One-step synthesis of PHEMA hydrogel films capable of generating highly ordered wrinkling patterns

Smart Wrinkled Interfaces: Patterning, Morphing, and Coding of Polymer Surfaces by Dynamic Anisotropic Wrinkling

In contrast to traditional static surfaces, smart patterned surfaces with periodical and reversible morphologies offer limitless opportunities for encoding surface functions and properties on demand, facilitating their widespread application as functional building blocks in various devices. Advances in intelligently controlling the macroscopic properties of these smart surfaces have been accomplished through various techniques (such as three-dimensional printing, imprint lithography and femtosecond laser) and responsive materials. In contrast to the sophisticated techniques above, dynamic anisotropic wrinkling, taking advantage of dynamic programmable manipulation of surface wrinkling and its orientation, offers a powerful alternative for fabricating dynamic periodical patterns due to its spontaneous formation, versatility, convenient scale-up fabrication, and sensitivity to various stimuli. This review comprehensively summarizes recent advances in smart patterned surfaces with dynamic oriented wrinkles, covering design principles, fabrication techniques, representative types of physical and chemical stimuli, as well as fine-tuning of wrinkle dimensions and orientation. Finally, advanced applications of these smart patterned surfaces are presented, along with a discussion of current challenges and future prospects in this rapidly evolving field. This review would offer some insights and guidelines for designing and engineering novel stimuli-responsive smart wrinkled surfaces, thereby facilitating their sustainable development and progressing toward commercialization.

Unconstrained dynamic gel swelling generates transient surface deformations

Polymer gels are comprised of a three-dimensional, cross-linked network that can typically withstand the mechanical deformation associated with both swelling and de-swelling. Thus, gels can be designed with smart behaviors that require both stress generation and dissipation, making them well-suited to many applications including membrane technology, water capture devices, and drug delivery systems. In contrast to the fully swelled equilibrium state, limited research characterizes the unsteady-state swelling regime prior to equilibrium. It is in this regime where unique surface deformations can occur. Here we show how internal network constraints and external diffusive pressure can be leveraged to manipulate swelling kinetics and surface deformations in poly(ethylene glycol) gels during unconstrained, three-dimensional swelling. We find that increasing cross-linker molecular weight and swelling in ethanol, as opposed to water, are both effective routes to increase the time it takes to reach equilibrium but do so through different mechanisms. Networks with fewer internal constraints, manipulated via cross-linker chain-length, imbibe more solvent over a longer time. In contrast, swelling in ethanol reduces the amount of solvent imbibed by the network while increasing the time to reach equilibrium. Measurements of surface patterns during swelling establishes that an immediate, fast relaxation at the surface occurs during the first five minutes of swelling. However, the density and persistence of these features varies with solvent quality. These results establish a framework for how soft materials undergo dynamic deformation. Engineering transient surface properties while mitigating unwanted instabilities opens the door for emerging technologies such as smart anti-fouling and sensors.

Leveraging Microgels Prepared from Poly(ethylene glycol) Bisepoxide and Polyetheramine for Versatile Surface Structuring of Agarose Hydrogels

We demonstrate a macromer-type bisepoxide, poly(ethylene glycol) diglycidyl ether, polymerizing readily with a trifunctional polyetheramine Jeffamine T-403 in water to facilitate the development of a series of microgels abbreviated as PMG. Simply by varying the concentration of the as-prepared thermoresponsive intermediate prepolymer from 1 to 2 and 4%, hydrodynamic sizes of the resulting P1MG, P2MG, and P4MG are easily tuned in the submicrometer to micrometer range shown by the dynamic light scattering results. Besides size difference, these microgels also deform differently, where the drying-induced deformation effect is most severe for P1MG and least prominent for P4MG. Simple evaporative deposition of PMG into multilayer packing provides versatile and green options for microgel-mediated surface structuring of agarose hydrogels. Specifically, deformabile P1MG- and P2MG-derived coatings render agarose gel microwrinkle textures by buckling against swelling-induced surface instability. Conversely, stiffer P4MG microgels lead to a patchy patterned hierarchical coating on agarose, similar to the cracking effect in drying colloidal films. The straightforward microgel-on-macrogel strategy allows integration of both wrinkle and patchy patterns to generate Janus-type agarose gels, just by rationally arranging the coating sequence. Diversifying topographic features attainable through microgel-based coatings on hydrogels could potentially make the sustainable and biocompatible material of agarose a more compelling choice for bioapplications. Brief demonstrations of the broad applicability of P1MG toward wrinkling of proteinaceous and synthetic hydrogels further highlight promising prospects of the PMG microgel-on-macrogel functionalization strategy.

