Porous multilayer-coated PDMS stamps for protein printing

Conventional and emerging strategies for the fabrication and functionalization of PDMS-based microfluidic devices

Microfluidics is an emerging and multidisciplinary field that is of great interest to manufacturers in medicine, biotechnology, and chemistry, as it provides unique tools for the development of point-of-care diagnostics, organs-on-chip systems, and biosensors. Polymeric microfluidics, unlike glass and silicon, offer several advantages such as low-cost mass manufacturing and a wide range of beneficial material properties, which make them the material of choice for commercial applications and high-throughput systems. Among polymers used for the fabrication of microfluidic devices, polydimethylsiloxane (PDMS) still remains the most widely used material in academia due to its advantageous properties, such as excellent transparency and biocompatibility. However, commercialization of PDMS has been a challenge mostly due to the high cost of the current fabrication strategies. Moreover, specific surface modification and functionalization steps are required to tailor the surface chemistry of PDMS channels (e.g. biomolecule immobilization, surface hydrophobicity and antifouling properties) with respect to the desired application. While significant research has been reported in the field of PDMS microfluidics, functionalization of PDMS surfaces remains a critical step in the fabrication process that is difficult to navigate. This review first offers a thorough illustration of existing fabrication methods for PDMS-based microfluidic devices, providing several recent advancements in this field with the aim of reducing the cost and time for mass production of these devices. Next, various conventional and emerging approaches for engineering the surface chemistry of PDMS are discussed in detail. We provide a wide range of functionalization techniques rendering PDMS microchannels highly biocompatible for physical or covalent immobilization of various biological entities while preventing non-specific interactions.

Effect of Functional Groups of Self-Assembled Monolayers on Protein Adsorption and Initial Cell Adhesion

Surface modification plays a vital role in regulating protein adsorption and subsequently cell adhesion. In the present work, we prepared nanoscaled modified surfaces using silanization and characterized them using Fourier-transform infrared spectroscopy (FTIR), water contact angle (WCA), and atomic force microscopy (AFM). Five different (amine, octyl, mixed, hybrid, and COOH) surfaces were prepared based on their functionality and varying wettability and their effect on protein adsorption and initial cell adhesion was investigated. AFM analysis revealed nanoscale roughness on all modified surfaces. Fetal bovine serum (FBS) was used for protein adsorption experiment and effect of FBS was analyzed on initial cell adhesion kinetics (up to 6 h) under three different experimental conditions: (a) with FBS in media, (b) with preadsorbed FBS on surfaces, and (c) incomplete media, i.e., without FBS. Various cell features such as cell morphology/circularity, cell area and nuclei size were also studied for the above stated conditions at different time intervals. The cell adhesion rate as well as cell spread area were highest in the case of surfaces with preadsorbed FBS. We observed higher surface coverage rate by adhering cells on hybrid (rate, 0.073 h^-1) and amine (0.072 h^-1) surfaces followed by COOH (0.062 h^-1) and other surfaces under preadsorbed FBS condition. Surface treated with cells in incomplete media exhibited least adhesion rate, poor cell spreading and improper morphology. Furthermore, we found that initial cell adhesion rate and Δadhered cells (%) linearly increased with the change in α-helix content of adsorbed FBS on surfaces. Among all the modified surfaces and under all three experimental conditions, hybrid surface exhibited excellent properties for supporting cell adhesion and growth and hence can be potentially used as surface modifiers in biomedical applications to design biocompatible surfaces.

Covalent layer-by-layer films: chemistry, design, and multidisciplinary applications

Covalent layer-by-layer (LbL) assembly is a powerful method used to construct functional ultrathin films that enables nanoscopic structural precision, componential diversity, and flexible design. Compared with conventional LbL films built using multiple noncovalent interactions, LbL films prepared using covalent crosslinking offer the following distinctive characteristics: (i) enhanced film endurance or rigidity; (ii) improved componential diversity when uncharged species or small molecules are stably built into the films by forming covalent bonds; and (iii) increased structural diversity when covalent crosslinking is employed in componential, spacial, or temporal (labile bonds) selective manners. In this review, we document the chemical methods used to build covalent LbL films as well as the film properties and applications achievable using various film design strategies. We expect to translate the achievement in the discipline of chemistry (film-building methods) into readily available techniques for materials engineers and thus provide diverse functional material design protocols to address the energy, biomedical, and environmental challenges faced by the entire scientific community.

