In recent years, implantable sensors have been extensively researched since they allow localized sensing at an area of interest (e.g., within the vicinity of a surgical site or other implant). They allow unobtrusive and potentially continuous sensing, enabling greater specificity, early warning capabilities, and thus timely clinical intervention. Wireless remote interrogation of the implanted sensor is typically achieved using radio frequency (RF), inductive coupling or ultrasound through an external device. Two categories of implantable sensors are available, namely active and passive. Active sensors offer greater capabilities, such as on-node signal and data processing, multiplexing and multimodal sensing, while also allowing lower detection limits, the possibility to encode patient sensitive information and bidirectional communication. However, they require an energy source to operate. Battery implantation, and maintenance, remains a very important constraint in many implantable applications even though energy can be provided wirelessly through the external device, in some cases. On the other hand, passive sensors offer the possibility of detection without the need for a local energy source or active electronics. They also offer significant advantages in the areas of system complexity, cost and size. In this review, implantable passive sensor technologies will be discussed along with their communication and readout schemes. Materials, detection strategies and clinical applications of passive sensors will be described. Advantages over active sensor technologies will be highlighted, as well as critical aspects related to packaging and biocompatibility.

Additive manufacturing, also known as 3D printing or digital fabrication technology, is emerging as a fast-expanding technology for the fabrication of prototypes and products in a variety of applications. This is mainly due to the advantages of 3D printing including the ease of manufacturing, the use of reduced material quantities minimizing material waste, low-cost mass production as well as energy efficiency. Polylactic acid (PLA) is a natural thermoplastic polyester that is produced from renewable resources and is routinely used to produce 3D-printed structures. One important feature that makes PLA appealing is that its properties can be modulated by the inclusion of nano or microfillers. This is of special importance for 3D-printed triboelectric nanogenerators since it can enhance the performance of the devices. In this work we investigate the influence of SiC micron-sized particles on the mechanical, electrical, and optical properties of a PLA-SiC composite for potential application in triboelectric energy harvesting. Our result show that the ultimate tensile strength of the pure PLA and 1%-doped PLA decreases with the number of fatigue cycles but increases by about 10% when SiC doping increases to 2% and 3%, while the strain at max load was about 3% independent of doping and the effective hardness was increased reaching a plateau at about 2 wt% SiC, about 40% above the value for pure PLA. Our results show that the mechanical properties of PLA can be enhanced by the inclusion of SiC, depending on the concentration of SiC. In addition, the same behavior is observed for the dielectric constant of the composite material increases as the SiC concentration increases, while the optical properties of the resulting composite are strongly dependent on the concentration of SiC.

Metal-Insulator-Metal (MIM) capacitors with a high capacitance density and low non-linearity coefficient using a single-layer dielectric of barrier-type anodic alumina (Al₂O₃) and an imprinted bottom Al electrode are presented. Imprinting of the bottom electrode aimed at increasing the capacitor effective surface area by creating a three-dimensional MIM capacitor architecture. The bottom Al electrode was only partly nanopatterned so as to ensure low series resistance of the MIM capacitor. With a 3 nm thick anodic Al₂O₃ dielectric, the capacitor with the imprinted electrode showed a 280% increase in capacitance density compared to the flat electrode capacitor, reaching a value of 20.5 fF/μm². On the other hand, with a 30 nm thick anodic Al₂O₃ layer, the capacitance density was 7.9 fF/μm² and the non-linearity coefficient was as low as 196 ppm/V². These values are very close to reaching all requirements of the last International Technology Roadmap for Semiconductors for MIM capacitors [ITRS, http://www.itrs2.net/2013-itrs.html for ITRS Roadmap (2013)], and they are achieved by a single-layer dielectric instead of the complicated dielectric stacks of the literature. The obtained results constitute a real progress compared to previously reported results by our group for MIM capacitors using imprinted electrodes.

Metal–insulator–metal (MIM) micro-capacitors for use in integrated energy storage applications are presented. A new, simple and batch Si processing compatible method for the creation of high aspect ratio metallic 3D structures on the surface of a Si substrate is described. The method consists of creating an array of Si nanopillars and then depositing Al at a small angle off the vertical while rotating the sample. Using this method, the effective area of the samples is increased by a factor of 3.8. Various capacitors are created using the described 3D structures as the lower electrode, with anodic alumina and atomic layer deposited HfO₂ as the dielectric. Al and Cu top electrodes are also investigated. Large values of capacitance densities as high as 3.2 μF cm⁻² are achieved. All capacitors are demonstrated to possess small values of series resistances and stable operation up to a frequency of 10⁵ Hz. These results make the presented MIM capacitors exceed the state-of-the-art while maintaining a simple and integrable fabrication scheme which renders them very interesting for energy storage applications where operational frequencies larger than 1 kHz are required, as is the case in several vibrational energy harvesters.

Barrier-type anodic alumina thin films are interesting for use in high capacitance density metal-insulator-metal capacitors due to their excellent dielectric properties at small thickness. This thickness is easily controlled by the anodization voltage. In previous papers we studied the main parameters of interest of the Al/barrier-type anodic alumina/Al structure for use in RF applications and showed the great potential of barrier-type anodic alumina in this respect. In this paper, we investigated in detail charging/discharging processes and leakage current of the above dielectric material. Two different sets of metal-insulator-metal capacitors were studied, namely, with the top Al electrode being either e-gun deposited or sputtered. The dielectric constant of the barrier-type anodic alumina was found at 9.3. Low leakage current was observed in all samples studied. Furthermore, depending on the film thickness, field emission following the Fowler-Nordheim mechanism was observed above an applied electric field. Charging of the anodic dielectric was observed, occurring in the bulk of the anodic layer. The stored charge was of the order of few μC/cm² and the calculated trap density ∼2 × 10¹⁸ states/cm³, the most probable origin of charge traps being, in our opinion, positive electrolyte ions trapped in the dielectric during anodization. We do not think that oxygen vacancies play an important role, since their existence would have a more important impact on the leakage current characteristics, such as resistive memory effects or significant changes during annealing, which were not observed. Finally, discharging characteristic times as high as 5 × 10⁹ s were measured.