Components of an integrated microfluidic device for continuous glucose monitoring with responsive insulin delivery

Microneedles-Based Theranostic Platform: From the Past to the Future

Fully integrated theranostic devices are highly esteemed in clinical applications, offering immense potential in real-time disease monitoring and personalized care. Microneedles (MNs), as innovative and wearable devices, boast important advantages in biosensing and therapy, thus holding significant promise in the advancement of diagnostic and therapeutic platforms. Encouragingly, advancements in electrochemical sensing technology, micronano fabrication, and biocompatible materials are propelling momentum for MNs-based closed-loop systems, enhancing detection capabilities, biocompatibility, and cost-effectiveness. Moreover, the notable progress in integrating MN chips with other biochips signifies a frontier for growth. Successful clinical trials in target molecule monitoring and drug delivery domains herald excellent clinical translational prospects for the aforementioned theranostic platform. Finally, we delineate both challenges and opportunities in the development of integrated diagnostic and therapeutic MN systems, including continuous monitoring, intelligent control algorithms, safety, and regulatory considerations.

Conformational-switch biosensors as novel tools to support continuous, real-time molecular monitoring in lab-on-a-chip devices

Recent years have seen continued expansion of the functionality of lab on a chip (LOC) devices. Indeed LOCs now provide scientists and developers with useful and versatile platforms across a myriad of chemical and biological applications. The field still fails, however, to integrate an often important element of bench-top analytics: real-time molecular measurements that can be used to "guide" a chemical response. Here we describe the analytical techniques that could provide LOCs with such real-time molecular monitoring capabilities. It appears to us that, among the approaches that are general (i.e., that are independent of the reactive or optical properties of their targets), sensing strategies relying on binding-induced conformational change of bioreceptors are most likely to succeed in such applications.

Stimuli-Responsive Microneedles as a Transdermal Drug Delivery System: A Demand-Supply Strategy

Microneedles are one of the most prominent approaches capable of physically disrupting the stratum corneum without devastating the deeper tissues to deliver both small molecules and macromolecules into the viable epidermis/dermis for local/systemic effects. Over the past two decades, microneedles have caught the attention of many researchers because of their outstanding advantages over oral and parenteral drug delivery systems such as self-administration, pain-free, steady-plasma concentration maintenance, avoidance of first-pass hepatic biotransformation, and so on. So far, scientists have reported various types of microneedle patches to deliver the loaded therapeutics as soon as the microneedles are inserted into the skin, regardless of the demand for therapeutics to treat a specific condition. This way of drug delivery can lead to potential risks such as poor therapeutic efficacy or drug overdose. The stimuli-responsive microneedles are the most predominant tool to achieve the on-demand/need-based drug delivery, leading to safe and effective treatment. Various natural and synthetic polymers that can undergo significant transitions such as swelling, shrinking, dissolution, or disintegration play a pivotal role in the development of stimuli-responsive microneedles. The current Review provides brief information about the history, emergence, type, and working principles of microneedles. Furthermore, it selectively discusses various exogenous and endogenous stimuli-responsive microneedles along with their mechanism of action involved in treating different disease conditions. Collaterally, the emergence of "closed-loop" combinatorial stimuli-responsive microneedle patches for precise delivery of therapeutics is meticulously canvassed. Subsequently, it covers the patents of different stimuli-responsive microneedles and further highlights the existing challenges and future perspectives concerning clinical application and large-scale production.

Advances in the Design of Transdermal Microneedles for Diagnostic and Monitoring Applications

Due to their intrinsic advantages over classical hypodermic needles, microneedles have received much attention over the last two decades and will likely soon appear in clinics. Although the vast majority of research is focused on designing microneedles for the painless delivery of drugs, their applications for diagnostic purposes have also provided promising results. In this paper, the main advances in the field of microneedles for diagnostic and patient monitoring purposes are introduced and critically discussed.

Microneedle electrodes toward an amperometric glucose-sensing smart patch

Here, efforts toward the development of a microneedle-based glucose sensor or "smart patch" for intradermal glucose sensing are described. Metallic microneedle array electrodes, conducting polymers, and glucose oxidase form the sensor platform. This work represents the first steps toward the development of painless, transdermal-sensing devices for continuous glucose monitoring.

Materials for diabetes therapeutics

This review is focused on the materials and methods used to fabricate closed-loop systems for type 1 diabetes therapy. Herein, we give a brief overview of current methods used for patient care and discuss two types of possible treatments and the materials used for these therapies-(i) artificial pancreases, comprised of insulin producing cells embedded in a polymeric biomaterial, and (ii) totally synthetic pancreases formulated by integrating continuous glucose monitors with controlled insulin release through degradable polymers and glucose-responsive polymer systems. Both the artificial and the completely synthetic pancreas have two major design requirements: the device must be both biocompatible and be permeable to small molecules and proteins, such as insulin. Several polymers and fabrication methods of artificial pancreases are discussed: microencapsulation, conformal coatings, and planar sheets. We also review the two components of a completely synthetic pancreas. Several types of glucose sensing systems (including materials used for electrochemical, optical, and chemical sensing platforms) are discussed, in addition to various polymer-based release systems (including ethylene-vinyl acetate, polyanhydrides, and phenylboronic acid containing hydrogels).

