Peter M. Johnson

Spherical indentation testing of poroelastic relaxations in thin hydrogel layers

In this work, we present the Poroelastic Relaxation Indentation (PRI) testing approach for quantifying the mechanical and transport properties of thin layers of poly(ethylene glycol) hydrogels with thicknesses on the order of 200 μm. Specifically, PRI characterizes poroelastic relaxation in hydrogels by indenting the material at fixed depth and measuring the contact area-dependent load relaxation process as a function of time. With the aid of a linear poroelastic theory developed for thin or geometrically confined swollen polymer networks, we demonstrate that PRI can quantify the water diffusion coefficient, shear modulus and average pore size of the hydrogel layer. This approach provides a simple methodology to quantify the material properties of thin swollen polymer networks relevant to transport phenomena.

Improving performance of dental resins by adding titanium dioxide nanoparticles

OBJECTIVE The objective of this study is to improve the performance of dental resins by adding a small amount of titanium dioxide nanoparticles (TiO₂ NPs), which have outstanding mechanical properties and unique photoactivities. METHODS Acrylic acid modified TiO₂ NPs (AP25) were prepared and added to a mixture of bis-phenol-A-dimethacrylate and triethylene glycol dimethacrylate (mass ratio 1:1) at seven mass fractions. Disks made of these resins were subjected to FTIR microspectroscopy, nanoindentation, microindentation, and 3-point bending to determine the degree of vinyl conversion (DC) modulus and hardness. The shear bond strengths (SBS) of dentin adhesives containing various amount of AP25 were also examined. RESULTS The DC increased as a function of mass fraction of AP25 and reached a plateau at 0.1%. The DC of the resin mixture was improved by ≈7% up to 91.7 ± 0.8%. The elastic modulus and hardness of the composites increased initially as more AP25 were added, and decreased after reached the maximum value at approximately 0.06% mass fraction of AP25. The maximum elastic modulus was ≈48% higher than that of the NP-free resin, and the maximum hardness was more than twice higher than that of the NP-free resin. Using these resin composites as dental adhesives, the mean SBS using resins with 0.1% mass fraction of AP25 was ≈30% higher than those using NP-free resin. SIGNIFICANCE By adding a small amount of AP25 to the resin, the DC and the mechanical properties of resins were improved dramatically. These findings could lead to better performing dental adhesives.

Fabrication and characterization of a multilayered optical tissue model with embedded scattering microspheres in polymeric materials.

We report on a novel fabrication approach to build multilayered optical tissue phantoms that serve as independently validated test targets for axial resolution and contrast in scattering measurements by depth-resolving optical coherent tomography (OCT) with general applicability to a variety of three-dimensional optical sectioning platforms. We implement a combinatorial bottom-up approach to prepare monolayers of light-scattering microspheres with interspersed layers of transparent polymer. A dense monolayer assembly of monodispersed microspheres is achieved via a combined methodology of polyelectrolyte multilayers (PEMs) for particle-substrate binding and convective particle flux for two-dimensional crystal array formation on a glass substrate. Modifications of key parameters in the layer-by-layer polyelectrolyte deposition approach are applied to optimize particle monolayer transfer from a glass substrate into an elastomer while preserving the relative axial positioning in the particle monolayer. Varying the dimensions of the scattering microspheres and the thickness of the intervening transparent polymer layers enables different spatial frequencies to be realized in the transverse dimension of the solid phantoms. Step-wise determination of the phantom dimensions is performed independently of the optical system under test to enable precise spatial calibration, independent validation, and quantitative dimensional measurements.

Modeling enzymatic kinetic pathways for ring-opening lactone polymerization.

A unified kinetic pathway for the enzyme-catalyzed polymerization and degradation of poly(ε-caprolactone) was developed. This model tracks the complete distribution of individual chain lengths, both enzyme-bound and in solution, and successfully predicts monomer conversion and the molecular mass distribution as a function of reaction time. As compared to reported experimental data for polymerization reactions, modeled kinetics generate similar trends, with ring-opening rates and water concentration as key factors to controlling molecular mass distributions. Water is critically important by dictating the number of linear chains in solution, shifting the molecular mass distribution at which propagation and degradation equilibrate. For the enzymatic degradation of poly(ε-caprolactone), the final reaction product is also consistent with the equilibrium dictated by the propagation and degradation rates. When the modeling framework described here is used, further experiments can be designed to isolate key reaction steps and provide methods for improving the efficiency of enzyme polymerization.

