Low‐Distortion Surface Functionalization of Polymeric Microstructures

Abstract

Polymeric microstructures can be immersed in an organic solution containing a reactive linker to functionalize the surface for attachment of nanoparticles, fluorescent dyes, proteins, DNA, and other species. However, organic media can swell and distort polymeric microstructures. Swelling and distortion can be irreversible when the linkers are themselves low‐molecular‐weight organics like ethylenediamine, which can bind to unreacted monomer groups or other functionalities within the polymer matrix. This work introduces an alternative approach for surface functionalization based on aqueous processing using higher‐molecular‐weight amines, which causes less distortion, but is comparably effective. The processes are compared by aminating crosslinked SU‐8 thin films and 3D “woodpile” microstructures, electrolessly depositing copper onto these primed surfaces, and measuring the amount of copper deposited and the degree of distortion caused by amination. The method provides a new route to low‐distortion SU‐8 microstructures and identifies a path for improving related processing with other polymeric materials and structure types.

Femtosecond‐Laser Direct Writing of Metallic Micro/Nanostructures: From Fabrication Strategies to Future Applications

“…This metallization method is not limited to silver plating but is also feasible in the case of other metals. Later, selective copper coating onto SU-8 microstructures was achieved in a similar fashion, [88,105] demonstrating the universal nature of this approach.…”

Fabrication of Functional Microdevices in SU-8 by Multi-Photon Lithography

“…For example, acrylic polymers swell when immersed in polar solvents like alcohols and much less in water. Although polymerized SU-8 can swell in water due to the hydroxyl groups formed upon polymerization, the dimensional variation is typically minimal and saturates in minutes [ 82 , 83 ]. This makes SU-8 an ideal material for microfluidic application where aqueous solutions are typically used.…”

“…Amination can be carried out by simply immersing the SU-8 surface in a solution of an alkyl diamine (e.g., ethylenediamine) in ethanol. Kuebler et al reported that this procedure causes irreversible shrinkage and distortion in SU-8 micro-lattices fabricated by MPL because as the ethanol solution of ethylenediamine infuses and swells the structure, it can react with unpolymerized oligomers and wash them away, causing irreversible distortion ( Figure 13 b) [ 83 ]. To remedy this problem, the authors investigated aqueous amination solutions which resulted in much lower irreversible distortion upon amination ( Figure 13 c) [ 83 ].…”

“…Kuebler et al reported that this procedure causes irreversible shrinkage and distortion in SU-8 micro-lattices fabricated by MPL because as the ethanol solution of ethylenediamine infuses and swells the structure, it can react with unpolymerized oligomers and wash them away, causing irreversible distortion ( Figure 13 b) [ 83 ]. To remedy this problem, the authors investigated aqueous amination solutions which resulted in much lower irreversible distortion upon amination ( Figure 13 c) [ 83 ]. Water is not a good solvent for SU-8 and similar polymers compared to alcohols and therefore does not swell the SU-8 polymer as much [ 82 ].…”

“…Additionally, the authors reported that using higher-molecular-weight alternatives to ethylenediamine could hinder diffusion of the amine into the cross-linked polymer and limit amination to the SU-8 surface. SU-8 structures aminated using tetraethylenepentamine (TEPA) in aqueous solution ( Figure 13 c) showed no further distortion compared to the as-fabricated sample ( Figure 13 a) without comprising the surface density of amine group used for metallization [ 83 ].…”