A Rapid Prototyping Approach for Multi-Material, Reversibly Sealed Microfluidics.

Microfluidic organ-on-chip models recapitulate increasingly complex physiological phenomena to study tissue development and disease mechanisms, where there is a growing interest in retrieving delicate biological structures from these devices for downstream analysis. Standard bonding techniques, however, often utilize irreversible sealing, making sample retrieval unfeasible or necessitating destructive methods for disassembly. To address this, several commercial devices employ reversible sealing techniques, though integrating these techniques into early-stage prototyping workflows is often ignored because of the variation and complexity of microfluidic designs. Here, we demonstrate the concerted use of rapid prototyping techniques, including 3D printing and laser cutting, to produce multi-material microfluidic devices that can be reversibly sealed. This is enhanced via the incorporation of acrylic components directly into polydimethylsiloxane channel layers to enhance stability, sealing, and handling. These acrylic components act as a rigid surface separating the multiple mechanical seals created between the bottom substrate, the microfluidic features in the device, and the fluidic interconnect to external tubing, allowing for greater design flexibility. We demonstrate that these devices can be produced reproducibly outside of a cleanroom environment and that they can withstand ~1 bar pressures that are appropriate for a wide range of biological applications. By presenting an accessible and low-cost method, we hope to enable microfluidic prototyping for a broad range of biomedical research applications.

Breaking barriers: exploring mechanisms behind opening the blood-brain barrier.

The blood-brain barrier (BBB) is a selectively permeable membrane that separates the bloodstream from the brain. While useful for protecting neural tissue from harmful substances, brain-related diseases are difficult to treat due to this barrier, as it also limits the efficacy of drug delivery. To address this, promising new approaches for enhancing drug delivery are based on disrupting the BBB using physical means, including optical/photothermal therapy, electrical stimulation, and acoustic/mechanical stimulation. These physical mechanisms can temporarily and locally open the BBB, allowing drugs and other substances to enter. Focused ultrasound is particularly promising, with the ability to focus energies to targeted, deep-brain regions. In this review, we examine recent advances in physical approaches for temporary BBB disruption, describing their underlying mechanisms as well as evaluating the utility of these physical approaches with regard to their potential risks and limitations. While these methods have demonstrated efficacy in disrupting the BBB, their safety, comparative efficacy, and practicality for clinical use remain an ongoing topic of research.

Periodic Rayleigh streaming vortices and Eckart flow arising from traveling-wave-based diffractive acoustic fields.

Recent studies have demonstrated that periodic time-averaged acoustic fields can be produced from traveling surface acoustic waves (SAWs) in microfluidic devices. This is caused by diffractive effects arising from a spatially limited transducer. This permits the generation of acoustic patterns evocative of those produced from standing waves, but instead with the application of a traveling wave. While acoustic pressure fields in such systems have been investigated, acoustic streaming from diffractive fields has not. In this work we examine this phenomenon and demonstrate the appearance of geometry-dependent acoustic vortices, and demonstrate that periodic, identically rotating Rayleigh streaming vortices result from the imposition of a traveling SAW. This is also characterized by a channel-spanning flow that bridges between adjacent vortices along the channel top and bottom. We find that the channel dimensions determine the types of streaming that develops; while Eckart streaming has been previously presumed to be a distinguishing feature of traveling-wave actuation, we show that Rayleigh streaming vortices also results. This has implications for microfluidic actuation, where traveling acoustic waves have applications in microscale mixing, separation, and patterning.

Mechanisms and Applications of Neuromodulation Using Surface Acoustic Waves-A Mini-Review.

The study of neurons is fundamental for basic neuroscience research and treatment of neurological disorders. In recent years ultrasound has been increasingly recognized as a viable method to stimulate neurons. However, traditional ultrasound transducers are limited in the scope of their application by self-heating effects, limited frequency range and cavitation effects during neuromodulation. In contrast, surface acoustic wave (SAW) devices, which are producing wavemodes with increasing application in biomedical devices, generate less self-heating, are smaller and create less cavitation. SAW devices thus have the potential to address some of the drawbacks of traditional ultrasound transducers and could be implemented as miniaturized wearable or implantable devices. In this mini review, we discuss the potential mechanisms of SAW-based neuromodulation, including mechanical displacement, electromagnetic fields, thermal effects, and acoustic streaming. We also review the application of SAW actuation for neuronal stimulation, including growth and neuromodulation. Finally, we propose future directions for SAW-based neuromodulation.