Wrinkled Interfaces: Taking Advantage of Anisotropic Wrinkling to Periodically Pattern Polymer Surfaces

Periodically patterned surfaces can cause special surface properties and are employed as functional building blocks in many devices, yet remaining challenges in fabrication. Advancements in fabricating structured polymer surfaces for obtaining periodic patterns are accomplished by adopting "top-down" strategies based on self-assembly or physico-chemical growth of atoms, molecules, or particles or "bottom-up" strategies ranging from traditional micromolding (embossing) or micro/nanoimprinting to novel laser-induced periodic surface structure, soft lithography, or direct laser interference patterning among others. Thus, technological advances directly promote higher resolution capabilities. Contrasted with the above techniques requiring highly sophisticated tools, surface instabilities taking advantage of the intrinsic properties of polymers induce surface wrinkling in order to fabricate periodically oriented wrinkled patterns. Such abundant and elaborate patterns are obtained as a result of self-organizing processes that are rather difficult if not impossible to fabricate through conventional patterning techniques. Focusing on oriented wrinkles, this review thoroughly describes the formation mechanisms and fabrication approaches for oriented wrinkles, as well as their fine-tuning in the wavelength, amplitude, and orientation control. Finally, the major applications in which oriented wrinkled interfaces are already in use or may be prospective in the near future are overviewed.

Study on the Interaction of Plasma-Polymerized Hydrogel Coatings with Aqueous Solutions of Different pH

Amphiphilic hydrogels from mixtures of 2-hydroxyethyl methacrylate and 2-(diethylamino)ethyl methacrylate p(HEMA-co-DEAEMA) with specific pH sensitivity and hydrophilic/hydrophobic structures were designed and polymerized via plasma polymerization. The behavior of plasma-polymerized (pp) hydrogels containing different ratios of pH-sensitive DEAEMA segments was investigated concerning possible applications in bioanalytics. In this regard, the morphological changes, permeability, and stability of the hydrogels immersed in solutions of different pHs were studied. The physico-chemical properties of the pp hydrogel coatings were analyzed using X-ray photoelectron spectroscopy, surface free energy measurements, and atomic force microscopy. Wettability measurements showed an increased hydrophilicity of the pp hydrogels when stored in acidic buffers and a slightly hydrophobic behavior after immersion in alkaline solutions, indicating a pH-dependent behavior. Furthermore, the pp (p(HEMA-co-DEAEMA) (ppHD) hydrogels were deposited on gold electrodes and studied electrochemically to investigate the pH sensitivity of the hydrogels. The hydrogel coatings with a higher ratio of DEAEMA segments showed excellent pH responsiveness at the studied pHs (pH 4, 7, and 10), demonstrating the importance of the DEAEMA ratio in the functionality of pp hydrogel films. Due to their stability and pH-responsive properties, pp (p(HEMA-co-DEAEMA) hydrogels are conceivable candidates for functional and immobilization layers for biosensors.

Swelling-Activated, Soft Mechanochemistry in Polymer Materials

Swelling in polymer materials is a ubiquitous phenomenon. At a molecular level, swelling is dictated by solvent-polymer interactions, and has been thoroughly studied both theoretically and experimentally. Favorable solvent-polymer interactions result in the solvation of polymer chains. For polymers in confined geometries, such as those that are tethered to surfaces, or for polymer networks, solvation can lead to swelling-induced tensions. These tensions act on polymer chains and can lead to stretching, bending, or deformation of the material both at the micro- and macroscopic scale. This Invited Feature Article sheds light on such swelling-induced mechanochemical phenomena in polymer materials across dimensions, and discusses approaches to visualize and characterize these effects.

