Programmable Chemical Gradient Patterns by Soft Grayscale Lithography

Abstract

A method for fabricating chemical gradients on planar and nonplanar substrates using grayscale lithography is reported. Compliant grayscale amplitude masks are fabricated using a vacuum‐assisted microfluidic filling protocol that employs dilutions of a carbon‐black‐containing polydimethylsiloxane emulsion (bPDMS) within traditional clear PDMS (cPDMS) to create planar, fully self‐supporting mask elements. The mask is then placed over a surface functionalized with a hydrophobic coumarin‐based photocleavable monolayer, which exposes a polar group upon irradiation. The mask serves to modulate the intensity of incident UV light, thereby controlling the density of molecules cleaved. The resulting molecular‐level grayscale patterns are characterized by condensation microscopy and imaging mode time‐of‐flight secondary‐ion mass spectrometry (ToF‐SIMS). Due to the inherent flexibility of this technique, the photofuse as well as the gradient patterns can be designed for a wide range of applications; in this paper two proof‐of‐concept demonstrations are shown. The first utilizes the ability to control the resulting contact angle of the surface for the fabrication of a passive pressure‐sensitive microfluidic gating system. The second is a model surface modification process that utilizes the functional groups deprotected during the photocleavage to pattern the deposition of moieties with complementary chemistry. The spatial layout, resolution, and concentration of these covalently linked molecules follow the gradient pattern created by the grayscale mask during exposure. The programmable chemical gradient fabrication scheme presented in this work allows explicit engineering of both surface properties that dictate nonspecific interactions (surface energy, charge, etc.) and functional chemistry necessary for covalent bonding.

Mechanical approach to chemical transport

“…This situation arises in practical applications. Experiments are coming online that can expose nonhomogeneous systems such as live cells or electrochemical interfaces to programmable spatiotemporal patterns of temperature, chemical composition, osmotic pressure, light exposure, and other variables (18)(19)(20)26).…”

“…The approach does not consider convection or Fourier heat flow in the system, and transport phenomena or chemical reactions should not perturb the externally imposed temperature gradients. Nevertheless, the general model presented here may be used to describe the processes besides traditional physical chemistry applications such as live cell kinetics, electrochemical transport, or spatially patterned perturbation of self-assembled monolayers with simultaneous reactions and diffusion (18)(19)(20)27). Furthermore, the random walk view of consumption (31) and various economic optimization problems can be formulated based on Lagrangians with logarithmic terms.…”

“…We believe that such a model could be useful for describing transport in electrochemistry, spatially heterogeneous kinetics, biophysics, and other areas where position-dependent forces are applied to a system. For example, temperature jumps and photoexcitation patterned with spatial light modulators, chemical and osmotic pressure gradients, or other complex perturbations can nowadays be applied to nonhomogeneous systems ranging from selfassembled monolayers to live cells (18)(19)(20) to induce transport phenomena.…”

Recent Advances in Functional Polymers Containing Coumarin Chromophores

“…The irreversible response of (coumarin-4-yl)methyl derivatives upon light exposure has been particularly exploited as a tool for temporally and spatially controlled probes of cell-based processes [45,46] as well as for the generation of patterned surfaces in different field of applications including bioanalytical science, cell biology, tissue engineering, etc. [47][48][49][50][51]. Moreover, its ability to undergo photolysis by nonresonant two-photon excitation ( Figure 3), at high light intensity, facilitates salient properties of this chromophore such as: (a) "phototherapeutic window" between 650 and 950 nm with lower scattering and reduced phototoxic effect [46] and (b) orthogonal protecting group based on wavelengthselective response [51].…”

High‐Precision Temperature‐Controllable Metal‐Coated Polymeric Molds for Programmable, Hierarchical Patterning

“…Lithography has been a key element in the field of planar processing, which is used in optical devices, lab‐on‐a‐chip, biomedical applications and advanced semiconductor manufacturing . In particular, conventional photolithography, which creates micro‐ and nanoscale structures with high‐throughput and reproducibility at the wafer scale, has greatly contributed to state‐of‐the‐art manufacturing processes in the semiconductor and electronics industry.…”