments and higher rate dynamic experiments) as well as traditionally non-SFA type experiments (e.g. scanning probe microscopy and atomic force microscopy) and for studying different types of systems.

Interaction forces and adhesion of supported myelin lipid bilayers modulated by myelin basic protein

Force–distance measurements between supported lipid bilayers mimicking the cytoplasmic surface of myelin at various surface coverages of myelin basic protein (MBP) indicate that maximum adhesion and minimum cytoplasmic spacing occur when each negative lipid in the membrane can bind to a positive arginine or lysine group on MBP. At the optimal lipid/protein ratio, additional attractive forces are provided by hydrophobic, van der Waals, and weak dipolar interactions between zwitterionic groups on the lipids and MBP. When MBP is depleted, the adhesion decreases and the cytoplasmic space swells; when MBP is in excess, the bilayers swell even more. Excess MBP forms a weak gel between the surfaces, which collapses on compression. The organization and proper functioning of myelin can be understood in terms of physical noncovalent forces that are optimized at a particular combination of both the amounts of and ratio between the charged lipids and MBP. Thus loss of adhesion, possibly contributing to demyelination, can be brought about by either an excess or deficit of MBP or anionic lipids.

Relating domain size distribution to line tension and molecular dipole density in model cytoplasmic myelin lipid monolayers

We fit the size distribution of liquid-ordered (Lo) domains measured from fluorescence images of model cytoplasmic myelin monolayers with an equilibrium thermodynamic expression that includes the competing effects of line tension, λ, dipole density difference, Δm, and the mixing entropy. From these fits, we extract the line tension, λ, and dipole density difference, Δm, between the Lo and liquid-disordered (Ld) phases. Both λ and Δm decrease with increasing surface pressure, , although λ/Δm2 remains roughly constant as the monolayer approaches the miscibility surface pressure. As a result, the mean domain size changed little with surface pressure, although the polydispersity increased significantly. The most probable domain radius was significantly smaller than that predicted by the energy alone, showing that the mixing entropy promotes a greater number of smaller domains. Our results also explain why domain shapes are stable; at equilibrium, only a small fraction of the domains are large enough to undergo theoretically predicted shape fluctuations. Monolayers based on the composition of myelin from animals with experimental allergic encephalomyelitis had slightly lower values of λ and Δm, and a higher area fraction of domains, than control monolayers at all . While it is premature to generalize these results to myelin bilayers, our results show that the domain distribution in myelin may be an equilibrium effect and that subtle changes in surface pressure and composition can alter the distribution of material in the monolayer, which will likely also alter the interactions between monolayers important to the adhesion of the myelin sheath.

Implantable Silk Composite Microneedles for Programmable Vaccine Release Kinetics and Enhanced Immunogenicity in Transcutaneous Immunization

Microneedle vaccines mimic several aspects of cutaneous pathogen invasion by targeting antigen to skin-resident dendritic cells and triggering local inflammatory responses in the skin, which are correlated with enhanced immune responses. Here, we tested whether control over vaccine delivery kinetics can enhance immunity through further mimicry of kinetic profiles present during natural acute infections. An approach for the fabrication of silk/poly(acrylic acid) (PAA) composite microneedles composed of a silk tip supported on a PAA base is reported. On brief application of microneedle patches to skin, the PAA bases rapidly dissolved to deliver a protein subunit vaccine bolus, while also implanting persistent silk hydrogel depots into the skin for a low-level sustained cutaneous vaccine release over 1-2 weeks. Use of this platform to deliver a model whole-protein vaccine with optimized release kinetics resulted in >10-fold increases in antigen-specific T-cell and humoral immune responses relative to traditional parenteral needle-based immunization.

Lipid-protein interactions alter line tensions and domain size distributions in lung surfactant monolayers.

