Christian Peters

An Integrated Microrobotic Platform for On‐Demand, Targeted Therapeutic Interventions

The presented microrobotic platform combines together the advantages of self-folding NIR light sensitive polymer bilayers, magnetic alginate microbeads, and a 3D manipulation system, to propose a solution for targeted, on-demand drug and cell delivery. First feasibility studies are presented together with the potential of the full design.

Degradable Magnetic Composites for Minimally Invasive Interventions: Device Fabrication, Targeted Drug Delivery, and Cytotoxicity Tests

Superparamagnetic nanoparticles and a functional, degradable polymer matrix based on poly(ethylene glycol) are combined to enable fully degradable magnetic microdevices for minimally invasive biomedical applications. A bioinspired helical microrobot platform mimicking Escherichia coli bacteria is fabricated and actuated using weak rotating magnetic fields. Locomotion based on corkscrew propulsion, targeted drug delivery, and low-degradation-product cytotoxicity are demonstrated.

Shape-Switching Microrobots for Medical Applications: The Influence of Shape in Drug Delivery and Locomotion

The effect of dynamic shape switching of hydrogel bilayers on the performance of self-folding microrobots is investigated for navigation in body orifices and drug release on demand. Tubular microrobots are fabricated by coupling a thermoresponsive hydrogel nanocomposite with a poly(ethylene glycol)diacrylate (PEGDA) layer, to achieve spontaneous and reversible folding from a planar rectangular structure. Graphene oxide (GO) or silica-coated superparamagnetic iron oxide nanoparticles are dispersed in the thermoresponsive hydrogel matrix to provide near-infrared (NIR) light sensitivity or magnetic actuation, respectively. The NIR light-responsive microstructures are fabricated for triggered drug delivery while magnetic nanocomposite-based microrobots are used to analyze the role of shape in locomotion. Experimental analysis and computational simulations of tubular structures show that drug release and motility can be optimized through controlled shape change. These concepts are finally applied to helical microrobots to show a possible way to achieve autonomous behavior.

Inkjet Printing of High Aspect Ratio Superparamagnetic SU-8 Microstructures with Preferential Magnetic Directions

Structuring SU-8 based superparamagnetic polymer composite (SPMPC) containing Fe3O4 nanoparticles by photolithography is limited in thickness due to light absorption by the nanoparticles. Hence, obtaining thicker structures requires alternative processing techniques. This paper presents a method based on inkjet printing and thermal curing for the fabrication of much thicker hemispherical microstructures of SPMPC. The microstructures are fabricated by inkjet printing the nanoparticle-doped SU-8 onto flat substrates functionalized to reduce the surface energy and thus the wetting. The thickness and the aspect ratio of the printed structures are further increased by printing the composite onto substrates with confinement pedestals. Fully crosslinked microstructures with a thickness up to 88.8 μm and edge angle of 112° ± 4° are obtained. Manipulation of the microstructures by an external field is enabled by creating lines of densely aggregated nanoparticles inside the composite. To this end, the printed microstructures are placed within an external magnetic field directly before crosslinking inducing the aggregation of dense Fe3O4 nanoparticle lines with in-plane and out-of-plane directions.

Superparamagnetic swimming microrobots with adjusted magnetic anisotropy

We present the fabrication of soft-magnetic helical micro robots employing two-photon polymerization of a superparamagnetic polymer composite. The proposed fabrication method allows for adjusting the magnetic easy axis independent from the helical shape by aligning the embedded superparamagnetic nanoparticles prior to composite crosslinking. In contrast to conventional, shape-anisotropic helical micro swimmers, the proposed devices possess a magnetic easy axis perpendicular to their helical axis and thus benefit from a significant performance increase including a wobbling-free swimming behavior at low speeds as well as an increase in forward velocity of more than 250%. The presented results highlight the importance of the magnetic easy axis for actuation purposes and imply increased performance for the entire class of superparamagnetic polymer actuators.

