More Like this

1. pH-Dependent Friction of Polyacrylamide Hydrogels

   https://doi.org/10.1007/s11249-023-01779-4  

   Chau, Allison L.; Pugsley, Conor D.; Miyamoto, Madeleine E.; Tang, Yongkui; Eisenbach, Claus D.; Mates, Thomas E.; Hawker, Craig J.; Valentine, Megan T.; Pitenis, Angela A. (September 2023, Tribology Letters)

   Abstract Polyacrylamide hydrogels are widely used in biomedical applications due to their tunable mechanical properties and charge neutrality. Our recent tribological investigations of polyacrylamide gels have revealed tunable and pH-dependent friction behavior. To determine the origins of this pH-responsiveness, we prepared polyacrylamide hydrogels with two different initiating chemistries: a reduction–oxidation (redox)-initiated system using ammonium persulfate (APS) andN,N,N′N′-tetramethylethylenediamine (TEMED) and a UV-initiated system with 2-hydroxy-4′-(2-hydroxyethoxy)-2-methylpropiophenone (Irgacure 2959). Hydrogel swelling, mechanical properties, and tribological behavior were investigated in response to solution pH (ranging from ≈ 0.34 to 13.5). For polyacrylamide hydrogels in sliding contact with glass hemispherical probes, friction coefficients decreased fromµ = 0.07 ± 0.02 toµ = 0.002 ± 0.002 (redox-initiated) and fromµ = 0.05 ± 0.03 toµ = 0.003 ± 0.003 (UV-initiated) with increasing solution pH. With hemispherical polytetrafluoroethylene (PTFE) probes, friction coefficients of redox-initiated hydrogels similarly decreased fromµ = 0.06 ± 0.01 toµ = 0.002 ± 0.001 with increasing pH. Raman spectroscopy measurements demonstrated hydrolysis and the conversion of amide groups to carboxylic acid in basic conditions. We therefore propose that the mechanism for pH-responsive friction in polyacrylamide hydrogels may be credited to hydrolysis-driven swelling through the conversion of side chain amide groups into carboxylic groups and/or crosslinker degradation. Our results could assist in the rational design of hydrogel-based tribological pairs for biomedical applications from acidic to alkaline conditions. Graphical abstract 

   more » « less
2. Tunable mid-infrared frequency combs from nanophotonic parametric oscillators

   https://doi.org/10.1364/CLEO_AT.2022.JTh6B.6  

   Roy, Arkadev; Ledezma, Luis; Costa, Luis; Gray, Robert; Sekine, Ryoto; Guo, Qiushi; Liu, Mingchen; Briggs, Ryan M.; Marandi, Alireza (January 2022, CLEO: Applications and Technology 2022)

   We demonstrate mid-infrared frequency combs from synchronously-pumped optical parametric oscillators in nanophotonic lithium niobate at 19 GHz. With a picosecond pump at ~1µm, the output can be tuned between 1.5 and 3.3µm supporting sub-picosecond pulses. 

   more » « less
3. Magnetic field-induced emissivity tuning of InSb-based metamaterials in the terahertz frequency regime

   https://doi.org/10.1364/OME.433003  

   Caratenuto, Andrew; Chen, Fangqi; Tian, Yanpei; Antezza, Mauro; Xiao, Gang; Zheng, Yi (August 2021, Optical Materials Express)

   This work demonstrates the magnetic field-induced spectral properties of metamaterials incorporating both indium antimonide (InSb) and tungsten (W) in the terahertz (THz) frequency regime. Nanostructure materials, layer thicknesses and surface grating fill factors are modified, impacting light-matter interactions and consequently modifying thermal emission. We describe and validate a method for determining spectral properties of InSb under an applied direct current (DC) magnetic field, and employ this method to analyze how these properties can be tuned by modulating the field magnitude. Notably, an InSb-W metamaterial exhibiting unity narrowband emission is designed, suitable as an emitter for wavelengths around 55µm (approximately 5.5 THz), which is magnetically tunable in bandwidth and peak wavelength. 