3D Diamond Electrode Array for High-Acuity Stimulation in Neural Tissue.

Innovations in micro- and nanofabrication technologies enable the manufacture of multielectrode arrays for use in neuromodulation and neural recording. Multielectrode arrays make possible medical implants such as pacemakers, deep-brain stimulators, or visual and hearing aids, to treat numerous neural disorders. An optimal neural interface requires a high density of electrodes to precisely record from and stimulate the nervous system while minimizing the overall size of the array. For example, people with retinal degenerative diseases can benefit from retinal prostheses implanted inside the eye. However, at present the visual acuity provided by such implants is well below the threshold for functional vision, mainly due to the limited spatial resolution. In this work, we present a design of 3D nanostructured conductive diamond electrodes, integrated within a polycrystalline diamond housing, offering a high electrode density and count, which simultaneously satisfies spatial resolution and biocompatibility goals. The array is composed of height adjustable pillar electrodes that are 80 µm in diameter and separated by 150 µm. A holistic characterization of the electrodes was performed and the device tested for stimulation performance in a whole-mounted retina. Electrochemical testing showed impedance of 20 kΩ and a wide water window of 2.47 V. The pillar structure allows the distance between the electrodes and the retinal ganglion cells to be reduced which is key to more confined stimulation at lower current levels, leading to potentially higher-acuity stimulation without damaging retinal tissue.

Hybrid diamond/ carbon fiber microelectrodes enable multimodal electrical/chemical neural interfacing.

Implantable medical devices are now in regular use to treat or ameliorate medical conditions, including movement disorders, chronic pain, cardiac arrhythmias, and hearing or vision loss. Aside from offering alternatives to pharmaceuticals, one major advantage of device therapy is the potential to monitor treatment efficacy, disease progression, and perhaps begin to uncover elusive mechanisms of diseases pathology. In an ideal system, neural stimulation, neural recording, and electrochemical sensing would be conducted by the same electrode in the same anatomical region. Carbon fiber (CF) microelectrodes are the appropriate size to achieve this goal and have shown excellent performance, in vivo. Their electrochemical properties, however, are not suitable for neural stimulation and electrochemical sensing. Here, we present a method to deposit high surface area conducting diamond on CF microelectrodes. This unique hybrid microelectrode is capable of recording single-neuron action potentials, delivering effective electrical stimulation pulses, and exhibits excellent electrochemical dopamine detection. Such electrodes are needed for the next generation of miniaturized, closed-loop implants that can self-tune therapies by monitoring both electrophysiological and biochemical biomarkers.

Improved visual acuity using a retinal implant and an optimized stimulation strategy.

OBJECTIVE: Retinal prosthetic devices hold great promise for the treatment of retinal degenerative diseases such as retinitis pigmentosa and age-related macular degeneration. Through electrical stimulation of the surviving retinal neurons, these devices evoke visual signals that are then relayed to the brain. Currently, the visual prostheses used in clinical trials have few electrodes, thus limiting visual acuity. Electrode arrays with high electrode densities have been developed using novel technologies, including diamond growth and laser machining, and these may provide a more promising route to achieve high visual acuity in blind patients. APPROACH: Here, we studied the potential spatial resolution of electrical stimulation using diamond electrodes. We did this by labeling retinal ganglion cells in whole mount retina with a calcium indicator in wild-type rats and those with retinal degeneration. We imaged the ganglion cell responses to a range of stimulation parameters, including pulse duration and return electrode configuration. MAIN RESULTS: With sub-retinal stimulation, in which electrodes were in contact with the intact or degenerated photoreceptor layer, we found that biphasic pulses of 0.1 ms phase duration and a local return configuration was the most effective in confining the retinal ganglion cell activation patterns, while also remaining within the safety limits of the materials and providing the best power efficiency. SIGNIFICANCE: These results provide an optimized stimulation strategy for retinal implants, which if implemented in a retinal prosthetic is expected to improve the achievable visual acuity.

The Effects of Phase Durations on the Spatial Responses of Retinal Ganglion Cells to Epi- and Sub-Retinal Electrical Stimulation.

