Self-folding single cell grippers.

Given the heterogeneous nature of cultures, tumors, and tissues, the ability to capture, contain, and analyze single cells is important for genomics, proteomics, diagnostics, therapeutics, and surgery. Moreover, for surgical applications in small conduits in the body such as in the cardiovascular system, there is a need for tiny tools that approach the size of the single red blood cells that traverse the blood vessels and capillaries. We describe the fabrication of arrayed or untethered single cell grippers composed of biocompatible and bioresorbable silicon monoxide and silicon dioxide. The energy required to actuate these grippers is derived from the release of residual stress in 3-27 nm thick films, did not require any wires, tethers, or batteries, and resulted in folding angles over 100° with folding radii as small as 765 nm. We developed and applied a finite element model to predict these folding angles. Finally, we demonstrated the capture of live mouse fibroblast cells in an array of grippers and individual red blood cells in untethered grippers which could be released from the substrate to illustrate the potential utility for in vivo operations.

Bio-origami hydrogel scaffolds composed of photocrosslinked PEG bilayers.

We describe the self-folding of photopatterned poly (ethylene glycol) (PEG)-based hydrogel bilayers into curved and anatomically relevant micrometer-scale geometries. The PEG bilayers consist of two different molecular weights (MWs) and are photocrosslinked en masse using conventional photolithography. Self-folding is driven by differential swelling of the two PEG bilayers in aqueous solutions. We characterize the self-folding of PEG bilayers of varying composition and develop a finite element model which predicts radii of curvature that are in good agreement with empirical results. Since we envision the utility of bio-origami in tissue engineering, we photoencapsulate insulin secreting ß-TC-6 cells within PEG bilayers and subsequently self-fold them into cylindrical hydrogels of different radii. Calcein AM staining and ELISA measurements are used to monitor cell proliferation and insulin production respectively, and the results indicate cell viability and robust insulin production for over eight weeks in culture.

Differentially photo-crosslinked polymers enable self-assembling microfluidics.

An important feature of naturally self-assembled systems such as leaves and tissues is that they are curved and have embedded fluidic channels that enable the transport of nutrients to, or removal of waste from, specific three-dimensional regions. Here we report the self-assembly of photopatterned polymers, and consequently microfluidic devices, into curved geometries. We discover that differentially photo-crosslinked SU-8 films spontaneously and reversibly curve on film de-solvation and re-solvation. Photolithographic patterning of the SU-8 films enables the self-assembly of cylinders, cubes and bidirectionally folded sheets. We integrate polydimethylsiloxane microfluidic channels with these SU-8 films to self-assemble curved microfluidic networks.

Self-folding immunoprotective cell encapsulation devices.

Cell encapsulation therapy (CET) provides an attractive means to transplant cells without the need for immunosuppression. The cells are immunoisolated by surrounding them with a synthetic, semipermeable nanoporous membrane that allows selective permeation of nutrients and therapeutics while isolating the cells from hostile immune components. This communication describes the fabrication and in vitro characterization of lithographically structured and self-folded containers for immunoprotective cell encapsulation. Lithographic patterning ensured identical shapes, sizes, tunable porosity, and precise volumetric control, whereas self-folding enabled transformation of two-dimensional porous membranes into cubes, ensuring that pores were present in all three dimensions for adequate diffusion of O(2) and other nutrients to encapsulated cells. We fabricated containers with varying pore sizes and observed that pores sizes of approximately 78 nm were sufficient to significantly inhibit diffusion of IgG (the smallest antibody) and permit adequate diffusion of insulin, highlighting the possibility to utilize these containers to develop a lithographically structured bioartificial pancreas. FROM THE CLINICAL EDITOR: In this paper, a novel immunoisolation technique is presented to enable cell transplant survival by surrounding them with a synthetic, semipermeable nanoporous membrane that allows selective permeation of nutrients and therapeutics while isolating the cells from hostile immune components. This method may pave the way to effective pancreatic islet cell transplantation.

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.

Three-dimensional microwell arrays for cell culture.

We propose the concept of three-dimensional (3D) microwell arrays for cell culture applications and highlight the importance of oxygen diffusion through pores in all three dimensions to enhance cell viability.

Enzymatically triggered actuation of miniaturized tools.

We demonstrate a methodology that utilizes the specificity of enzyme-substrate biomolecular interactions to trigger miniaturized tools under biocompatible conditions. Miniaturized grippers were constructed using multilayer hinges that employed intrinsic strain energy and biopolymer triggers, as well as ferromagnetic elements. This composition obviated the need for external energy sources and allowed for remote manipulation of the tools. Selective enzymatic degradation of biopolymer hinge components triggered closing of the grippers; subsequent reopening was achieved with an orthogonal enzyme. We highlight the utility of these enzymatically triggered tools by demonstrating the biopsy of liver tissue from a model organ system and gripping and releasing an alginate bead. This strategy suggests an approach for the development of smart materials and devices that autonomously reconfigure in response to extremely specific biological environments.

Directed growth of fibroblasts into three dimensional micropatterned geometries via self-assembling scaffolds.

We describe the use of conventional photolithography to construct three dimensional (3D) thin film scaffolds and direct the growth of fibroblasts into three distinct and anatomically relevant geometries: cylinders, spirals and bi-directionally folded sheets. The scaffolds were micropatterned as two dimensional sheets which then spontaneously assembled into specific geometries upon release from the underlying substrate. The viability of fibroblasts cultured on these self-assembling scaffolds was verified using fluorescence microscopy; cell morphology and spreading were studied using scanning electron microscopy. We demonstrate control over scaffold size, radius of curvature and folding pitch, thereby enabling an attractive approach for investigating the effects of these 3D geometric factors on cell behaviour.

Effects of a surfactant monolayer on the measurement of equilibrium interfacial tension of a drop in extensional flow.

The effect of surfactant monolayer concentration on the measurement of interfacial surface tension using transient drop deformation methods is studied using the Boundary Integral Method. Emulsion droplets with a surfactant monolayer modeled with the Langmuir equation of state initially in equilibrium are suddenly subjected to axisymmetric extensional flows until a steady state deformation is reached. The external flow is then removed and the retraction of the drops to a spherical equilibrium shape in a quiescent state is simulated. The transient response of the drop to the imposed flow is analyzed to obtain a characteristic response time, tau(s)( *). Neglecting the initial and final stages, the retraction process can be closely approximated by an exponential decay with a characteristic time, tau(r)( *). The strength of the external flow on each model drop is increased in order to investigate the coupled effect of deformation and surfactant distribution on the characteristic relaxation time. Different model drops are considered by varying the internal viscosity and the equilibrium surfactant concentrations from a surfactant free state (clean) to high concentrations approaching the maximum packing limit. The characteristic times obtained from the simulated drop dynamics both in extension and retraction are used to determine an apparent surface tension employing linear theory. In extension the apparent surface tension under predicts the prescribed equilibrium surface tension. The error increases monotonically with the equilibrium surfactant concentration and diverges as the maximum packing limit is approached. In retraction the apparent surface tension under predicts the prescribed equilibrium surface tension depends non-monotonically on the equilibrium surfactant concentration. The error is highest for moderate surfactant concentrations and decreases as the maximum packing limit is approached. It was found that the difference between the prescribed surface tension and the apparent surface tension increased as the viscosity ratio decreased. Differences as large as 40% were seen between the prescribed surface tension and the apparent surface tension predicted by the linear theory.