SKR-1, a homolog of Skp1 and a member of the SCF SEL-10 complex, regulates sex-determination and LIN-12/Notch signaling in C. elegans

Sex-determination in Caenorhabditis elegans requires regulation of gene transcription and protein activity and stability. sel-10 encodes a WD40-repeat-containing F-box protein that likely mediates the ubiquitin-mediated degradation of important sex-determination factors. Loss of sel-10 results in a mild masculinization of hermaphrodites, whereas dominant alleles of sel-10, such as sel-10(n1074), cause a more severe masculinization, including a reversal of the life versus death decision in sex-specific neurons. To investigate about how sel-10 regulates sex-determination, we conducted a sel-10(n1074) suppressor screen and isolated a weak loss-of-function allele of skr-1, one of 21 Skp1-related genes in C. elegans. Skp1, Cullin, and F-box proteins, such as SEL-10, are components of the SCF E3 ubiquitin-ligase complex. We present genetic evidence that the sel-10(n1074) masculinization phenotype is dependent upon skr-1 and cul-1 activity. Furthermore, we show that the SKR-1(M140I) weak loss-of-function mutation interferes with SKR-1/SEL-10 binding. Unexpectedly, we found that the G567E substitution in SEL-10 caused by the n1074 allele impairs the binding of SEL-10 to SKR-1 and the dimerization of SEL-10, which may be important for SEL-10 function. Our results suggest that SKR-1, CUL-1 and SEL-10 constitute an SCF E3 ligase complex that plays an important role in modulating sex-determination and LIN-12/Notch signaling in C. elegans.

In situ monitoring of enzyme-catalyzed (co)polymerizations by Raman spectroscopy

In situ, fiber optic-based Raman spectroscopy provided real time monitoring of enzyme-catalyzed ring-opening homo- and copolymerizations of e-caprolactone (e-CL) and δ-valerolactone (δ-VL). A custom designed reactor equipped with in situ fiber optic probe was used to measure monomer conversion as a function of time. The results from the in situ technique were in good agreement with those determined by offline 1H NMR analysis. Monomer reactivity ratios for the lipase-catalyzed copolymerization of e-CL and δ-VL were estimated using the Kelen-Tudos method as re-CL = 0.38 and rδ-VL = 0.29.

Poroelastic relaxation of polymer-loaded hydrogels

Recently, Poroelastic Relaxation Indentation (PRI) was developed as a simple indentation-based approach for quantifying solvent diffusion in polymer gel layers. In this work, we extend the PRI approach to study the poroelastic relaxations of polymer-loaded hydrogels. We observe that PRI can quantify the diffusion coefficients of solvent and polymer solutions within the hydrogel separately via control of the indentation depth, i.e., the compressive strain. Specifically, the water diffusion coefficient is measured for strains above a critical value whereas the polymer solution diffusion coefficient is measured for strains below the critical value. With the aid of the Flory–Rehner theory, we show that this strain-dependent diffusion is related to the extent of deformation of the hydrogel network. Beyond a critical compressive strain, this deformation leads to a significant reduction in the water volume fraction relative to that of PEG solution within the hydrogel, thus enabling the measurement of the diffusion coefficients separately by simply adjusting the indentation depth.

Surface indentation arrays for high-throughput analysis of viscoelastic material properties.

Viscoelastic relaxation processes factor into polymer performance and stability throughout an application lifetime, controlled by the polymer network structure and dynamics which occur over a wide spectrum of time scales. In this work, we detail the design and operation of an independent array of surface indenters which can measure the creep response at multiple points on a polymer substrate. Samples with composition and temperature gradients are used to exhibit the ability to measure viscoelastic properties under unique conditions for each indentation. Methacrylate photopolymer systems are measured at different compositions and crosslink densities simultaneously within an indenter array to increase the measurement throughput, with a measured creep compliance ranging from 10(-9) Pa(-1) to 10(-5) Pa(-1). The application of temperature gradients allows for the viscoelastic measurements to be assembled onto a master curve using time-temperature superposition.

On state vs. channel quantum extension problems: exact results for UxUxU symmetry

We develop a framework which unifies seemingly different extension (or ‘joinability’) problems for bipartite quantum states and channels. This includes known extension problems such as optimal quantum cloning and quantum marginal problems as special instances. Central to our generalization is a variant of the Jamiolkowski isomorphism between bipartite states and linear transformations, which we term the homocorrelation map: in contrast to the better-known Choi isomorphism which emphasizes the preservation of the positivity constraint, use of the Jamiolkowski isomorphism allows one to characterize the preservation of the statistical correlations of bipartite states and quantum channels. The resulting homocorrelation map thus acquires a natural operational interpretation. We define and analyze state-joining, channel-joining, and local-positive-joining problems in three-party settings with collective symmetry, obtaining exact analytical characterizations in low dimensions. We find that bipartite quantum states are limited in the degree to which their measurement outcomes may agree, whereas quantum channels are limited in the degree to which their measurement outcomes may disagree. Loosely speaking, quantum mechanics enforces an upper bound on the strength of positive correlation across two subsystems at a single time, as well as on the strength of negative correlation between the state of a single system across two instants of time. We argue that these general statistical bounds inform the quantum joinability limitations, and show that they are in fact sufficient for the three-party -invariant setting.