-

Introduction

Active stents consist of an electrically active component integrated into a stent and are intended to be used for monitoring physiological values such as blood pressure. Currently, there is no wireless, miniaturized, and functionalized system which is integrated into a stent, incorporating all requirements. Relevant are mechanical aspects during implantation procedure and body movement, plus long-term stability of all functionalities within the body.

The Chip-in-Foil technology represents a new approach for integrating an electrical circuit into a stent. Polyimide (PI) is used to apply and simultaneously encapsulate platinum conductors. This enables the manufacturing of a conformable circuit adaptable to the geometry of an existing stent.

Methods

Finite Element Analysis is utilized to understand the interaction and mechanical load conditions of a PI-based substrate braided into a Nitinol stent. Investigating the mechanical load, simplified models were generated in COMSOLTM in form of a cutout of the repetitive braiding pattern, specifically the PI-sample and stent strut. Conducted load represents the force generated by physiological blood pressure.

Results

The material behavior of the metal traces encapsulated in PI under load of up to 120 mmHg shows maximum stresses at the edges. Crossing points of the PI-sample and the Nitinol strut result due to braiding. The load onto the PI-sample was investigated by three cases: over, under and in-between two stent struts. The modelled case of the PI “in-between” the stent struts shows maximum stresses of approximately 2.2 ∙ 10-4 MPa. All cases show stresses at the interface between PI and Nitinol.

Conclusion

First insights of possible predetermined breaking points at the crossing points of a simplified integrated electrical circuit braided into a stent were investigated by computational modelling. The occurring stresses can be categorized as relatively low but must be experimentally validated in order to draw conclusions about the stability under cyclic load conditions.

Introduction

In the case of miniaturized neural interfaces, the surfaces are protected with biocompatible insulating polymers except for the microelectrodes. Here, area-selective etching offers the potential to replace error-prone and cost-intensive lithography steps, as the opening above the electrodes can be achieved by catalytic etching. Oxygen diffuses through parylene-C to dissociate atomically on the platinum electrode surface. The resulting reaction partner can etch the parylene-C above it. Parylene-C can be deposited from the gas phase and thus offers an excellent basis for the production of 3D electrodes and geometries.

Methods

Silicon wafers were coated with titanium/platinum and then coated with approximately 250 nm parylene-C. The wafers were placed in a recipient in which the table temperature could be adjusted and in-situ ellipsometry was possible. The wafers were tested for etching under different conditions (oxygen flow, temperature, pressure). Tests were carried out in a vacuum and at atmospheric pressure.

Results

Under a process pressure of up to 250 mTorr and gas flow of up to 100 sccm of oxygen and at temperatures of up to 300 °C, no catalytic etching could be detected using ellipsometry. Catalytic etching only occurred at temperatures above 230 °C and under atmospheric pressure (approx. 750 Torr). Thus, in addition to the temperature, the pressure was crucial for catalytic etching. An etching rate of approx. 11 nm/min could be calculated for the parylene-C on the platinum sample. No etching took place on the silicon sample under the same conditions.

Conclusion

The experiments have shown that selective etching of parylene-C on platinum is possible. This opens up new possibili-ties for the production and encapsulation of 3D electrodes. Not only can the electrode surfaces of platinum be selec-tively opened, but they can also be subsequently selectively coated using the atomic layer deposition process.

In this study, a passive control strategy of inductive power transfer (IPT) using ferroelectric multilayer ceramic chip capacitors (MLCCs) is presented. The required system parameters, i.e., ferroelectric hysteresis, frequency of the IPT, and voltage range across the MLCCs are reported. The receiver circuit consists only of a parallel resonant circuit, a half-wave rectifier and a load; the passive control of the IPT is achieved exclusively by the nonlinear properties of the ferroelectric MLCCs. The stabilization of the secondary output voltage ULoad at constant load is experimentally evaluated for an inductive coupling factor k between 10 % and 30 % for three nonlinear MLCCs #1, #2 and #3. With our proposed passive control strategy ULoad is maintained at -1.2 % and +0.6 % around a median value of 17.3 V (17.1 - 17.4 V) for k between 20 % and 30 % using MLCC #1, ±0.9 % around a median value of 11.2 V (11.1 - 11.3 V) for k between 10 % and 18 % using MLCC #2, and -1.6% and +2.6 % around a median value of 5.03 V (4.95 - 5.16 V) for k between 16 % and 30 % using MLCC #3. The proposed control principle is particularly advantageous for highly miniaturized microimplants, as it allows IPT control without additional semiconductors, sensors and vulnerable communication channels.