Protein synthesis in a device with nanoporous membranes and microchannels

Cell-free protein synthesis (CFPS) is an alternative approach to cell-based recombinant protein production. It involves in vitro transcription and translation in a cell-free medium. In this work, we implemented CFPS in a plastic array device. Each unit in the array consisted of an inner well and an outer well. Two synthesis steps, gene transcription and protein translation, took place in the inner well, in which a cell-free medium was used to provide ribosomes and additional components necessary for protein synthesis. The outer well was concentric to the inner well and it functioned as a nutrient reservoir. A nanoporous membrane was sandwiched between the inner and outer wells for retaining the synthesized proteins and removing the reaction byproducts. A microfluidic channel was employed to connect these two wells for supplying fresh nutrients for longer reaction time and higher expression yield. Synthesis of luciferase was shown to last 8 times longer and yield 10 times more proteins than in a conventional container. The device also enables more than 2 orders of magnitude reduction in reagent consumption compared to a bench-top instrument. The effects of the membrane pore size and microfluidic channel on the protein production yield were also studied. The array device has potential to become a platform for parallel protein expression for proteomics applications, matching high-throughput gene discovery.

Droplet-based microdialysis-Concept, theory, and design considerations

The capability of continuously sampling the extracellular fluid opens up a wide range of applications of microdialysis in biological, pharmaceutical, and clinical studies. Existing microdialysis, however, faces challenges in sampling analytes with fast clearance and limited diffusivity because sampling resolution is limited by device size. Size reduction in probes and interconnected cannulae is a promising solution to improve temporal and spatial resolution. But the back pressure produced by resistance to laminar flows will be magnified in smaller channels, raising a concern as to whether it is feasible to operate continuous perfusion for miniaturized microdialysis. We demonstrate that a 10-fold smaller channel will exhibit 100-fold larger back pressure in response to the increase in the flow rate to maintain the relative recovery. In order to overcome the foreseen back pressure issue, this paper discusses a new concept using discrete droplets instead of continuous flows to operate dialysis in a miniaturized probe. This conceptual design is referred to as droplet-based digital microdialysis, in which droplets are produced, controlled and advanced within microchannels at a rate that in theory should allow for analytes to equilibrate with the extracellular fluid under no flow conditions. Expecting that a digital droplet design will entirely eliminate back pressure by introducing air between droplets, we numerically compare the equilibration kinematics of droplets to that of continuous flow. Results suggest equilibration of low molecular weight analytes between intermittently stationary droplets and the extracellular fluid in a few seconds. Considerations in design, prototyping, calibration and quantification, and the integration with other devices are suggested.

On-chip microdialysis system with flow-through glucose sensing capabilities

BACKGROUND

Microdialysis is a sampling technique based on controlling the mass transfer rate of different-sized molecules across a semipermeable membrane. Because the dialysis process has minimal effects on the surrounding fluid, it is viewed as a tool for continuous monitoring of human metabolites. In diabetes treatment, microdialysis probes have been used as sampling systems coupled to a glucose biosensor but may struggle to obtain high recoveries of analytes, as the sampling housing, probes, and glucose sensors are fabricated as separate pieces and then assembled, resulting in a large dead volume, which limits sensing frequency. An in situ combination of a miniaturized microdialysis probe with an integrated glucose sensor could help solve some of these problems.

METHOD

The system was fabricated by bonding a 6-mum-thick polycarbonate track-etch membrane with 100-nm-diameter pores onto microfluidic channels with the electrochemical glucose sensing electrodes patterned within the microchannels.

RESULTS

In vitro experiments demonstrating glucose microdialysis with continuous sensing were conducted. The permeability of glucose to the polycarbonate membrane with a 100-nm-diameter pore size was obtained to be 5.44 mum/s. Glucose recovery of 99% was observed using this microdialysis system at a perfusion flow rate of 0.5 microl/min. Experiments monitoring fluctuating glucose concentrations in the time domain at 99% recovery were also performed. The lag time was measured to be 210 seconds with 45 seconds contributed by mass transfer limitations and the rest from dead volume within the experimental setup.

CONCLUSION

The electrochemical sensing component was able to continuously track concentration changes in the reservoir. This system is expected to have the proper sensitivity to track physiologically relevant concentration changes of glucose with a lag time of less than 1 minute and minimal amplitude reduction for continuous glucose monitoring for diabetes treatment.

Recent progress in analytical instrumentation for glycemic control in diabetic and critically ill patients

Implementing strict glycemic control can reduce the risk of serious complications in both diabetic and critically ill patients. For this reason, many different analytical, mainly electrochemical and optical sensor approaches for glucose measurements have been developed. Self-monitoring of blood glucose (SMBG) has been recognised as being an indispensable tool for intensive diabetes therapy. Recent progress in analytical instrumentation, allowing submicroliter samples of blood, alternative site testing, reduced test time, autocalibration, and improved precision, is comprehensively described in this review. Continuous blood glucose monitoring techniques and insulin infusion strategies, developmental steps towards the realization of the dream of an artificial pancreas under closed loop control, are presented. Progress in glucose sensing and glycemic control for both patient groups is discussed by assessing recent published literature (up to 2006). The state-of-the-art and trends in analytical techniques (either episodic, intermittent or continuous, minimal-invasive, or noninvasive) detailed in this review will provide researchers, health professionals and the diabetic community with a comprehensive overview of the potential of next-generation instrumentation suited to either short- and long-term implantation or ex vivo measurement in combination with appropriate body interfaces such as microdialysis catheters.

Microneedles and transdermal applications

With the limitations of oral drug delivery and the pain and needle phobias associated with traditional injections, drug delivery research has focused on the transdermal delivery route. A formidable barrier to transdermal drug delivery is the stratum corneum, the superficial layer of the skin. In the last 10 years, microneedles were proposed as a mechanical tool to pierce through the stratum corneum, in order to create drug delivery channels without stimulating underlying pain nerves. Since then, the field of microneedles has rapidly evolved to spawn a plethora of potential transdermal applications. In this review, the authors provide an overview of the progress in microneedle research and design, and the advancements that have been made in employing this technology for transdermal applications.