Anthracene dimer cross-linked, washing- and sterilization-free hydrogel films for multicellular spheroid generation

Three-dimensional multicellular spheroids are better in vitro cell models than two-dimensional cell monolayers; however, their applications are limited by their difficult production. PHEMA hydrogel films with honeycomb-like wrinkled patterns have been developed for high-throughput generation of multicellular spheroids with a uniform shape and size; however, the films are prepared by polymerization of the HEMA monomer, and should be washed extensively before use. Here to synthesize washing-free PHEMA hydrogel films, linear anthracene-functionalized PHEMA polymers were first synthesized by the free-radical copolymerization of HEMA and an anthracene-containing vinyl monomer. Smooth films were then prepared from the linear polymers in the wells of cell culture plates by solution casting. They were cross-linked via photo-dimerization of anthracene groups by exposing to 365 nm UV light, and patterned spontaneously by adding water to swell them. The swelling degree of the films and hence the shape of the wrinkled patterns could be facilely adjusted by adjusting the anthracene content in the linear polymers. If necessary, the cross-link density of the film and thus the shape of the wrinkled patterns could be callbacked by irradiating with 254 nm UV light, thanks to the reversibility of the photo-dimerization of anthracene. The as-prepared films exhibit a high biocompatibility and support cell growth well even without washing and sterilization. The high biocompatibility is attributed to the facts that no low molecular weight monomer, crosslinker and initiator were involved in the film preparation, and the photo-dimerization of anthracene groups produces no by-products. The films are sterilized simultaneously when cross-linked with UV irradiation, thus avoiding an additional sterilization step. PHEMA films with long-range ordered hexagonal wrinkled patterns were selected to generate multicellular spheroids of tumor cell lines. The resulting spheroids exhibit a narrow size distribution and high cell viability. Preliminary tests demonstrated their potential in the screening of anti-cancer drugs.

Fabricating scalable, personalized wound dressings with customizable drug loadings via 3D printing

In recent times, 3D printing has been gaining traction as a fabrication platform for customizable drug dosages as a form of personalized medicine. While this has been recently demonstrated as oral dosages, there is potential to provide the same customizability and personalization as topical applications for wound healing. In this paper, the application of 3D printing to fabricate hydrogel wound dressings with customizable architectures and drug dosages was investigated. Chitosan methacrylate was synthesized and mixed with Lidocaine Hydrochloride and Levofloxacin respectively along with a photoinitiator before being used to print wound dressings of various designs. These designs were then investigated for their effect on drug release rates and profiles. Our results show the ability of 3D printing to customize drug dosages and drug release rates through co-loading different drugs at various positions and varying the thickness of drug-free layers over drug-loaded layers in the wound dressing respectively. Two scale-up approaches were also investigated for their effects on drug release rates from the wound dressing. The influence that each wound dressing design has on the release profile of drugs was also shown by fitting them with drug release kinetic models. This study thus shows the feasibility of utilizing 3D printing to fabricate wound dressings with customizable shapes, drug dosage and drug release rates that can be tuned according to the patient's requirements.

Diels-Alder Cross-Linked, Washing-Free Hydrogel Films with Ordered Wrinkling Patterns for Multicellular Spheroid Generation

