In this review article, we focus on the various types of materials used in biomedical implantable devices, including the polymeric materials used as substrates and for the packaging of such devices. Polymeric materials are used because of the ease of fabrication, flexibility, and their biocompatible nature as well as their wide range of mechanical, electrical, chemical, and thermal behaviors when combined with different materials as composites. Biocompatible and biostable polymers are extensively used to package implanted devices, with the main criteria that include gas permeability and water permeability of the packaging polymer to protect the electronic circuit of the device from moisture and ions inside the human body. Polymeric materials must also have considerable tensile strength and should be able to contain the device over the envisioned lifetime of the implant. For substrates, structural properties and, at times, electrical properties would be of greater concern. Section 1 gives an introduction of some medical devices and implants along with the material requirements and properties needed. Different synthetic polymeric materials such as polyvinylidene fluoride, polyethylene, polypropylene, polydimethylsiloxane, parylene, polyamide, polytetrafluoroethylene, poly(methyl methacrylate), polyimide, and polyurethane have been examined, and liquid crystalline polymers and nanocomposites have been evaluated as biomaterials that are suitable for biomedical packaging (section 2). A summary and glimpse of the future trend in this area has also been given (section 3). Materials and information used in this manuscript are adapted from papers published between 2010 and 2015 representing the most updated information available on each material.

Although direct ink writing (DIW) is a versatile 3D printing technique, progress in DIW has been constrained by the stringent rheological requirements for printable conductive nanocomposites, particularly at smaller length scales. In this work, we overcome these challenges using an aqueous nanocomposite ink with polydimethylsiloxane (PDMS) submicrobeads and an electrochemically-derived graphene oxide (EGO) nanofiller. This nanocomposite ink possesses a thixotropic, self-supporting viscoelasticity. It can be easily extruded through very small nozzle openings (as small as 50 µm) allowing for the highest resolution PDMS DIW reported to date. With a mild thermal annealing, the DIW-printed device exhibits low resistivity (1660 Ω•cm) at a low percolation threshold of EGO (0.83 vol%) owing to the unique nanocomposite structure of graphene-wrapped elastomeric beads. The nanocomposite ink was used to print wearable, macro-scale strain sensing patches, as well as remarkably small, micron-scale pressure sensors. The large-scale strain sensors have excellent performance over a large working range (up to 40% strain), with high gauge factor (20.3), and fast responsivity (83 ms) while the micron-scale pressure sensors demonstrated high pressure sensitivity (0.31 kPa -1 ) and operating range (0.248-500 kPa). Ultrahigh resolution, multimaterial layer-by-layer deposition allows the engineering of microscale features into the devices, features which can be used to tune the piezoresistive mechanism and degree of piezoresistivity.

Focusing and separation of particles such as cells at a high throughput are extremely attractive for biomedical applications. Particle manipulation based on inertial effects requires a high flow speed, and thus suits well to high-throughput applications. Recently, inertial focusing and separation using curvilinear microchannels have been attracting a great interest due to the linear structure for parallelization, small device footprint, superior particle focusing performance and easy implementation of particle separation. However, the curvature direction of these microchannels is alternating, leading to the variation in both the magnitude and direction of the induced secondary flow. The accumulation of this variation along the channel causes unpredictable behaviour of particles. This paper systematically investigates the inertial focusing phenomenon in low-aspect-ratio symmetric sinusoidal channels. First, we comprehensively studied the effects of parameters such as viscosity, flow condition, particle size, and geometric dimension of microchannel on differential particle focusing. We found that particle inertial focusing is generally independent of fluid kinematic viscosity, but highly dependent on particle size, flow condition and channel dimension. Next, we derived an explicit scaling factor and included all four dimensionless parameters (particle blockage ratio, curvature ratio and Dean number, and channel aspect ratio) in a single operational map to illustrate the particle focusing patterns. Finally, we proposed a rational guideline to intuitively instruct the design of channel dimensions for separation of a given particle mixture.

We introduce an effective method to actively induce droplet generation using negative pressure. Droplets can be generated on demand using a series of periodic negative pressure pulses. Fluidic network models were developed using the analogy to electric networks to relate the pressure conditions for different flow regimes. Experimental results show that the droplet volume is correlated to the pressure ratio with a power law of 1.3. Using a pulsed negative pressure at the outlet, we are able to produce droplets in demand and with a volume proportional to the pulse width.