SU-8-based immunoisolative microcontainer with nanoslots defined by nanoimprint lithography

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.

Modulating the foreign body response of implants for diabetes treatment

Diabetes Mellitus is a group of diseases characterized by high blood glucose levels due to patients' inability to produce sufficient insulin. Current interventions often require implants that can detect and correct high blood glucose levels with minimal patient intervention. However, these implantable technologies have not reached their full potential in vivo due to the foreign body response and subsequent development of fibrosis. Therefore, for long-term function of implants, modulating the initial immune response is crucial in preventing the activation and progression of the immune cascade. This review discusses the different molecular mechanisms and cellular interactions involved in the activation and progression of foreign body response (FBR) and fibrosis, specifically for implants used in diabetes. We also highlight the various strategies and techniques that have been used for immunomodulation and prevention of fibrosis. We investigate how these general strategies have been applied to implants used for the treatment of diabetes, offering insights on how these devices can be further modified to circumvent FBR and fibrosis.

Anodic Imprint Lithography: Direct Imprinting of Single Crystalline GaAs with Anodic Stamp

Anodic imprint lithography patterns the GaAs substrate electrochemically by applying a voltage through a predefined anodic stamp. This newly devised technique performs anodic etching in a stamping manner. Stamps that serve as anodic electrodes are fabricated precisely, and the patterns can be imprinted continuously on GaAs substrates. The anodic current locally oxidizes the GaAs through the metal attached to the stamp, and the GaAs oxides are subsequently removed by an acid in the solution. The process is simplified because the metal catalyst is not left on the substrate and the use of an oxidizing agent is not required. Anodic imprint lithography integrates the lithography and etching steps without the use of a polymer resist. Predefined anodic stamps with fin, pillar, and mesh arrays clearly imprinted trenches, holes, and embossed disk arrays on the GaAs substrates, respectively. Anodic imprints replace photons and electrons in conventional lithography with electrochemical stamping, which can simplify existing techniques that are highly complex for extreme nanopatterning.

Polyallylamine as an Adhesion Promoter for SU-8 Photoresist

Resist lithography is an important microfabrication technique in the electronics industry. In this, patterns are transferred by irradiation onto a photosensitive polymer. SU-8 has emerged as a favorite photoresist for High Aspect Ratio (HAR) lithography, showing high chemical and mechanical stability and biocompatibility. Unfortunately, its poor adhesion to substrates is a drawback, with possible solutions being the use of low-viscosity SU-8, surface modification with a low molecular weight adsorbate like hexamethyldisilazane (HMDS), or a commercial adhesion promotion reagent. However, HMDS and the commercial reagent require surface dehydration and/or curing, and a modified form of SU-8 is not always desirable. Here, we demonstrate the use of a water-soluble, amine-containing polymer, polyallylamine (PAAm), which spontaneously adsorbs to silica surfaces, as a simple, easy-to-apply, and reactive adhesion promoter for SU-8. Conditions for the use of PAAm are explored, and the resulting materials are characterized by X-ray photoelectron spectroscopy (XPS), spectroscopic ellipsometry (SE), and wetting.

Membranes to achieve immunoprotection of transplanted islets

Transplantation of islet or beta cells is seen as the cure for type 1 diabetes since it allows physiological regulation of blood glucose levels without requiring any compliance from the patients. In order to circumvent the use of immunosuppressive drugs (and their side effects), semipermeable membranes have been developed to encapsulate and immunoprotect transplanted cells. This review presents the historical developments of immunoisolation and provides an update on the current research in this field. A particular emphasis is laid on the fabrication, characterization and performance of membranes developed for immunoisolation applications.

BioMEMS in drug delivery

The drive to design micro-scale medical devices which can be reliably and uniformly mass produced has prompted many researchers to adapt processing technologies from the semiconductor industry. By operating at a much smaller length scale, the resulting biologically-oriented microelectromechanical systems (BioMEMS) provide many opportunities for improved drug delivery: Low-dose vaccinations and painless transdermal drug delivery are possible through precisely engineered microneedles which pierce the skin's barrier layer without reaching the nerves. Low-power, low-volume BioMEMS pumps and reservoirs can be implanted where conventional pumping systems cannot. Drug formulations with geometrically complex, extremely uniform micro- and nano-particles are formed through micromolding or with microfluidic devices. This review describes these BioMEMS technologies and discusses their current state of implementation. As these technologies continue to develop and capitalize on their simpler integration with other MEMS-based systems such as computer controls and telemetry, BioMEMS' impact on the field of drug delivery will continue to increase.

In vitro and in vivo evaluation of SU-8 biocompatibility

SU-8 negative photoresist is a high tensile strength polymer that has been used for a number of biomedical applications that include cell encapsulation and neuronal probes. Chemically, SU-8 comprises, among other components, an epoxy based monomer and antimony salts, the latter being a potential source of cytotoxicity. We report on the in vitro and in vivo evaluation of SU-8 biocompatibility based on leachates from various solvents, at varying temperatures and pH, and upon subcutaneous implantation of SU-8 substrates in mice. MTT cell viability assay did not exhibit any cytotoxic effects from the leachates. The hemolytic activity of SU-8 is comparable to that of FDA approved implant materials such as silicone elastomer, Buna-S and medical steel. In vivo histocompatibility study in mice indicates a muted immune response to subcutaneous SU-8 implants.

