Surface modification of SU-8 for enhanced cell attachment and proliferation within microfluidic chips

Rapid Prototyping of Organ-on-a-Chip Devices Using Maskless Photolithography

Organ-on-a-chip (OoC) and microfluidic devices are conventionally produced using microfabrication procedures that require cleanrooms, silicon wafers, and photomasks. The prototyping stage often requires multiple iterations of design steps. A simplified prototyping process could therefore offer major advantages. Here, we describe a rapid and cleanroom-free microfabrication method using maskless photolithography. The approach utilizes a commercial digital micromirror device (DMD)-based setup using 375 nm UV light for backside exposure of an epoxy-based negative photoresist (SU-8) on glass coverslips. We show that microstructures of various geometries and dimensions, microgrooves, and microchannels of different heights can be fabricated. New SU-8 molds and soft lithography-based polydimethylsiloxane (PDMS) chips can thus be produced within hours. We further show that backside UV exposure and grayscale photolithography allow structures of different heights or structures with height gradients to be developed using a single-step fabrication process. Using this approach: (1) digital photomasks can be designed, projected, and quickly adjusted if needed; and (2) SU-8 molds can be fabricated without cleanroom availability, which in turn (3) reduces microfabrication time and costs and (4) expedites prototyping of new OoC devices.

Biocompatibility of SU-8 and Its Biomedical Device Applications

SU-8 is an epoxy-based, negative-tone photoresist that has been extensively utilized to fabricate myriads of devices including biomedical devices in the recent years. This paper first reviews the biocompatibility of SU-8 for in vitro and in vivo applications. Surface modification techniques as well as various biomedical applications based on SU-8 are also discussed. Although SU-8 might not be completely biocompatible, existing surface modification techniques, such as O₂ plasma treatment or grafting of biocompatible polymers, might be sufficient to minimize biofouling caused by SU-8. As a result, a great deal of effort has been directed to the development of SU-8-based functional devices for biomedical applications. This review includes biomedical applications such as platforms for cell culture and cell encapsulation, immunosensing, neural probes, and implantable pressure sensors. Proper treatments of SU-8 and slight modification of surfaces have enabled the SU-8 as one of the unique choices of materials in the fabrication of biomedical devices. Due to the versatility of SU-8 and comparative advantages in terms of improved Young’s modulus and yield strength, we believe that SU-8-based biomedical devices would gain wider proliferation among the biomedical community in the future.

Chemistries for Making Additive Nanolithography in OrmoComp Permissive for Cell Adhesion and Growth

Two-photon lithography allows writing of arbitrary nanoarchitectures in photopolymers. This design flexibility opens almost limitless possibilities for biological studies, but the acrylate-based polymers frequently used do not allow for adhesion and growth of some types of cells. Indeed, we found that lithographically defined structures made from OrmoComp do not support E18 murine cortical neurons. We reacted OrmoComp structures with several diamines, thereby rendering the surfaces directly permissive for neuron attachment and growth by presenting a surface coating similar to the traditional cell biology coating achieved with poly-d-lysine (PDL) and laminin. However, in contrast to PDL-laminin coatings that cover the entire surface, the amine-terminated OrmoComp structures are orthogonally modified in deference to the surrounding glass or plastic substrate, adding yet another design element for advanced biological studies.

The crossing and integration between microfluidic technology and 3D printing for organ-on-chips

Organ-on-chips were designed to simulate the real tissue or organ microenvironment by precise control of the cells, the extracellular matrix and other micro-environmental factors to clarify physiological or pathological mechanisms. The organ chip is mainly based on the poly(dimethylsiloxane) (PDMS) microfluidic devices, whereas the conventional soft lithography requires a cumbersome manufacturing process, and the complex on-chip tissue or organ chip also depends on the complicated loading process of the cells and biomaterials. 3D printing can efficiently design and automatically print micrometre-scale devices, while bio-printing can also precisely manipulate cells and biomaterials to create complex organ or tissue structures. In recent years, the popularization of 3D printing has provided more possibilities for its application to 3D printed organ-on-chips. The combination of 3D printing and microfluidic technology in organ-on-chips provides a more efficient choice for building complex flow channels or chambers, as well as the ability to create biological structures with a 3D cell distribution, heterogeneity and tissue-specific function. The fabrication of complex, heterogeneous 3D printable biomaterials based on microfluidics also provides new assistance for building complex organ-on-chips. Here, we discuss the recent advances and potential applications of 3D printing in combination with microfluidics to organ-on-chips and provide outlooks on the integration of the two technologies in building efficient, automated, modularly integrated, and customizable organ-on-chips.

Optical µ-Printing of Cellular-Scale Microscaffold Arrays for 3D Cell Culture

Guiding cell culture via engineering extracellular microenvironment has attracted tremendous attention due to its appealing potentials in the repair, maintenance, and development of tissues or even whole organs. However, conventional biofabrication technologies are usually less productive in fabricating microscale three-dimensional (3D) constructs because of the strident requirements in processing precision and complexity. Here we present an optical µ-printing technology to rapidly fabricate 3D microscaffold arrays for 3D cell culture and cell-scaffold interaction studies on a single chip. Arrays of 3D cubic microscaffolds with cubical sizes matching the single-cell size were fabricated to facilitate cell spreading on suspended microbeams so as to expose both apical and basal cell membranes. We further showed that the increasing of the cubical size of the microscaffolds led to enhanced spreading of the seeded human mesenchymal stem cells and activation of mechanosensing signaling, thereby promoting osteogenesis. Moreover, we demonstrated that the spatially selective modification of the surfaces of suspended beams with a bioactive coating (gelatin methacrylate) via an in-situ printing process allowed tailorable cell adhesion and spreading on the 3D microscaffolds.