The size distribution of domains in phase-separated lung surfactant monolayers influences monolayer viscoelasticity and compressibility which, in turn, influence monolayer collapse and set the compression at which the minimum surface tension is reached. The surfactant-specific protein SP-B decreases the mean domain size and polydispersity as shown by fluorescence microscopy. From the images, the line tension and dipole density difference are determined by comparing the measured size distributions with a theory derived by minimizing the free energy associated with the domain energy and mixing entropy. We find that SP-B increases the line tension, dipole density difference, and the compressibility modulus at surface pressures up to the squeeze-out pressure. The increase in line tension due to SP-B indicates the protein avoids domain boundaries due to its solubility in the more fluid regions of the film.

Frictional Properties of Surfactant-Coated Rod-Shaped Nanoparticles in Dry and Humid Dodecane †

We have investigated the effects of humidity (water content or activity from 0 to approximately 0.98) on the frictional properties of surfactant-coated ZnS nanoparticles of various shapes, specifically, nanorods and nanowires, dispersed in an organic solvent (dodecane). The friction coefficients were found to be sensitive to even trace amounts of water, increasing logarithmically with time after the systems were exposed to humid air, doubling after 2-4 h of exposure time to air of relative humidity approximately 98%. We also show that increasing the humidity caused noticeable effects on the interactions of the nanoparticles, increasing their adhesion and aggregation through capillary forces. These effects should be considered in the design of organic solvents containing nanoparticles with physisorbed surfactants, for example, lube oils with nanoparticles additives, particularly those exposed to atmospheric conditions.

Mechanomutable and reversibly swellable polyelectrolyte multilayer thin films controlled by electrochemically induced pH gradients

We present a new strategy to electrochemically control the swelling state and mechanical properties of a polyelectrolyte multilayer thin film. While a number of pH-responsive polymer films and hydrogels have been developed, biological systems typically will not tolerate substantial deviations in pH. Therefore, to apply such pH-responsive systems for biomedical or other sensitive applications, we developed an electrochemical approach to alter local pH, while maintaining a constant, mild, bulk pH. The polymer film investigated in this work comprises polyallylamine hydrochloride (PAH) and sulfonated polystyrene (SPS) assembled at high pH (>9.0), which is known to exhibit a large pH-induced swelling transition; however, relatively extreme bulk pH values (pH 10.5 to deswell) are required to manipulate the film. Here, we apply negative electric potentials to gold electrodes coated with the film; the potential induces the reduction of dissolved oxygen, which generates hydroxide ions at the electrode surface and raises the local pH. The in situ swelling state and mechanical properties of the film have been probed with a number of techniques. Overall, we have attained reversible 300% volume changes in the polymer thin films, and have reversibly altered the mechanical properties over an order of magnitude (shear modulus between 1.9 MPa and 230 kPa, loss modulus between 620 kPa and 92 kPa, and effective indentation modulus between 19.2 MPa and 3.16 MPa). We maintain that electrochemical control over local pH is a promising strategy to manipulate pH-responsive polymer systems for biomedical and other applications.

Studies of Bilayers and Vesicle Adsorption to Solid Substrates: Development of a Miniature Streaming Potential Apparatus (SPA)