A Highly Sensitive Humidity Sensor Based on Ultrahigh-Frequency Microelectromechanical Resonator Coated with Nano-Assembled Polyelectrolyte Thin Films

We developed a highly sensitive humidity sensor based on the combination of ultrahigh-frequency film bulk acoustic resonator (FBAR) and nano-assembled polyelectrolyte (PET) thin films. The water molecule absorption efficiency was optimized by forming loosely-packed PET nanostructures. Then, the humidity sensing characteristics were analyzed in terms of sensitivity, linearity, reversibility, stability and detection limit. As a result, PET-coated FBAR exhibits excellent humidity sensitivity of 2202.20 Hz/ppm, which is five orders of magnitude higher than quartz crystal microbalance (QCM). Additionally, temperature dependence was investigated with the result that PET-coated FBAR possessed a higher sensitivity at low temperature. Furthermore, we realized the selective detection of water vapor from volatile organic compounds (VOCs) with respect to the polarity property. Owing to the high sensitivity, miniaturized size and ultrahigh operating frequency, PET-coated FBAR is uniquely favorable as a wireless humidity sensor node to integrate into wireless sensor networks (WSNs).

Self-assembled Biodegradable Nanoparticles and Polysaccharides as Biomimetic ECM Nanostructures for the Synergistic effect of RGD and BMP-2 on Bone Formation

Producing biomimetic extracellular matrix (ECM) is an effective approach to improve biocompatibility of medical devices. In this study, biomimetic ECM nanostructures are constructed through layer-by-layer self-assembling positively charged chitosan (Chi), negatively charged oxidized sodium alginate (OAlg), and positively charged bovine serum albumin (BSA)-based nanoparticles. The BSA-based nanoparticles in the self-assembled films not only result in porous nanostructures similar to natural ECM, but also preserve the activity and realize the sustained release of Bone morphogenetic protein-2 (BMP-2). The results of bone marrow stem cells (BMSCs) culture demonstrate that the penta-peptide glycine-arginine-glycine-aspartate-serine (GRGDS) grafted Chi/OAlg films favor cell adhesion and proliferation. GRGDS and BMP-2 in biomimetic ECM nanostructures synergistically promote BMSC functions and new bone formation. The RGD and BMP incorporated biomimetic ECM coatings could be applied on a variety of biomedical devices to improve the bioactivity and biocompatibility.

Tuning the resonant frequency of resonators using molecular surface self-assembly approach

In this work, a new method to tune the resonant frequency of microfabricated resonator using molecular layer-by-layer (LbL) self-assembly approach is demonstrated. By simply controlling the polymer concentration and the number of layers deposited, precisely tuning the frequency of microfabricated resonators is realized. Due to its selective deposition through specific molecular recognitions, such technique avoids the high-cost and complex steps of conventional semiconductor fabrications and is able to tune individual diced device. Briefly, film bulk acoustic resonator (FBAR) is used to demonstrate the tuning process and two types of LbL deposition methods are compared. The film thickness and morphology have been characterized by UV-vis reflection spectra, ellipsometer and AFM. As a result, the maximum resonant frequency shift of FBAR reaches more than 20 MHz, meaning 1.4% tunability at least. The minimum frequency shift is nearly 10 kHZ per bilayer, indicating 7 ppm tuning resolution. Pressure cooker test (PCT) is performed to evaluate the reliability of LbL coated FBAR. Furthermore, applications for wireless broadband communication and chemical sensors of LbL coated FBAR have been demonstrated.

