The trends of miniaturization and increasing performance of electronics and sensors enable new fields of applications. Especially in the medical context, a variety of different examples for the monitoring of vital functions on the base of intelligent implants and plasters can be found. However, the currently used materials for adhesives and plasters often cause impairments of the normal skin function as well as severe skin irritations. To address this issue, a new approach is presented, which targets the realization of adhesive properties by micro-structuring biocompatible materials, and hence, avoid the usage of chemical adhesives at all. A particular advantage of this new approach is the structuring of materials that simultaneously meet the requirements given by the application as a functionalized medical plaster as well as the ones given by the use as a substrate for flexible electronics, namely the compatibility with established fabrication technologies.
Die Trends der Miniaturisierung und Funktionserweiterung im Bereich der Elektronik und Sensorik eröffnen neue Anwendungsgebiete. Insbesondere im Bereich Medizin finden sich vielfältige Beispiele für die Messung von Vitalfunktionen mit Hilfe von intelligenten Implantaten und Pflastern. Jedoch beeinträchtigen die dabei für Kleber und Pflaster eingesetzten Materialien häufig die Funktion der Haut und führen insbesondere beim Entfernen der Pflaster nicht selten zu Hautirritationen und -schäden. Um dieses Problem zu adressieren wird der neue Ansatz vorgestellt, durch Mikrostrukturierung von biokompatiblen Materialien adhäsive Eigenschaften zu realisieren und dadurch gänzlich auf die Verwendung von Klebern zu verzichten. Besonders vorteilhaft ist dabei die Strukturierung von Materialien, die durch ihre Eigenschaften sowohl als funktionalisiertes medizinisches Pflaster als auch – bedingt durch ihre Kompatibilität mit den etablierten Technologien – als Substrat für flexible Elektronik dienen können.
1 Introduction
Medical wearables, such as sensory plasters, that need direct contact with the human body to monitor patients’ vital signals require a biocompatible, reliable and long-term stable attachment to the skin. The attachment layer of a medical wearable is expected to be skin friendly, i. e. it would not impede normal functions of human skin (e. g. sweating, shedding and breathing) or provoke skin reactions, would stay conformal to the skin and can be easily attached and removed. Some strong adhesives that are currently used in medical plasters are painful to remove and their frequent removal may cause severe discomfort, skin reaction and allergic response. These symptoms are summarized as Medical Adhesive-Related Skin Injuries (MARSI).
The long-term reliability of the attachment layer is a key subject because some sensory plasters are expected to remain functional on the skin for a long period for monitoring chronic conditions. Beside the major advantage of having an adhesive-free attachment layer, the sensory plasters built upon these layers, which is in direct contact to the body, would benefit from a higher sensitivity and reduced noise due to the absence of an intervening medium in the signal path. An adhesive-free skin complaint and flexible attachment layer is an ideal solution for such issues.
Beside the requirement for biocompatibility and biostability, the ideal material for these applications should also be able to act as a substrate material for flexible electronics and hence, need to be compatible with the established fabrication technologies and microsystem technologies, respectively.
Parylene refers to the thermoplastic polymer family of Poly(p-xylylene), which combines a variety of unique properties: biostability and biocompatibility according to ISO 10993, chemical inertness against all common acids, bases and solvents, optical transparency, a high flexibility as well as a comparably good thermal stability and barrier properties. Due to its chemical inertness, Parylene is particularly compatible with established microsystem technologies and can act as a substrate material for flexible electronics and sensors (Fig. 1 and Fig. 2). Utilizing the biocompatibility of flexible Parylene based electronics medical wearables can be realized, e. g. for monitoring vital signals of patients. Hence, it would be highly beneficial to realize an adhesive-free flexible attachment layer directly on Parylene.
Fig. 1: Metal electrodes on a free-standing Parylene substrate
Fig. 2: Flexible Parylene based pH sensor
2 Experimental
Within a joint research project with the Institute of Microstructure Technology as part of the Karlsruhe Nano and Micro Facility (KNMF) located at the Karlsruhe Institute of Technology (KIT), the surface of Parylene is rendered by hot-embossing to various forms that enable a physically driven attachment. The hot-embossing process, which is chosen utilizing the thermoplastic properties of the polymer, is depicted in Figur 3. Doing so, a movable traverse containing the substrate to be patterned is pressed against a fixed frame containing the micro-structured mold insert (shim) at a defined temperature and pressure. The required shims with the inverted pattern are fabricated using the flow of microsystem technologies given in Figure 4. Particularly, direct laser writing of the inverted pattern into negative tone photoresist is used followed by subsequent metallization (Cr/Au evaporation) and thick nickel electroforming. After substrate removal (by wet chemical etching or lift-off) end resist removal (by plasma etching), microstructured nickel shims of 94 mm diameter and 0.8 mm to 1.0 mm thickness are fabricated and fixed on steel plates afterwards.
