ICMCTF2016 Session TS1: Biointerfaces

Interfacial films are widely recognized to be significant contributors to surface forces in friction and adhesion – they control whether a surface is ‘sticky’ or ‘slippery’. These films are influenced by local chemical and ambient environment as well as repetitive applied stresses. Conventional approaches to examining interfaces are destructive in nature – separating the surfaces and performing ex situ examination of the contacts. Further, time dependent surface changes like adsorption/desorption kinetics and adhesive secretion and curing in live organisms can’t be examined “after the fact” and instead require real-time measures of molecule-surface interactions. At NRL, we have made significant progress in developing in situ methods to demonstrate the chemical, mechanical and rheological processes in interfaces. These approaches have been advantageous in elucidating the solid lubrication mechanisms governing friction and wear of solid lubricant coatings. We are now applying and extending these approaches to examine underwater adhesion in marine organisms.

We are using a broad suite of interfacial spectroscopies, coupled with modern bioinformatics, biochemistry, chemistry and materials science approaches to deconstruct the adhesion mechanisms of barnacles. Acorn barnacles develop complex, protective shells surrounding their soft tissues and secrete a permanent adhesive under the shell, making them extremely difficult to remove from marine surfaces like ships. We find that barnacles use a multifaceted, hierarchical materials approach to building their adhesive interface. Both the organic (protein-based adhesive) and inorganic (calcium carbonate shell) materials are secreted and deposited into hierarchically organized structures with features ranging from nm to mm size scales. I will describe how we have elucidated new insights into the physical and chemical mechanisms the barnacle uses to cure its adhesive underwater.

One of the most important engineered biological processes is the activated sludge treatment of municipal and industrial wastewaters. In order to optimize the performance of the treatment plants it is essential to understand the initial steps of floc formation. We have developed a three-dimensional Individual-based Model (IbM) to simulate the formation and growth of the floc at micro-scale.

The IbM consists of two parts: one deals with the growth and behaviour of individual microorganisms as autonomous agents; the other deals with the substrate and product diffusion and reaction. Each microorganism grows by consuming the substrate and nutrients, and divides when a critical mass is reached. Heterotrophic and autotrophic organisms, extracellular polymeric substance, and dead microorganisms are considered as particulate components (rigid spheres) in a Lagrangian coordinate system. The soluble components are carbon substrate, oxygen, ammonia, nitrite, and nitrate. The particulate and soluble components are coupled through carbon oxidation, nitrification, and denitrification processes.

The growth is modelled using Monod kinetics. The pressure buildup due to the growth of biomass is released by maintenance of a minimum distance between the neighbouring organisms by using a shoving mechanism. The substrate and nutrients transport is governed by the diffusion-reaction equation which is solved in an Eulerian frame.

A sensitivity analysis of the model would be important for guiding experimental analysis, model reduction, and parameter estimation. As such, a global sensitivity analysis is performed for a floc to evaluate the importance of various input parameters such as kinetic and stoichiometric parameters as well as mass transfer and substrate and nutrient concentrations. The input parameters are sampled according to the Latin hypercube sampling technique and the sensitivity is estimated by computing partial rank correlation coefficients between each input parameter and each outcome variable such as floc mass and size.

This study has revealed that only about half of the input parameters considered have most significant effect on floc formation. The present results further indicate that the total floc mass and its size are mostly sensitive to the kinetic parameters of heterotrophic organisms. The floc mass and size are not very sensitive to the nutrient transfer rates as diffusion is much faster than biomass growth. In addition, this study has demonstrated that the shoving mechanism would significantly affect the floc size.

Direct coupling of Raman spectrometer with nanoindentation instrument provides the capability for full mechanical characterization of the material at the nanoscale and its direct correlation to the localized chemical composition. The vibrational (phonon) states of molecules detected using Raman spectroscopy give a molecular fingerprint of the physical state of a matter. We demonstrate the advanced productivity and benefits of coupled instruments on polydimethylsiloxane (PDMS) films used for lab-on-chip design.

PDMS physical properties can be tuned by variety of fabrications process in favor of proliferation and movement of cells. UV irradiation is widely used for modification of surface chemistry resulting in variation in surface mechanical properties. UV exposure creates benzophenone radicals that react with silicon hydrides present in the PDMS elastomer.