Three-dimensional (3D) multicellular spheroids are a new generation in vitro cell model, however, their applications are severely limited by difficulties in their generation. Here patterned poly(2-hydroxyethyl methacrylate) (PHEMA) hydrogel films were synthesized for their generation. Instead of polymerization of HEMA monomers in the presence of a cross-linker, here the PHEMA films were synthesized by cross-linking furan-functionalized linear PHEMA, PHEMA-furan, and maleimide-functionalized linear PHEMA, PHEMA-mal, via Diels-Alder (DA) reaction between furan and maleimide groups. A thermal treatment temperature of 75 °C was chosen for the cross-linking reaction. The occurrence of DA reaction was confirmed by IR spectra. Using this method, cross-linked PHEMA films with smooth surface were successfully synthesized in situ in the well of cell culture plates. The films were then patterned by simply adding water to swell them. Highly ordered, honeycomb-like wrinkling patterns were successfully obtained by adjusting the furan and maleimide contents in the precursor linear polymers. The patterned hydrogel films were used to generate multicellular spheroids. Guided by the patterns, 3D spheroids with narrow size distribution, tunable size, and high cell viability were successfully obtained. The patterned PHEMA films reported here exhibited a lot of advantages. The patterning method was quite simple and required no template or special equipment. They were synthesized in situ in commercial cell culture plates. Particularly, thanks to the clean nature of the DA reaction, no low molecular weight monomer, cross-linker, initiator, or catalyst, which were potentially cytotoxic, was involved in the film synthesis, and no byproduct was produced and left in the film. The resulting films presented a high biocompatibility, allowing the avoidance of the tedious washing step. The films synthesized here were expected to have high potential for massive production of well-defined multicellular spheroids.

Design of Bio-Conjugated Hydrogels for Regenerative Medicine Applications: From Polymer Scaffold to Biomolecule Choice

Bio-conjugated hydrogels merge the functionality of a synthetic network with the activity of a biomolecule, becoming thus an interesting class of materials for a variety of biomedical applications. This combination allows the fine tuning of their functionality and activity, whilst retaining biocompatibility, responsivity and displaying tunable chemical and mechanical properties. A complex scenario of molecular factors and conditions have to be taken into account to ensure the correct functionality of the bio-hydrogel as a scaffold or a delivery system, including the polymer backbone and biomolecule choice, polymerization conditions, architecture and biocompatibility. In this review, we present these key factors and conditions that have to match together to ensure the correct functionality of the bio-conjugated hydrogel. We then present recent examples of bio-conjugated hydrogel systems paving the way for regenerative medicine applications.

Brushing the surface: cascade reactions between immobilized nanoreactors

Functionalization of hard or soft surfaces with, for example, ligands, enzymes or proteins, is an effective and practical methodology for the development of new applications. We report the assembly of two types of nanoreactors based upon poly(dimethylsiloxane)-block-poly(2-methyl-2-oxazoline) (PDMS-b-PMOXA) diblock copolymers as scaffold, uricase and lactoperoxidase as bio-catalysts located within the nanoreactors, and melittin as the biopores inserted into the hydrophobic shell. The nanoreactors were immobilized on poly(2-hydroxyethyl methacrylate)-co-poly(2-aminoethyl methacrylate hydrochloride) (PHEMA-co-P(2-AEMA·HCl) brushes-grafted wafer surfaces by utilizing the strong supramolecular interactions between biotin and streptavidin. The (PHEMA-co-P(2-AEMA·HCl) brushes on silicon surfaces were prepared by a surface initiating atom transfer radical polymerization (ATRP) "graft-from" technique. Cascade reactions between different surface-anchored nanoreactors were demonstrated by converting Amplex® Red to the fluorescent probe resorufin by using the H2O2 produced from uric acid and H2O. The detailed properties of the nanoreactors on the functionalized surface including the binding behaviours and cascade reactions were investigated using emission spectroscopy, transmission electron microscopy (TEM), light scattering (LS), atomic force microscopy (AFM) and a quartz crystal microbalance (QCM-D). The results are proof-of-principle for the preparation of catalytically functional engineered surface materials and lay the foundation for applying this advanced functional surface material in biosensing, implanting and antimicrobial materials preparation.