Retinal prostheses have the potential to restore vision to blind patients that have retinitis pigmentosa or similar hereditary degenerative disorders, by electrically stimulating surviving retinal neurons through implanted electrode arrays. Current retinal prostheses provide limited visual acuity and one challenge is to spatially control neural activation following electrical stimulation. Most of the retinal prostheses are either epi-retinal - in front of the retinal ganglion cell layer, or sub-retinal - behind photoreceptor layer. In this study, we performed calcium imaging of ganglion cells from whole mounted retinas and compared the spread of neural activation between epi-retinal stimulation with a fiber electrode and sub-retinal stimulation with a disk electrode. We investigated the effects of phase durations on the spatial resolution of biphasic stimulation. Our results suggest that with fiber electrode epi-retinal stimulation, the axon bundles activation can lead to significant spread of stimulation, and cannot be avoided simply by changing the phase durations. However, sub-retinal stimulation with very short pulses (phase duration 0.033ms) can effectively confine the activation of retinal ganglion cells.

The influence of different abutment materials on tissue regeneration after surgical treatment of peri-implantitis - a randomized controlled preclinical study.

OBJECTIVES: This study aimed to assess the impact of different abutment materials on peri-implant tissue regeneration after surgical treatment of peri-implantitis in a large animal model. MATERIAL AND METHODS: Titanium implants (n = 51) were inserted in the upper and lower jaw of eight beagle dogs and a peri-implant infection was induced. After two months the peri-implant infection was surgically treated and abutments with different surfaces (Ti-2: n = 14; CoCrMb: n = 13; Ag-modified Ti-4: n = 14; Ti-4 control: n = 10) were applied. Clinical attachment level (CAL), modified sulcus bleeding index (mBI), bleeding on probing (BoP), and the sulcus fluid flow rate (SFFR) were determined 4, 8, and 12 weeks after surgical treatment to document the peri-implant tissue reaction. RESULTS: Superior levels for CAL and mBI were found with the Ti-4 control and the Ag-modified abutments, with the Ag-modified abutments showing the best values after 12 weeks. Lowest SFFR values compared with the other treatment groups underlined the superior soft tissue reaction adjacent to Ag-modified abutments. After 12 weeks inferior CAL, SFFR, BOP and mBI values were documented for the Ti-2 surface. CONCLUSION: Within limitations of the study, Ag-modified abutments lead to superior tissue reactions. Further studies are needed to investigate the properties of abutment materials.

Exploring the Limits of Cell Adhesion under Shear Stress within Physiological Conditions and beyond on a Chip.

Cell adhesion processes are of ubiquitous importance for biomedical applications such as optimization of implant materials. Here, not only physiological conditions such as temperature or pH, but also topographical structures play crucial roles, as inflammatory reactions after surgery can diminish osseointegration. In this study, we systematically investigate cell adhesion under static, dynamic and physiologically relevant conditions employing a lab-on-a-chip system. We screen adhesion of the bone osteosarcoma cell line SaOs-2 on a titanium implant material for pH and temperature values in the physiological range and beyond, to explore the limits of cell adhesion, e.g., for feverish and acidic conditions. A detailed study of different surface roughness Rq gives insight into the correlation between the cells' abilities to adhere and withstand shear flow and the topography of the substrates, finding a local optimum at Rq = 22 nm. We use shear stress induced by acoustic streaming to determine a measure for the ability of cell adhesion under an external force for various conditions. We find an optimum of cell adhesion for T = 37 °C and pH = 7.4 with decreasing cell adhesion outside the physiological range, especially for high T and low pH. We find constant detachment rates in the physiological regime, but this behavior tends to collapse at the limits of 41 °C and pH 4.

A novel tool for dynamic cell adhesion studies--the De-Adhesion Number Investigator DANI.

For an optimal implementation of materials, such as, e.g. medical implants in living environments, a thorough characterization of cell adhesion, kinetics and strength is required, as well as a prerequisite e.g. for bone integration. Here we present a miniaturized (~100 µl) lab-on-a-chip implant hybrid system which allows quantification of cell adhesion under dynamic conditions mimicking those of physiological relevance. Surface acoustic waves are excited and used on optical transparent chips to induce micro acoustic streaming and to create a microfluidic shear spectrum ranging from 0 to ~35 s(-1). We demonstrate its potential for a time-efficient, dynamic screening test of new implant materials using a model of an osseointegration with SAOS-2 cells. The upside-down orientation also allows utilization of the micro reactor on non-transparent materials like titanium and diamond-like-carbon (DLC).