Applications of Semiconductor Fabrication Methods to Nanomedicine: A Review of Recent Inventions and Techniques

We live in a world of convergence where scientific techniques from a variety of seemingly disparate fields are being applied cohesively to the study and solution of biomedical problems. For instance, the semiconductor processing field has been primarily developed to cater to the needs of the ever decreasing transistor size and cost while increasing functionality of electronic circuits. In recent years, pioneers in this field have equipped themselves with a powerful understanding of how the same techniques can be applied in the biomedical field to develop new and efficient systems for the diagnosis, analysis and treatment of various conditions in the human body. In this paper, we review the major inventions and experimental methods which have been developed for nano/micro fluidic channels, nanoparticles fabricated by top-down methods, and in-vivo nanoporous microcages for effective drug delivery. This paper focuses on the information contained in patents as well as the corresponding technical publications. The goal of the paper is to help emerging scientists understand and improvise over these inventions.

Nanocomposite microcontainers

Versatile all-nanocomposite capped microcontainers are made using layer-by-layer (LBL) assembly. The microcontainers can act as inert packaging with slow/controlled release for virtually any type of encapsulating material based on clay nanocomposites 3D molded by PDMS templates and capped with another LBL film.

Biofriendly bonding processes for nanoporous implantable SU-8 microcapsules for encapsulated cell therapy

Mechanically robust, cell encapsulating microdevices fabricated using photolithographic methods can lead to more efficient immunoisolation in comparison to cell encapsulating hydrogels. There is a need to develop adhesive bonding methods which can seal such microdevices under physiologically friendly conditions. We report the bonding of SU-8 based substrates through (i) magnetic self assembly, (ii) using medical grade photocured adhesive and (iii) moisture and photochemical cured polymerization. Magnetic self-assembly, carried out in biofriendly aqueous buffers, provides weak bonding not suitable for long term applications. Moisture cured bonding of covalently modified SU-8 substrates, based on silanol condensation, resulted in weak and inconsistent bonding. Photocured bonding using a medical grade adhesive and of acrylate modified substrates provided stable bonding. Of the methods evaluated, photocured adhesion provided the strongest and most stable adhesion.

Self-folding micropatterned polymeric containers

We demonstrate self-folding of precisely patterned, optically transparent, all-polymeric containers and describe their utility in mammalian cell and microorganism encapsulation and culture. The polyhedral containers, with SU-8 faces and biodegradable polycaprolactone (PCL) hinges, spontaneously assembled on heating. Self-folding was driven by a minimization of surface area of the liquefying PCL hinges within lithographically patterned two-dimensional (2D) templates. The strategy allowed for the fabrication of containers with variable polyhedral shapes, sizes and precisely defined porosities in all three dimensions. We provide proof-of-concept for the use of these polymeric containers as encapsulants for beads, chemicals, mammalian cells and bacteria. We also compare accelerated hinge degradation rates in alkaline solutions of varying pH. These optically transparent containers resemble three-dimensional (3D) micro-Petri dishes and can be utilized to sustain, monitor and deliver living biological components.

Encapsulated cell grafts to treat cellular deficiencies and dysfunction

Cell transplantation provides a therapeutic alternative to whole organ transplantation in the management of diseases arising from the absence or failure of specialized cells. Though allogenic transplantation is favorable in terms of graft acceptance, xenotransplantation can provide a potentially unlimited source of cells and can overcome shortage of human donors. Effective immunoisolation of the xenografts is critical for their long term survival and function. Encapsulation of cells in polymeric matrices, organic or inorganic, provides a physical selectively permeable barrier between the host and the graft, thereby immunoisolating the graft. Microencapsulation of cells in alginate hydrogels has been pervasive, but this approach does not provide precise control over porosity, whereas micro- and nano-fabrication technologies can provide precise and reproducible control over porosity. We highlight both encapsulation approaches in this review, with their relative advantages and challenges. We also highlight the therapeutic potential of encapsulated cells for treating a variety of diseases, detailing the xenotransplantation of pancreatic islets in diabetes therapy as well as the grafting of engineered cells that facilitate localized enzyme-prodrug therapy of pancreatic cancer.

Inhibited cell spreading on polystyrene nanopillars fabricated by nanoimprinting and in situ elongation

Polymer nanopillars (40-80 nm in diameter and 100 nm in pitch) were fabricated at high density over large areas directly on bulk tissue culture polystyrene plates using nanoimprint lithography. Nanoporous Si molds for imprinting were generated by transfer from an anodic alumina membrane. Ultrahigh aspect ratio polymer nanopillars were formed in a novel procedure using controlled elongation of the imprinted pillars during mold release. The resulting nanopillar arrays show significant changes in surface wettability upon brief O(2) plasma treatment. Human dermal fibroblasts were cultured on the nanopillar surfaces in order to study cell-substrate interaction at the nanoscale. The nanopillar topography shows strong effects on the cell morphology, with pillars of widely varying aspect ratios and surface energies resisting cell spreading. This effect on cell behavior can be rationalized in terms of the cells' requirement to form micron-scale focal adhesions. The study indicates that at the nanoscale, physical factors can supersede the effects of chemical factors on the cell-substratum interaction.

Making a case: nanofabrication techniques in encapsulated cell therapy for people with diabetes

A nanoporous immunoisolative case/capsule that encases/encapsulates insulin-secreting cells vastly expands the source of therapeutic cells available for grafting in people with diabetes, including cells from animal sources, stem cells, and genetically engineered cells. These encapsulated cellular grafts potentially provide an endogenous, renewable, and long-term source of insulin without the need for pharmacological immunosuppression. Micro- and nanofabrication techniques used principally in the semiconductor industry can play a positive role in encapsulated cell therapy. Many of these techniques do not have direct applicability in cell encapsulation, but can be leveraged to develop processes suitable for this application. This commentary highlights the salient features of an effective cell encapsulation system, enumerates limitations of existing encapsulation schemes, and touches upon progress in key areas of encapsulation technology; one example of how micro- and nanofabrication technology may be used to develop a more effective platform for cell encapsulation is presented. This commentary urges further exploration and expansion of techniques used traditionally in electronics and optics for cell-based therapy in people with diabetes.