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1. Salt- and pH-induced swelling of a poly(acrylic acid) brush via quartz crystal microbalance w/dissipation (QCM-D)

   https://doi.org/10.1039/c9sm01289c  

   Hollingsworth, Nisha R.; Wilkanowicz, Sabina I.; Larson, Ronald G. (October 2019, Soft Matter)

   We infer the swelling/de-swelling behavior of weakly ionizable poly(acrylic acid) (PAA) brushes of 2–39 kDa molar mass in the presence of KCl concentrations from 0.1–1000 mM, pH = 3, 7, and 9, and grafting densities σ = 0.12–2.15 chains per nm 2 using a Quartz Crystal Microbalance with Dissipation (QCM-D), confirming and extending the work of Wu et al. to multiple chain lengths. At pH 7 and 9 (above the p K a ∼ 5), the brush initially swells at low KCl ionic strength (<10 mM) in the “osmotic brush” regime, and de-swells at higher salt concentrations, in the “salted brush” regime, and is relatively unaffected at pH 3, below the p K a , as expected. At pH 7, at low and moderate grafting densities, our results in the high-salt “salted brush” regime ( C s > 10 mM salt) agree with the predicted scaling H ∼ Nσ +1/3 C s −1/3 of brush height H , while in the low-salt “osmotic brush” regime ( C s < 10 mM salt), we find H ∼ Nσ +1/3 C s +0.28–0.38 , whose dependence on C s agrees with scaling theory for this regime, but the dependence on σ strongly disagrees with it. The predicted linearity in the degree of polymerization N is confirmed. The new results partially confirm scaling theory and clarify where improved theories and additional data are needed. 

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2. Strongly coupled plasmonic metal nanoparticles with reversible pH-responsiveness and highly reproducible SERS in solution

   https://doi.org/10.1039/d3nr05071h  

   Wei, Zichao; Vandergriff, Audrey; Liu, Chung-Hao; Liaqat, Maham; Nieh, Mu-Ping; Lei, Yu; He, Jie (January 2024, Nanoscale)

   We report a facile method to prepare polymer-grafted plasmonic metal nanoparticles (NPs) that exhibit pH-responsive surface-enhanced Raman scattering (SERS). The concept is based on the use of pH- responsive polymers, such as poly(acrylic acid) (PAA) and poly(allylamine hydrochloride) (PAH), as multi- dentate ligands to wrap around the surface of NPs instead of forming polymer brushes. Upon changing the solvent quality, the grafted pH-responsive polymers would drive reversible aggregation of NPs, leading to a decreased interparticle distance. This creates numerous hot spots, resulting in a secondary enhancement of SERS as compared to the SERS from discrete NPs. For negatively charged PAA-grafted NPs, the SERS response at pH 2.5 showed a secondary enhancement of up to 104-fold as compared to the response for discrete NPs at pH 12. Similarly, positively charged PAH-grafted AuNPs showed an oppo- site response to pH. We demonstrated that enhanced SERS with thiol-containing and charged molecular probes was indeed from the pH-driven solubility change of polymer ligands. Our method is different from the conventional SERS sensors in the solid state. With pH-responsive polymer-grafted NPs, SERS can be performed in solution with high reproducibility and sensitivity but without the need for sample pre-con- centration. These findings could pave the way for innovative designs of polymer ligands for metal NPs where polymer ligands do not compromise interparticle plasmon coupling. 

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3. Molecular simulations and understanding of antifouling zwitterionic polymer brushes

   https://doi.org/10.1039/D0TB00520G  

   Liu, Yonglan; Zhang, Dong; Ren, Baiping; Gong, Xiong; Xu, Lijian; Feng, Zhang-Qi; Chang, Yung; He, Yi; Zheng, Jie (January 2020, Journal of Materials Chemistry B)

   Zwitterionic materials are an important class of antifouling biomaterials for various applications. Despite such desirable antifouling properties, molecular-level understanding of the structure–property relationship associated with surface chemistry/topology/hydration and antifouling performance still remains to be elucidated. In this work, we computationally studied the packing structure, surface hydration, and antifouling property of three zwitterionic polymer brushes of poly(carboxybetaine methacrylate) (pCBMA), poly(sulfobetaine methacrylate) (pSBMA), and poly((2-(methacryloyloxy)ethyl)phosporylcoline) (pMPC) brushes and a hydrophilic PEG brush using a combination of molecular mechanics (MM), Monte Carlo (MC), molecular dynamics (MD), and steered MD (SMD) simulations. We for the first time determined the optimal packing structures of all polymer brushes from a wide variety of unit cells and chain orientations in a complex energy landscape. Under the optimal packing structures, MD simulations were further conducted to study the structure, dynamics, and orientation of water molecules and protein adsorption on the four polymer brushes, while SMD simulations to study the surface resistance of the polymer brushes to a protein. The collective results consistently revealed that the three zwitterionic brushes exhibited stronger interactions with water molecules and higher surface resistance to a protein than the PEG brush. It was concluded that both the carbon space length between zwitterionic groups and the nature of the anionic groups have a distinct effect on the antifouling performance, leading to the following antifouling ranking of pCBMA > pMPC > pSBMA. This work hopefully provides some structural insights into the design of new antifouling materials beyond traditional PEG-based antifouling materials. 

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4. Rational hydrogel design to improve brain modulus matching for implantation

   https://doi.org/10.1016/j.matlet.2025.138187  

   Garifo, Molli; Bethel, Keturah; Davis, Eric M; Larsen, Jessica (April 2025, Materials Letters)

   Treating the brain is challenging due to the restrictive blood–brain barrier, and modulus-mismatched implants often cause problems. Herein, we have fabricated copolymer hydrogels from thermoresponsive polymer, poly(N-isopropylacrylamide) (PNIPAAm), -r-hydrophilic polymer, poly(acrylic acid) (PAA), which are injectable and transform into soft implants above their lower critical solution temperature (LCST). PAA concentration can be leveraged to tune the LCST and viscosity of the PNIPAAm–r–PAA hydrogel in solution. Furthermore, the Young’s moduli of these materials, ranging from 1-4 kPa, are close to rat and human brain tissue, potentially leading to less inflammation and rejection due to significant modulus mismatch. 

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5. Finding the sweet spot: a library of hydrogels with tunable degradation for tissue model development

   https://doi.org/10.1039/d1tb02436a  

   Pandala, Narendra; LaScola, Michael A.; Hinton, Zachary; Korley, La Shanda; Lavik, Erin (March 2022, Journal of Materials Chemistry B)

   In vitro models are valuable tools for applications including understanding cellular mechanisms and drug screening. Hydrogel biomaterials facilitate in vitro models by mimicking the extracellular matrix and in vivo microenvironment. However, it can be challenging for cells to form tissues in hydrogels that do not degrade. In contrast, if hydrogels degrade too much or too quickly, tissue models may be difficult to assess in a high throughput manner. In this paper, we present a poly(allylamine) (PAA) based synthetic hydrogel system which can be tuned to control the mechanical and chemical cues provided by the hydrogel scaffold. PAA is a polycation with several biomedical applications, including the delivery of small molecules, nucleic acids, and proteins. Based on PAA and poly(ethylene glycol) (PEG), we developed a synthetic non-degradable system with potential applications for long-term cultures. We then created a second set of gels that combined PAA with poly- l -lysine (PLL) to generate a library of semi-degradable gels with unique degradation kinetics. In this work, we present the hydrogel systems’ synthesis, characterization, and degradation profiles along with cellular data demonstrating that a subset of gels supports the formation of endothelial cell cord-like structures. 

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