Entropic Effects in Solvent-Free Bidisperse Polymer Brushes Investigated Using Density Functional Theories.

Solvent-free polymer-functionalized nanoparticles form a special type of colloid composed of inorganic cores self-suspended by their grafted coronas. In the absence of intervening solvent molecules, the fluidity of the system is provided by these tethered polymers as they fill the space. Here, we study the structure and interaction of neighboring polymer-grafted surfaces in the solvent-free condition using mean-field density functional theories. For opposing flat surfaces, the brush configuration and the associated energy landscape are semianalytically investigated given the incompressibility of the tethered entropic chains. The effect of brush polydispersity (including variations in both chain length and surface grafting density) is considered by two bidisperse models corresponding to different physical scenarios: one for opposing brushes uniformly mixed with two species at a fixed grafting density, and the other for opposing brushes with distinct chain lengths and grafting densities. The space-filling capabilities of the neighboring coronas differ not only by their ratio of radii of gyration for the composing polymers but also by their ratio of grafting densities. We show that the system energy depicts a steric repulsion as the brushes are compressed, which is typical for hairy particles in a solvent. However, as the interwall separation increases, the cooperative stretching of the chains leads to an entropic attraction between them, a unique characteristic of solventless systems. The corresponding brush profiles change from a bell-like shape to a more step-function-like feature as the interwall spacing increases significantly. The interwall separation associated with the overall free energy minimum therefore characterizes the favorable interparticle spacing for solvent-free polymer-functionalized particles. The limiting accessible parameter space of polymer sizes and grafting densities subjected to the space-filling constraint is comprehensively explored for representative interparticle spacing characterizing the compressed, relaxed, and stretched regimes for a given polymer species, respectively. Such information would be useful for guiding the design of experimental solvent-free polymer-functionalized nanoparticles.

Multifunctional and Continuous Gradients of Biointerfaces Based on Dual Reverse Click Reactions.

Chemical or biological gradients that are composed of multifunctional and/or multidirectional guidance cues are of fundamental importance for prospective biomaterials and biointerfaces. As a proof of concept, a general modification approach for generating multifunctional and continuous gradients was realized via two controlled and reversed click reactions, namely, thermo-activated thiol-yne and copper-free alkyne and azide click reactions. The cell adhesion property of fibroblasts was guided in a gradient with an enhancement, showing that the PEG molecule and RGD peptide were countercurrently immobilized to form such reversed gradients (with negating of the cell adhesion property). Using the gradient modification protocol to also create countercurrent distributions of FGF-2 and BMP-2 gradients, the demonstration of not only multifunctional but also gradient biointerfacial properties was resolved in time latencies on one surface by showing the manipulation in gradients toward proliferation and osteogenic differentiation for adipose-derived stem cells.

Multifaceted and route-controlled "click" reactions based on vapor-deposited coatings.

"Click" reactions provide precise and reliable chemical transformations for the preparation of functional architectures for biomaterials and biointerfaces. The emergence of a multiple-click reaction strategy has paved the way for a multifunctional microenvironment with orthogonality and precise multitasking that mimics nature. We demonstrate a multifaceted and route-controlled click interface using vapor-deposited functionalized poly-para-xylylenes. Distinctly clickable moieties of ethynyl and maleimide were introduced into poly-para-xylylenes in one step via a chemical vapor deposition (CVD) copolymerization process. The advanced interface coating allows for a double-click route with concurrent copper(i)-catalyzed Huisgen 1,3-dipolar cycloaddition (CuAAC) and the thiol-maleimide click reaction. Additionally, double-click reactions can also be performed in a cascade manner by controlling the initiation route to enable the CuAAC and/or thiol-yne reaction using a mono-functional alkyne-functionalized poly-para-xylylene. The use of multifaceted coatings to create straightforward and orthogonal interface properties with respect to protein adsorption and cell attachment is demonstrated and characterized.