Angel Savov

A novel stretchable micro-electrode array (SMEA) design for directional stretching of cells

Stretchable micro-electrode arrays (SMEAs) are useful tools to study the electrophysiology of living cells seeded on the devices under mechanical stimulation. For such applications, the SMEAs are used as cell culture substrates; therefore, the surface topography and mechanical properties of the devices should be minimally affected by the embedded stretchable electrical interconnects. In this paper, a novel design and micro-fabrication technology for a pneumatically actuated SMEA are presented to achieve stretchability with minimal surface area dedicated to the electrical interconnects and a well-defined surface strain distribution combined with integrated diverse micro-patterns to enable alignment and directional stretching of cells. The special mechanical design also enables the SMEA to have a prolonged electro-mechanical fatigue life time required for long-term cyclic stretching of the cell cultures (stable resistance of electrical interconnects for more than 160 thousand cycles of 20% stretching and relaxing). The proposed fabrication method is based on the state of the art micro-fabrication techniques and materials and circumvents the processing problems associated with using unconventional methods and materials to fabricate stretchable electrode arrays. The electrochemical impedance spectroscopy characterization of the SMEA shows 4.5 MΩ impedance magnitude at 1 kHz for a TiN electrode 12 um in diameter. Cell culture experiments demonstrate the robustness of the SMEAs for long-term culturing experiments and compatibility with inverted fluorescent microscopy.

Ultra-Stretchable Interconnects for High-Density Stretchable Electronics

The exciting field of stretchable electronics (SE) promises numerous novel applications, particularly in-body and medical diagnostics devices. However, future advanced SE miniature devices will require high-density, extremely stretchable interconnects with micron-scale footprints, which calls for proven standardized (complementary metal-oxide semiconductor (CMOS)-type) process recipes using bulk integrated circuit (IC) microfabrication tools and fine-pitch photolithography patterning. Here, we address this combined challenge of microfabrication with extreme stretchability for high-density SE devices by introducing CMOS-enabled, free-standing, miniaturized interconnect structures that fully exploit their 3D kinematic freedom through an interplay of buckling, torsion, and bending to maximize stretchability. Integration with standard CMOS-type batch processing is assured by utilizing the Flex-to-Rigid (F2R) post-processing technology to make the back-end-of-line interconnect structures free-standing, thus enabling the routine microfabrication of highly-stretchable interconnects. The performance and reproducibility of these free-standing structures is promising: an elastic stretch beyond 2000% and ultimate (plastic) stretch beyond 3000%, with <0.3% resistance change, and >10 million cycles at 1000% stretch with <1% resistance change. This generic technology provides a new route to exciting highly-stretchable miniature devices.

A stretchable Micro-Electrode Array for in vitro electrophysiology

A novel stretchable Micro-Electrode Array (MEA) for biomedical applications with robust spiraled interconnects and radial micro-grooves for cell alignment is presented, enabling the unique combination of alignment, mechanical stimulation, and electrical characterization of living cells. Many of the problems normally associated with the fabrication of interconnects and electrodes on stretchable PDMS (Polydimethylsiloxane) membranes have been eliminated by first fabricating the interconnects and then spin-coating the PDMS. Experiments supported by Finite Element simulations demonstrate the robustness of the new spiraled interconnect design. The alignment of cells to micro-molded grooves is demonstrated, allowing for directional stretching of the cells.

Living Chips and Chips for the living

In this paper we present a “polymer-last” approach for the fabrication of (partly) flexible and stretchable sensors assemblies. The “Flex-to-Rigid” (F2R) platform is based on this approach and especially designed for the fabrication of very small sensor systems which can be folded into, or around the tip of minimal invasive instruments such as laparoscopic instruments, catheters or guide-wires. As an example the fabrication and assembly of a combined pressure and flow sensor on the tip of a 360 μm diameter guide-wire is presented. The F2R platform uses standard silicon manufacturing equipment in a standard production environment and is therefore suitable for high volume production. Using the same “polymerlast” approach stretchable circuits have been fabricated. As an example a stretchable Micro-Electrode Array for electrophysiology is demonstrated. The paper ends with an outlook on an entire new field in micro fabrication where living cells are co-integrated with silicon bases micro-systems resulting in truly Living Chips.

A post processing approach for manufacturing high-density stretchable sensor arrays

A method for fabricating stretchable sensor arrays consisting of rigid islands attached to a stretchable membrane is proposed. The method utilizes a CMOS compatible post processing approach, where the sensor elements and the underlying electronics are fabricated first, followed by a sequence of etching steps and polymer spin coating that result in the formation of the stretchable array. The elements of the array are connected by free-standing interconnects that are prefabricated in the metal stack of the CMOS. The presented fabrication results demonstrate the feasibility of manufacturing of fine-pitch, high density arrays where the interconnects are completely separated from the stretchable substrate allowing for high stretchability.

A Platform for Mechano(-Electrical) Characterization of Free-Standing Micron-Sized Structures and Interconnects

A device for studying the mechanical and electrical behavior of free-standing micro-fabricated metal structures, subjected to a very large deformation, is presented in this paper. The free-standing structures are intended to serve as interconnects in high-density, highly stretchable electronic circuits. For an easy, damage-free handling and mounting of these free-standing structures, the device is designed to be fabricated as a single chip/unit that is separated into two independently movable parts after it is fixed in the tensile test stage. Furthermore, the fabrication method allows for test structures of different geometries to be easily fabricated on the same substrate. The utility of the device has been demonstrated by stretching the free-standing interconnect structures in excess of 1000% while simultaneously measuring their electrical resistance. Important design considerations and encountered processing challenges and their solutions are discussed in this paper.

A novel processing concept for reduction of substrate artifacts in ultrasound transducer arrays

Free-standing tungsten membranes supported by polyimide pillars were fabricated, demonstrating the general feasibility of the process. To reduce the warping due to the stress in the deposited tungsten layer four different membrane shapes were tested. The type B membrane shape gave the best result with the average peak to thorough distance of 162 nm. While the result is encouraging, the warping is still posing risk to the reliable operation of the CMUT since the gap of the transducer that is to be fabricated on the membrane is on the same order of magnitude.