Biointerface topography mediates the interplay between endothelial cells and monocytes

Endothelial cell (EC) monolayers located in the inner lining of blood vessels serve as a semipermeable barrier between circulating blood and surrounding tissues. The structure and function of the EC monolayer affect the recruitment and adhesion of monocytes, which plays a pivotal role in the development of inflammation and atherosclerosis. Here we investigate the effect of material wrinkled topographies on the responses of human umbilical vein endothelial cells (HUVECs) and adhesion of monocytes to HUVECs. It is found that HUVEC responses are non-linearly mediated by surface topographies with different dimensions. Specifically, more cell elongation and better cell orientation on the wrinkled surface with a 3.5 μm amplitude and 10 μm wavelength (W10) are observed compared to other surfaces. The proliferation rate of HUVECs on the W10 surface is higher than that on other surfaces due to more 5-ethynyl-2′-deoxyuridine (EdU) detected on the W10 surface. Also, greater expression of inflammatory cytokines from HUVECs and adhesion of monocytes to HUVECs on the W10 surface is shown than other surfaces due to greater expression of p-AKT and ICAM, respectively. This study offers a new in vitro system to understand the interplay between HUVEC monolayers and monocytes mediated by aligned topographies, which may be useful for vascular repair and disease modeling for drug testing.

This study offers a new in vitro system to understand the interplay between HUVEC monolayer and monocytes mediated by aligned topographies, which may be useful for vascular repair and disease modeling for drug testing.

Wrinkling of an Enteric Coating Induced by Vapor-Deposited Stimuli-Responsive Hydrogel Thin Films

In this contribution, we report on the thin-film synthesis of a thermoresponsive polymer onto another polymer used as an enteric coating in drug applications. In particular, we deposit cross-linked poly(N-vinylcaprolactam) (pNVCL) thin films by initiated chemical vapor deposition (iCVD) onto spin-coated Eudragit (EUD) layers. Already upon iCVD synthesis, the layered structure starts to form wrinkles at a minimum iCVD thickness of 30 nm. By changing the EUD layer thickness and the amount of cross-linking used during iCVD, the morphology of the wrinkles is demonstrated to be readily tunable. Laterally, the double-layer structures vary in morphology from being ultrasmooth to exhibiting up to a 3.5 μm wrinkle wavelength. The surface roughness and, thus, the wrinkles' height can be tailored from below 1 nm up to 100 nm. From the resulting wavelength of wrinkles, an estimation of the elastic modulus of pNVCL proves its tunability over a wide range of values thanks to the iCVD process. This study elucidates an uncomplicated way to tune the wrinkles' morphology and, thus, the surface area of a system that can be employed in drug delivery applications. Hence, an enteric coating of EUD together with an iCVD-synthesized thermoresponsive thin film is proposed as a promising composite encapsulation layer to outperform established systems in terms of tunability of the response to multiple external stimuli.

2.5D Hierarchical Structuring of Nanocomposite Hydrogel Films Containing Cellulose Nanocrystals

Although two-dimensional hydrogel thin films have been applied across many biomedical applications, creating higher dimensionality structured hydrogel interfaces would enable potentially improved and more biomimetic hydrogel performance in biosensing, bioseparations, tissue engineering, drug delivery, and wound healing applications. Herein, we present a new and simple approach to control the structure of hydrogel thin films in 2.5D. Hybrid suspensions containing cellulose nanocrystals (CNCs) and aldehyde- or hydrazide-functionalized poly(oligoethylene glycol methacrylate) (POEGMA) were spin-coated onto prestressed polystyrene substrates to form cross-linked hydrogel thin films. The films were then structured via thermal shrinking, with control over the direction of shrinking leading to the formation of biaxial, uniaxial, or hierarchical wrinkles. Notably, POEGMA-only hydrogel thin films (without CNCs) did not form uniform wrinkles due to partial dewetting from the substrate during shrinking. Topographical feature sizes of CNC-POEGMA films could be tuned across 2 orders of magnitude (from ∼300 nm to 20 μm) by varying the POEGMA concentration, the length of poly(ethylene glycol) side chains in the polymer, and/or the overall film thickness. Furthermore, by employing adhesive masks during the spin-coating process, structured films with gradient wrinkle sizes can be fabricated. This precise control over both wrinkle size and wrinkle topography adds a level of functionality that to date has been lacking in conventional hydrogel networks.

PHEMA hydrogel films crosslinked with dynamic disulfide bonds: synthesis, swelling-induced mechanical instability and self-healing

Undesired swelling-induced instability patterns on substrate-attached hydrogel films can be self-healed by the introduction of dynamic covalent bonds.