Ramakrishnan Ganesan

Multicomponent protein patterning of material surfaces

Micro- and nano-scale protein patterns have gained significant technological interest. While certain techniques for single-component protein patterning are well-established, multicomponent protein patterning approaches are a current topic of intensive research, which might enable complex biosensor systems and expand the knowledge in protein-protein and protein-cell or cell-cell interactions. Only a few patterning methods are suitable for the realization of three dimensional patterns, which are essential for many applications e.g. in the design of scaffolds for regenerative therapies. In this feature article representative approaches for creating multicomponent protein patterning are presented and their potential for tailoring microenvironments for cells on biomaterials surfaces is discussed.

Simple micropatterning of biomolecules on a diazoketo-functionalized photoresist

A simple strategy for the patterning of cells and biomolecules including DNA and protein was developed, employing novel diazoketo-functionalized photoresists in conjunction with a simplified lithographic process that does not require photoacid generator, post-exposure bake or development steps. The diazoketo functional groups present in the polymers undergo Wolff rearrangement upon UV light irradiation to generate carboxylic groups. This chemistry was used to create patterns of alternate hydrophilic/hydrophobic regions on the surface of diazoketo-functionalized polymers. The in vitro cell culture on the patterned surfaces of the polymer showed good alignment of cells on the UV-exposed regions, where carboxylic groups were predominant. Furthermore, cells were found to maintain their alignment during proliferation. In addition to this, the photogeneration of carboxylic groups by diazoketo functional groups was exploited for DNA and streptavidin patterning. For DNA patterning, amine-modified probe DNA was successfully immobilized on the photopatterned regions of the polymer by amide bond formation using EDC/NHS coupling chemistry. Successful hybridization with complementary DNA proved the selectivity and functionality of the immobilized probe DNA. Biotin-amine was also immobilized on the photopatterned regions of the diazoketo-functionalized polymer in a similar manner to that of probe DNA to study streptavidin patterning. Biotin-specific streptavidin binding proved the successful patterning of biotin-amine by amide bond formation. These results showed that diazoketo-functionalized photoresist is biocompatible and simple to use for biomolecular patterning, which can be used in further approaches for high-throughput screening assays.

Photobleachable silicon-containing molecular resist for deep UV lithography

A novel molecular resist material based on polyhedral oligomeric silsesquioxane, possessing diazoketo groups, was successfully synthesized for deep UV lithography. The initial lithographic evaluation of the molecular resist shows the potential of the new platform for the next generation resists.

Photosensitive polymer brushes grafted onto PTFE film surface for micropatterning of proteins

Surface grafting of photosensitive polymer brushes on a flexible polymer surface was performed using Ar ion irradiation, which generated peroxides on the polymer surface upon exposure to air. These peroxides were utilized as initiators for graft polymerization of a photosensitive diazoketo-functionalized methacrylate monomer. Upon UV light exposure, the diazoketo group was converted into the carboxylic acid group which was used to covalently immobilize amine-functionalized biotin in a patterned fashion. Successful binding of streptavidin to the biotin-linked regions proved the potential of the platform for biomolecular patterning applications on commercial polymer substrates.

Photoactive diazoketo-functionalized self-assembled monolayer for biomolecular patterning.

A diazoketo-functionalized alkoxysilane compound was synthesized, and its self-assembled monolayer (SAM) formation was studied on glass and silicon substrates. Infrared spectroscopy was employed to follow the photoreaction in this system, which proved that carboxylic groups were generated from diazoketo groups in the surface of the substrate. Photopatterning of biotin/streptavidin on the diazoketo-functionalized SAM shows the potential of this novel platform for biomolecular patterning.

Negative nanomolecular resists based on calix[4]resorcinarene

Negative working nanomolecular resists based on fully epoxy-protected tetra-Cmethylcalix[4]resorcinarene (epoxy C-4-R) and oxetanyl-protected tetra-methylcalix[4]resocinarene (oxetanyl C-4-R) have been developed. They were prepared by the reaction of C-4-R with epichlorohydrin or in the presence of trimethylamine. They can be coated on the silicon wafer by spin-coating method. A clear film cast from a 20 wt% epoxy C-4-R solution in chloroform showed high transparency to UV above 300 nm. A fine negative image featuring 0.8 μm of minimum line and space patterns was observed on the film of the photoresist exposed to 40 mJ/ cm2 of Near UV-light by the contact mode.