   more » « less
4. Graded index couplers for next generation chip-to-chip and fiber-to-chip photonic packaging

   https://doi.org/10.1088/2515-7647/ae1648  

   Weninger, Drew; Duessel, Christian; Serna, Samuel; Kimerling, Lionel; Agarwal, Anuradha (November 2025, Journal of Physics: Photonics)

   Abstract The transition towards designs which co-package electronic and photonic die together in data center switch packages has created a scaling path to Petabyte per second (Pbps) input/output (I/O) in such systems. In a co-packaged design, the scaling of bandwidth, cost, and energy will be governed by the number of optical I/O channels and the data rate per channel. While optical communication provide an opportunity to exploit wavelength division multiplexing to scale data rate, the limited 127 µm pitch of V-groove based single mode fiber arrays and the use of active alignment and bonding for their packaging present challenges to scaling the number of optical channels. Flip-chip optical couplers which allow for low loss, broadband operation and automated passive assembly represent a solution for continued scaling. In this paper, we propose a novel scheme to vertically couple between silicon based waveguides on separate chips using graded index couplers in combination with an evanescent coupler. Simulation results using a 3D finite-difference time-domain solver are presented, demonstrating coupling losses as low as 0.35 dB for a chip-to-chip gap of 11 µm; 1 dB vertical and lateral alignment tolerances of approximately 2.45 µm and ± 2.66 µm, respectively; and a possible 1 dB bandwidth of greater than 1500 nm. These results demonstrate the potential of our coupler as a universal interface in future co-packaged optics systems. 

   more » « less
5. Superlubricity of pH-responsive hydrogels in extreme environments

   https://doi.org/10.3389/fchem.2022.891519  

   Chau, Allison L.; Getty, Patrick T.; Rhode, Andrew R.; Bates, Christopher M.; Hawker, Craig J.; Pitenis, Angela A. (August 2022, Frontiers in Chemistry)

   Poly(acrylamide- co -acrylic acid) (P(AAm- co -AA)) hydrogels are highly tunable and pH-responsive materials frequently used in biomedical applications. The swelling behavior and mechanical properties of these gels have been extensively characterized and are thought to be controlled by the protonation state of the acrylic acid (AA) through the regulation of solution pH. However, their tribological properties have been underexplored. Here, we hypothesized that electrostatics and the protonation state of AA would drive the tribological properties of these polyelectrolyte gels. P(AAm- co -AA) hydrogels were prepared with constant acrylamide (AAm) concentration (33 wt%) and varying AA concentration to control the amount of ionizable groups in the gel. The monomer:crosslinker molar ratio (200:1) was kept constant. Hydrogel swelling, stiffness, and friction behavior were studied by systematically varying the acrylic acid (AA) concentration from 0–12 wt% and controlling solution pH (0.35, 7, 13.8) and ionic strength ( I = 0 or 0.25 M). The stiffness and friction coefficient of bulk hydrogels were evaluated using a microtribometer and borosilicate glass probes as countersurfaces. The swelling behavior and elastic modulus of these polyelectrolyte hydrogels were highly sensitive to solution pH and poorly predicted the friction coefficient ( µ ), which decreased with increasing AA concentration. P(AAm- co -AA) hydrogels with the greatest AA concentrations (12 wt%) exhibited superlubricity ( µ = 0.005 ± 0.001) when swollen in unbuffered, deionized water (pH = 7, I = 0 M) and 0.5 M NaOH (pH = 13.8, I = 0.25 M) ( µ = 0.005 ± 0.002). Friction coefficients generally decreased with increasing AA and increasing solution pH. We postulate that tunable lubricity in P(AAm- co -AA) gels arises from changes in the protonation state of acrylic acid and electrostatic interactions between the probe and hydrogel surface. 

   more » « less