Fig. 3: Process flow for micro structuring by hot-embossing
Fig. 4: Process flow for the fabrication of a Nickel shim with the inverted pattern
The hot-embossing process is demonstrated for several variations of the column dimensions. Different shims are realized, particularly varying the diameter of and the pitch between these columns. The hot-embossing process itself is optimized with respect to the embossing temperature, embossing force and pressure, respectively, as well as the demolding temperature. The obtained samples are characterized by Scanning Electron Microscopy (SEM) and profilometry in order to investigate their shape, morphology and dimensions, respectively.
The micro-patterning is demonstrated on Parylene C thin films which are deposited at room temperature on blocks of stainless steel (Fig. 5) using a Plasma Parylene LC 300 RW (Plasma Parylene Systems GmbH) equipment. Therefore the backside of the blocks is masked in order to obtain a coating on one side. The Parylene thickness is varied between 10 µm and 30 µm.
Fig. 5: Block of stainless steel used as an insert for the hot-embossing process
3 Results and discussion
SEM images of the hot-embossing results of Parylene C are depicted in Figure 6 for the realized Parylene pillars. Their dimensions are > 10 µm in width, height and pitch, respectively. The aspect ratio of the pillars is 0 to 1. For the hot-embossing process, an embossing temperature of 310 °C is identified to produce good results. Other process parameters to vary are the force and the demolding temperature, respectively. The process is performed in vacuum in order to avoid degeneration or oxidation of the Parylene C. Different challenges such as non-uniform patterning, delamination (Fig. 7) and torning-off (Fig. 8) or cracking the Parylene layer on the stainless steel blocks are overcome by optimizing the hot-embossing process. Additionally, a dependency of the hot-embossing result on the Parylene thickness is noticed.
Fig. 6: SEM image of the hot-embossed pillars on Parylene C
Fig. 7: SEM image of delaminated Parylene C due to not fully optimized demolding process
Fig. 8: SEM image of torned-off Parylene C due to not fully optimized demolding process
The profilometer measurements given in Figure 9 reveal the dimensions of the hot-embossing process. The micro-structured pillars show different forms at their top end: simple conic shapes (a), step-wise tops (b) and suction-cup like structures. The latter are helpful for the targeted adhesion properties, i. e. to obtain self-sticking (adhesive-free) polymer layers. Particularly, the successfully micro-structured layers fabricated by the hot-embossing process can be used as a generic adhesion layer for medical plasters and wearables in long-term use, or can be integrated with functional layers. Furthermore, the approach can be transferred to other materials such as Poly-dimethylsiloxan (PDMS).
Fig. 9: Results of profilometer scans on hot-embossed Parylene pillars: (a) simple pillars, (b) step-wise pillars, (c) suction cups
4 Conclusion
In conclusion, the feasibility of the realization of Parylene micro-structures by hot-embossing is confirmed and, hence, the way is paved for further process optimization. Nevertheless, unsolved issues remain with respect to the adhesion of Parylene to the shim, and the substrate as well as to the reproducibility. Within a new and subsequent KNMF project, currently the partners focus on further optimizing the process and a detailed characterization of the sticking properties of the fabricated structures. Furthermore, inspired by nature, research is carried out on the realization of hair-like sub-structures on top of the pillars in similarity to a Gecko’s foot.
Acknowledgements
This work was supported by the Karlsruhe Nano and Micro Facility (KNMF) at the Karlsruhe Institute of Technology (KIT), which is part of the Helmholtz Association of German Research Centers. Furthermore, this work was supported by the Fraunhofer Society. The authors thank all involved colleagues for their support and fruitful discussions.
Corresponding author
Franz Selbmann, Fraunhofer ENAS, Technologie-Campus 3, D-09126 Chemnitz
E-Mail: franz.selbmann@enas.fraunhofer.de
1 Fraunhofer Institute for Electronic Nano Systems, Chemnitz, Germany
2 TU Bergakademie Freiberg, Institute for Electronic and Sensor Materials, Freiberg, Germany
3 TU Chemnitz, Center for Microtechnologies, Chemnitz, Germany
4 Institute of Microstructure Technology and Karlsruhe Nano and Micro Facility, Karlsruhe Institute of Technology, Karlsruhe, Germany