Skin Conformal Polymer Electrodes for Clinical ECG and EEG Recordings

Preparation-free and skin compliant biopotential electrodes with high recording quality enable wearables for future healthcare and the Internet of Humans. Here, super-soft and self-adhesive electrodes are presented for use on dry and hairy skin without skin preparation or attachment pressure. The electrodes show a skin-contact impedance of 50 kΩ cm2 at 10 Hz that is comparable to clinical standard gel electrodes and lower than existing dry electrodes. Microstructured electrodes inspired by grasshopper feet adhere repeatedly to the skin with a force of up to 0.1 N cm-2 without further attachment even during strong movement or deformation of the skin. Skin compliance and adhesive properties of the electrodes result in reduction of noise and motion artifacts superior to other dry electrodes reaching the performance of commercial gel electrodes. The signal quality is demonstrated by recording a high-fidelity electrocardiograms of a swimmer in water. Furthermore, an electrode with soft macropillars is used to detect alpha activity in the electroencephalograms from the back of the head through dense hair. Compared to gel electrodes, the soft biopotential electrodes are nearly imperceptible to the wearer and cause no skin irritations even after hours of application. The electrodes presented here could combine unobtrusive and long-term biopotential recordings with clinical-grade signal performance.

Self-folding mobile microrobots for biomedical applications

The presented microrobotic platform combines the advantages of self-folding NIR light sensitive polymer bilayers, magnetic alginate microbeads, and a 3D manipulation system and introduces a solution for targeted, on-demand drug and cell delivery. First feasibility studies are presented together with the potential of the full design.

Superparamagnetic hydrogels for Two-Photon Polymerization and their application for the fabrication of swimming microrobots

This work reports on superparamagnetic hydrogel composites suitable for Two-Photon Polymerization (TPP) and their application for the fabrication of helical swimming microrobots, also known as Artificial Bacterial Flagella (ABFs). Suitable hydrogel compositions consisting of multifunctional monomers were identified based on percolation theory and prepared accordingly. Hydrogel ABFs 28 μm in length and 3 μm in diameter were fabricated and actuated in DI water. The investigated hydrogel (polyethylene glycol (PEG)) is of particular interest to the biomedical MEMS community. PEG can absorb/release water-soluble drugs and is highly resistant to protein adsorption. The combination of PEG and superparamagnetic nanoparticles enables corkscrew propulsion of ABFs, making them ideal candidates for biomedical applications in vivo and in vitro.

Visible light curing of Epon SU-8 based superparamagnetic polymer composites with random and ordered particle configurations.

The performance of superparamagnetic polymer composite microdevices is highly dependent on the magnetic particle content. While high loading levels are desired for many applications, the UV absorption of these nanoparticles limits the overall thickness of the fabricated microstructures and subsequently their capability of magnetic interaction. The combination of a visible-light-sensitive photoinitiator and particle self-organization is proposed to extend the exposure depth limitation in Epon SU-8 based superparamagnetic polymer composites. While superparamagnetic iron oxide particles strongly absorb i-line radiation required to cross-link the Epon SU-8 polymer matrix, we propose the utilization of H-Nu 470 photoinitiator to expand the photosensitivity of the composite toward the visible spectrum, where the dispersed nanoparticles are more transparent. The novel photoinitiator preserves the composites superparamagnetic properties as well as a homogeneous particle distribution. As a result, particle load or resist thickness can be more than doubled while maintaining exposure time. The self-organization of ordered magnetic structures allows for an additional increase in exposure depth of up to 40%, resulting in a 2.5-fold saturation magnetization.

Strain engineered 3D magnetic micro actuators with programmed magnetic anisotropy

This work combines programmed magnetic anisotropy and strain engineering in superparamagnetic polymer composites. The proposed approach enables tailored deformation of planar strips into three-dimensional (3D) micro actuators with shape-independent magnetic properties in a single wafer-scale fabrication step. The developed process allows for pattern feature sizes below five μm using a superparamagnetic polymer composite with a particle content of 10 %vol. The feasibility of this approach is demonstrated by fabricating helically shaped swimming microrobots, also known as Artificial Bacterial Flagella (ABF), with shape-independent magnetic properties and diameters as small as 100 μm. The necessity of decoupling the magnetic properties from the shape was confirmed by demonstrating that only ABFs with programmed anisotropy, i.e. shape-independent magnetic properties, are capable of performing desirable locomotion patterns.