Measured indentation curve clearly showed pronounced adhesion and drop of reduced elastic modulus with extending UV exposure time. Indentation results are correlated with variation in surface chemistry by means of in situ Raman spectroscopy. Cell spreading analysis with respect to obtained mechanical data was also performed. Additionally, the benefits of the direct integration/combination of Raman spectroscopy and nanomechanical testing will be presented and discussed.

Polylactic acid (PLA) is a thermoplastic, biodegradable polyester. It can be derived from natural resources such as corn starch, cassava starch or sugarcane. PLA can also be classified as a bioplastic. In industrial production PLA can be polymerized by two monomers: lactic acid and cyclic di-ester (lactide) with various metal catalysts in solution or suspension.

PLA is soluble in tetrahydrofuran, dioxane, chlorinated solvents and heated benzene. The selected solubility provides limited choices for dissolving PLA into solution using electrospinning process to fabricate nano-fibers. The advantage of using electrospinning is to create material with very high porosities and complex geometry, maintaining its biodegradability, enhancing bio-adhesion and extracellular chemical signal transduction for cell culture.

PLA can be manufactured by injection molding, extrusion, casting, and in most recent techniques such as electrospinning and three dimensional printing. This variety of different processes makes PLA accessible to a wide range of applications.

In this study, we will fabricate PLA nanofibers by electrospinning and then deposit poly-methyl-methacrylate (PMMA) thin film on its top by plasma polymerization. PMMA is a transparent, nontoxic thermoplastic polymer with highly stable physical and chemical properties. The combination of PMMA film and PLA nanofibers has dual purposes for cell culture. The PMMA thin film can protect PLA from degradation from chemical reagents and PLA provides a flexible support and cultural bed for PMMA.

In addition to fabrication, we also investigated the structure and surface morphology of the combined PMMA film and PLA fibers by SEM and AFM. These films are tested for their biocompatibility via cell culture for 3T3 fibroblasts (mouse embryonic). The 3T3 cell line is the standard fibroblast and commonly used for DNA transfection studies. The cell culture will be subjected to ELISA or MTT assays for the evaluation of cellular activity during culture on the PMMA and PLA films.

Poly(methyl methacrylate) (PMMA) commonly used as a shatter-proof glass for security reasons is a transparent thermoplastic polymer. It is also known as acrylic glass as well as Plexiglas, Perspex, Acrylite and Lucite among several other trademarks. PMMA, because of its nontoxic, stable physical and chemical properties, inexpensive and easy processing, makes it extensively used in industry. PMMA is an economical replacement to polycarbonate (PC) for applications without demand on high mechanical strength. Furthermore, PMMA does not contain the potentially unsafe bisphenol-A subunits found in polycarbonate.

Cyclopropylamine (CPA), on the other hand, is a chemical active and volatile small polymer in a liquid form at room temperature. The cyclopropylamine group can be found in a number of drugs, and could be bioactive to form distonic radicals cations by an oxidation process. In the pharmaceutical side, an N-substituted cyclopropylamine can be highly selective, irreversible monoamine oxidase-inhibiting antidepressants. For example, the monoamine structure of cyclopropylamine may be capable of crossing linking quinoprotein methylamine dehydrogenase to prevent its normal enzymatic functions.

One relatively new and yet important applications of PMMA is in the broad area of biomedical devices. Due to its properties mentioned above, PMMA is particularly suitable for cellular and tissue engineering as support materials or scaffold. Among different fabrication processes such as gelation, injection and casting, plasma polymerization is the one capable of depositing nano scale films on almost any substrate uniformly and rapidly. In this study, we investigated the structure, composition, surface and optical properties of deposited PMMA and CPA films on glass by radio frequency power plasma inside a vacuum chamber. During deposition, power input, mixed monomers (PMMA+Cyclopropylamine) in chamber pressure and Ar flow rate are fixed at an optimal values from our earlier trial studies. Optical emission spectroscopy (OES) is used to study the plasma characteristics under each deposition condition. After deposition, these films are characterized using FTIR, and surface profiler. Eventually, these films are applied to cell culture and the host is chosen to be 3T3 fibroblast (mouse embryonic) cells purchased from a cell line. The 3T3 cell line is the standard fibroblast and commonly used for DNA transfection studies. The cell culture will be subjected to ELISA or MTT assays for the evaluation of cellular activity during culture on the mixed PMMA and CPA films. The biological results shall be correlated with the process parameters and plasma conditions during deposition.