In situ synthesis of large-area single sub-10 nm nanoparticle arrays by polymer pen lithography

In order to take advantage of the unique properties of nanoparticles in integrated devices, it is desirable to position monodispersed nanoparticles on substrates with controlled placement. Herein, we utilize small molecules such as ethylene glycol (EG) or glycerol to facilitate the delivery of nanoparticle precursors to the substrates in the polymer pen lithography (PPL) process. Subsequently, large-area ordered single nanoparticle arrays, including sub-10 nm Ag nanoparticle, 30 nm Au nanoparticle and 80 nm Fe2O3 nanoparticle arrays have been synthesized in situ with controllable sizes and pitches.

Templated nanopores for robust functional surface porosity in poly(methyl methacrylate)

A novel "sink and etch" technique is used to generate stable surface nanoporosity in poly(methyl methacrylate). Layer-by-layer assembly is first used to conformally coat PMMA substrates with a uniform layer of silica nanoparticles. Thermal annealing is then applied to cause sinking and engulfment of the silica nanoparticles into the thermoplastic PMMA surface. By selectively etching away the layer of embedded silica nanoparticles, a conformal porous layer of inversely templated structure can be obtained in the PMMA surface. Characterization with atomic force microscopy shows that a variety of nanoporous surface morphologies can be achieved simply by controlling the duration and temperature of thermal annealing. The nanoporous surfaces consisting of either as assembled silica nanoparticles or templated inverse porosity in PMMA were compared in terms of their antireflective (AR) properties. Measuring AR properties provided a quantitative means to compare the stability of these porous AR surfaces before and after several cleaning cycles. Our results show that while both types of surface porosity can provide excellent AR properties (optimized for 300-400 nm), the porous layer generated by the "sink and etch" technique showed superior mechanical stability.

Surface patterning by microcontact chemistry

In this Feature Article we describe recent progress in covalent surface patterning by microcontact chemistry. Microcontact chemistry is a variation of microcontact printing based on the transfer of reactive "ink" molecules from a microstructured, elastomeric stamp onto surfaces modified with complementary reactive groups, leading to a chemical reaction in the area of contact. In comparison with other lithographic methods, microcontact chemistry has a number of advantageous properties including very short patterning times, low consumption of ink molecules, high resolution and large area patterning. During the past 5 years we and many others have investigated a set of different reactions that allow the modification of flat and also spherical surfaces in an effective way. Especially click-type reactions were found to be versatile for substrate patterning by microcontact chemistry and were applied for chemical modification of reactive self-assembled monolayers and polymer surfaces. Microcontact chemistry has already found broad application for the production of functional surfaces and was also used for the preparation of DNA, RNA, and carbohydrate microarrays, for the immobilization of proteins and cells and for the development of sensors.

Unconventional layer-by-layer assembly: surface molecular imprinting and its applications

Layer-by-layer assembly (LbL) is a rich, versatile, and powerful technique for fabricating multilayer thin films with controlled architecture and functions. Singly charged, uncharged, or water-repellent molecules cannot be used directly in conventional LbL assembly. This problem can be solved with unconventional LbL methods, by employing the preassembly of building blocks in solution and the use of these assemblies for LbL formation at the interface. This Concept summarizes different methods of unconventional LbL assembly, including electrostatic complex formation, hydrogen-bonded complexes, block-copolymer micelles, and π-π interaction complexes. These preassembly treatments endow the building blocks with enhanced abilities for advanced functionality, in particular, surface molecular imprinting, a new concept emerging from unconventional LbL. Molecular imprinting approaches are thus conceptually described based on different types of interactions and their great potential in applications is demonstrated by examples such as selective surface patterning and selective filtration.