We show, by natural occurring phenomena of charge separation near the solid-liquid interface in microchannels, that electricity can be generated by forcing water through a ceramic rod with no moving part and emission. A single hand push on a syringe is our source of power which easily generates a streaming potential of over 20 V and 30 µA. By means of streaming potentials, two capacitors were charged and discharged alternatively to light-up two Light-Emitting-Diodes in every ten seconds. From our specific choice of liquid/solid pair, an efficiency of 0.8% was obtained. A mobile-ion-drain method is also demonstrated to increase the streaming potential.A new miniature streaming potential apparatus (SPA) was developed to determine the streaming potentials (Psi(str)) and zeta potentials (zeta) of substrates under different ionic conditions while simultaneously visualizing the state of the surfaces, such as the adsorption of surfactants or polymers, using fluorescence microscopy and/or fluorescence recovery after photobleaching (FRAP). Experimental results with different surfaces show that the new SPA provides streaming potential values (hence zeta potentials) that agree with results obtained using traditional electrokinetic analyzers. Using the new SPA, the formation of supported lipid bilayers (SLBs) on glass from fluorescently labeled, unilamellar (approximately 100 nm diameter), charge neutral dimyristoyl-sn-glycero-3-phosphocholine (DMPC) vesicles was studied in aqueous electrolyte solutions at different lipid concentrations. Simultaneous zeta potential measurements and fluorescence imaging for measuring diffusion coefficients by confocal microscopy enabled us to precisely monitor the changes in the surface charge as well as in the surface morphology during SLB formation from vesicles. For a fixed incubation time of 5 min, both results revealed that the adsorption of intact vesicles and/or discrete bilayer patches were observed below a threshold concentration, above which the formation of continuous SLBs occurred leading to an estimate for the zeta-potential and for the diffusion coefficient of -9.1 +/- 1.6 mV and (1.1 +/- 0.02) x 10(-12) m(2)/s, respectively.

Adsorption Mechanism of Myelin Basic Protein on Model Substrates and Its Bridging Interaction between the Two Surfaces

Myelin basic protein (MBP) is an intrinsically disordered (unstructured) protein known to play an important role in the stability of myelins multilamellar membrane structure in the central nervous system. The adsorption of MBP and its capacity to interact with and bridge solid substrates has been studied using a surface forces apparatus (SFA) and a quartz crystal microbalance with dissipation (QCM-D). Adsorption experiments show that MBP molecules adsorb to the surfaces in a swollen state before undergoing a conformational change into a more compact structure with a thickness of ∼3 nm. Moreover, this compact structure is able to interact with nearby mica surfaces to form adhesive bridges. The measured adhesion force (energy) between two bridged surfaces is 1.0 ± 0.1 mN/m, (Ead = 0.21 ± 0.02 mJ/m(2)), which is slightly smaller than our previously reported adhesion force of 1.7 mN/m (Ead = 0.36 mJ/m(2)) for MBP adsorbed on two supported lipid bilayers (Lee et al., Proc. Natl. Acad. Sci. U.S.A. 2014, 111, E768-E775). The saturated surface concentration of compact MBP on a single SiO2 surface reaches a stable value of 310 ± 10 ng/cm(2) regardless of the bulk MBP concentration. A kinetic three-step adsorption model was developed that accurately fits the adsorption data. The developed model is a general model, not limited to intrinsically disordered proteins, that can be extended to the adsorption of various chemical compounds that undergo chemical reactions and/or conformational changes upon adsorbing to surfaces. Taken together with our previously published data (Lee et al., Proc. Natl. Acad. Sci. U.S.A. 2014, 111, E768-E775), the present results confirm that conformational changes of MBP upon adsorption are a key for strong adhesion, and that such conformational changes are strongly dependent on the nature of the surfaces.

Effect of Composition and Synthetic Route on the Microstructure of Biodegradable Diblock Copolymer, Poly(ε-caprolactone-co-L-lactide)-b-Poly(ethylene glycol)

Biodegradable poly(ε-caprolactone-co-L-lactide)-b-poly(ethylene glycol) (PCLA-b-PEG) copolymers were synthesized via solution polymerization by varying the feed composition of ε-caprolactone (ε-CL) and L-lactide (LLA) (ε-CL: LLA=10∶0, 7∶3, 5∶5, 3∶7, 0∶10). The feed ratio based on weight is in accordance with the copolymer composition except for the case of ε-CL: LLA=3:7 (C3L7), which was verified by1H-NMR. Two different approaches were used for the exceptional case, which is an extension of the reaction time or the sequential introduction of the monomer. A copolymer composition of ε-CL: LLA=3:7 could be obtained in either case. The chemical microstructure of PCLA-b-PEG was determined using the13C-NMR spectra and the effect of the sequential structure on the thermal properties and crystallinity were examined. Despite the same composition ratio of the copolymer, the microstructure can differ according to the reaction conditions.