Local pH-Responsive Diazoketo-Functionalized Photoresist for Multicomponent Protein Patterning

Selective surface immobilization of multiple biomolecule components, under mild conditions where they do not denature, is attractive for applications in biosensors and biotechnology. Here, we report on a biocompatible and pH-responsive photoresist containing diazoketo-functionalized methacrylate, methacrylic acid, and poly(ethylene glycol) methacrylate monomers, where the photolithographic process may be carried out in a local pH range to minimize biomolecular denaturation. The polymer is insoluble or sparsely soluble in pH 6.4 or more acidic solution or deionized water, but soluble in a basic solution, pH 7.9 or more. After UV exposure, however, carboxylic acid groups are generated by Wolff rearrangement and photodissociation of the diazoketo groups in the polymer chain, leading to dissolution of UV-exposed polymer at pH 6.4. Using the property of the pH-solubility switching, we demonstrate dual streptavidin patterning using only biological buffers, pH 6.4 and 7.9 solutions, and double exposure patterning to confirm the sustainability of the diazoketo groups in unexposed regions despite carrying out several wet processes.

Top surface imaging study by selective chemisorptions of poly(dimethyl siloxane) on diazoketo-functionalized polymeric surface

Top surface imaging (TSI) techniques using vapor or liquid phase silylation have been investigated extensively as alternatives to conventional resist processing. However, earlier imaging schemes such as diffusion enhanced silylated resist (DESIRE) and digital top surface imaging showed several difficulties limiting the successful application of such TSI approaches. In the case of DESIRE, additional CF4 plasma descum process was required to remove the thin layer of Si incorporated into the cross-linked regions, as some of the Si remained even in the unexposed regions. Also, difference in the cross-linking density and subsequent amount of silicon incorporation across the width of an optically projection printed feature led to non-uniform silylation profiles resulting the difficulty with critical dimension (CD) control of the feature and increased the LER of the overall process. In the case of digital TSI, even though it was developed to overcome these problems with the cross-linking-based silylation process, the concentration of active sites in the exposed polymer varies across the feature width due to the non-uniform energy deposition profile across a feature which results from the non-ideal aerial image produced using optical projection tools. In this study, we have used a diazoketo-functionalized polymer as the platform for the immobilization of amine-functionalized poly(dimethyl siloxane) (amine-PDMS). The diazoketo functional groups undergo Wolff rearrangement to generate carboxylic acid groups upon UV light exposure. This chemistry is exploited to create alternate hydrophilic/hydrophobic patterned regions by selective UV light exposure. The hydrophilic regions that contain carboxylic acid groups predominantly are further used to immobilize amine-PDMS by amide bond formation using carbodiimide coupling chemistry. Due to the high silicon content, the immobilized PDMS acts as the etch barrier for the subsequent oxygen plasma reactive ion etching (O2-RIE) process. Thus, a negative-tone pattern has been successfully generated using O2-RIE process. An amine-PDMS with a molecular weight of 900 was used in this study. Auger electron spectroscopy was employed to characterize the immobilization of amine-PDMS onto UV light exposed regions of diazoketo-functionalized polymer surface. Atomic force microscopy was used to study the surface smoothness after O2-RIE process. Scanning electron microscopy was used to image the pattern profiles formed after O2-RIE process. High resolution pattern profiles are obtained using the TSI process reported in this study.

Nonchemically amplified resists for deep-UV lithography

A novel monomer containing a diazoketo functional group was designed and synthesized. Polymers were synthesized using the diazoketo-functionalized monomer and their physical properties were evaluated. The polymers were synthesized by radical copolymerization of cholic acid 3-diazo-3-ethoxycarbonyl-2-oxo-propyl ester methacrylate, methyl methacrylate, and γ-butyrolacton-2-yl methacrylate. These polymers showed 0.7 &mgr;m line and space patterns using a mercury-xenon lamp in a contact printing mode.

Bilayer resists based on polyhedral oligomeric silsesquioxane for 193-nm lithography

A novel nanomolecular resist based on POSS substituted with diazodiketo-functionalized cholate derivatives was successfully synthesized as a candidate for 193-nm lithography. The diazodiketo group was introduced into the cholate derivatives to provide the solubility change and to eliminate the problems of chemically amplified resists. The decomposition temperature of the resist was found to be 130°C. The initial lithographic studies showed the feasibility of the resist to be used as a candidate for 193-nm lithography.