Surface molecular imprinted layer-by-layer film attached to a porous membrane for selective filtration

A surface molecular imprinted layer-by-layer (SMILbL) film was fabricated on a polyethersulfone (PES) porous membrane substrate for selective filtration of cations and anions. The LbL deposition procedure and the ultraviolet (UV) cross-linking of the modified membrane were monitored by attenuated total reflection-infrared (ATR-IR) spectra. The SMILbL-PES membrane with 4.5 bilayers of diazoresin (DAR)/poly(acrylic acid) complexed with 5,10,15,20-tetrakis(4-(trimethylammonio)-phenyl)-21H,23H-porphyrin tetratosylate (PAA-Por(4+)) effectively reduced the permeation velocity of Por(4+) after washing the Por(4+) template out. In comparison to a control film DAR/PAA-modified PES membrane (ConLbL-PES) in a dialysis experiment, the SMILbL-PES membrane exhibited better selectivity for permeation of 5,10,15,20-tetraphenyl-21H,23H-porphine-p,p',p″,p‴-tetrasulfonic acid tetrasodium hydrate (Por(4-)) against permeation of Por(4+). In pressure-driven transport experiments, the SMILbL-PES membrane showed a much longer blocking time for Por(4+) than for Por(4-), indicating the selective loading of Por(4+) into the SMILbL film. The surface charge of the SMILbL-PES membrane after Por(4+) loading was higher than that of other membranes, resulting in an enhanced rejection ability of the SMILbL-PES membrane to Por(4+) caused by Coulomb repulsion. A possible mechanism was proposed as follows. The binding sites generated through imprinting in the SMILbL-PES membrane enable loading of a larger amount of Por(4+). The stronger repulsion between Por(4+) and the SMILbL film may cause the main contribution to the selective rejection of Por(4+). It can be easily imagined that this concept can be extended to the construction of composite membranes from other imprinting systems.

Layer-by-layer assembly of salt-containing polyelectrolyte complexes for the fabrication of dewetting-induced porous coatings

The layer-by-layer (LbL) assembly of salt-containing nonstoichiometric polyelectrolyte complexes (PECs) with oppositely charged uncomplexed polyelectrolyte for the fabrication of dewetting-induced porous polymeric films has been systematically investigated. Salt-containing poly(allylamine hydrochloride) (PAH) and poly(acrylic acid) (PAA) complexes (noted as PAH-PAA) with a molar excess of PAH were LbL assembled with polyanion poly(sodium 4-styrenesulfonate) (PSS) to produce PSS/PAH-PAA films. The structure of the PAH-PAA complexes is dependent on the concentration of NaCl added to their aqueous dispersions, which can be used to tailor the structure of the LbL-assembled PSS/PAH-PAA films. Porous PSS/PAH-PAA films are fabricated when salt-containing PAH-PAA complexes with a large amount of added NaCl are used for LbL assembly with PSS. In-situ and ex-situ atomic force microscopy measurements disclose that the dewetting process composed of pore nucleation and pore growth steps leads to the formation of pores in the LbL-assembled PSS/PAH-PAA films. The present study provides a facile way to fabricate porous polymeric films by dewetting LbL-assembled polymeric films comprising salt-containing PECs.

Selectively erasable multilayer thin film by photoinduced disassembly

A multilayer film consisting of poly(acrylic acid) (PAA) and an azobenzene-containing surfactant (AzoTEA) was fabricated via a layer-by-layer assembly technique. The micellar structure favored by AzoTEA while in solution results in a bilayer structure when deposited on a substrate surface. This aggregation conversion behavior favors the deposition of AzoTEA and PAA in an alternating pattern to form a photoresponsive multilayer film. The molecular amphiphilicity of AzoTEA can be tuned by photoisomerization of the azobenzene moiety, which affects the aggregation behavior of AzoTEA in both films and solutions. The disassembly of AzoTEA aggregates caused by photoirradiation can induce the disassembly of the whole multilayer film. The AzoTEA-PAA multilayer films were found to be stable in pH 4 acetic acid (AcOH) solution, unless treated with UV radiation. On the basis of the different stability of multilayers with or without photoirradiation, when the multilayer films are selectively irradiated with UV light, the regions exposed to UV radiation disassembled after being immersed in pH 4 AcOH solution for 10 min but the regions not exposed to photoradiation are maintained on the substrate. Moreover, the plausible mechanism for the assembly and disassembly of these multilayer films and the confirmation of erasable films by atomic force microscopy are discussed.