Joonas J. Heikkinen’s research while affiliated with Aalto University and other places

Flexible and conductive silver electrodes were fabricated by inkjet printing on several different compositions of thiol-ene polymers. Conductive electrodes with resistivity down to 30 µΩcm and good adhesion of the electrodes were obtained by optimizing the printing parameters. The maximum printing resolution was 100 µm lines and 80 µm gaps between the lines. Printing on top of cross-linked off-stoichiometric thiol-ene polymer was tested for compositions ranging from 30 % thiol excess to 5 % allyl (‘ene’) excess. The roughness off the thiol-ene surfaces was shown to greatly improve the quality of the printed electodes: consistently high yield of conductive electrodes was obtained on rough surfaces (roughness ≈ 1 µm), whereas on smooth surfaces the electrodes were often cracked. The lowest resistivity values were obtained on electrodes printed on near stoichiometric thiol-ene substrates. The conductivity of the electrodes was retained after 5 % linear strain and after repeated bending with 1 mm radius of curvature, showing the potential for flexible sensors. The electrodes were also applied to electrical impedance-based monitoring of cell growth on thiol-ene surfaces, which showcased that the electrodes survive stressed cell culture conditions for at least 36 h.

Different types of carbon materials are biocompatible with neural cells and can promote maturation. The mechanism of this effect is not clear. Here we have tested the capacity of a carbon material composed of amorphous sp 3 carbon backbone, embedded with a percolating network of sp 2 carbon domains to sustain neuronal cultures. We found that cortical neurons survive and develop faster on this novel carbon material. After 3 days in culture, there is a precocious increase in the frequency of neuronal activity and in the expression of maturation marker KCC2 on carbon films as compared to a commonly used glass surface. Accelerated development is accompanied by a dramatic increase in neuronal dendrite arborization. The mechanism for the precocious maturation involves the activation of intracellular calcium oscillations by the carbon material already after 1 day in culture. Carbon-induced oscillations are independent of network activity and reflect intrinsic spontaneous activation of developing neurons. Thus, these results reveal a novel mechanism for carbon material-induced neuronal survival and maturation.

Biofouling imposes a significant threat for sensing probes used in vivo. Antifouling strategies commonly utilize a protective layer on top the electrode but this may compromise performance of the electrode....

Pattern formation of pyrolyzed carbon (PyC) and tetrahedral amorphous carbon (ta-C) thin films were investigated at micro- and nanoscale. Micro- and nanopillars were fabricated from both materials, and their biocompatibility was studied with cell viability tests. Carbon materials are known to be very challenging to pattern. Here we demonstrate two approaches to create biocompatible carbon features. The microtopographies were 2 μ m or 20 μ m pillars (1:1 aspect ratio) with three different pillar layouts (square-grid, hexa-grid, or random-grid orientation). The nanoscale topography consisted of random nanopillars fabricated by maskless anisotropic etching. The PyC structures were fabricated with photolithography and embossing techniques in SU-8 photopolymer which was pyrolyzed in an inert atmosphere. The ta-C is a thin film coating, and the structures for it were fabricated on silicon substrates. Despite different fabrication methods, both materials were formed into comparable micro- and nanostructures. Mouse neural stem cells were cultured on the samples (without any coatings) and their viability was evaluated with colorimetric viability assay. All samples expressed good biocompatibility, but the topography has only a minor effect on viability. Two μ m pillars in ta-C shows increased cell count and aggregation compared to planar ta-C reference sample. The presented materials and fabrication techniques are well suited for applications that require carbon chemistry and benefit from large surface area and topography, such as electrophysiological and -chemical sensors for in vivo and in vitro measurements.

In the current study, platinum - present as a negligible component (below 1 ppb, the detection limit of the HR-ICP-MS at the dilutions used) in real industrial hydrometallurgical process solutions was recovered by an electrodeposition − redox replacement (EDRR) method on pyrolysed carbon (PyC) electrode, a method not earlier applied to metal recovery. The recovery parameters of the EDRR process were initially investigated using a synthetic nickel electrolyte solution ([Ni] = 60 g/L, [Ag] = 10 ppm, [Pt] = 20 ppm, [H2SO4] = 10 g/L) and the results demonstrated an extraordinary increase of 3·10⁵ in the [Pt]/[Ni] on the electrode surface cf. synthetic solution. EDRR recovery of platinum on PyC was also tested with two real industrial process solutions that contained a complex multi-metal solution matrix: Ni as the major component (>140 g/L) and very low contents of Pt, Pd and Ag (i.e. <1 ppb, 117 ppb and 4 ppb, respectively). The selectivity of Pt recovery by EDRR on the PyC electrode was found to be significant – nanoparticles deposited on the electrode surface comprised on average of 90 wt-% platinum and a [Pt]/[Ni] enrichment ratio of 10¹¹ compared to the industrial hydrometallurgical solution. Furthermore, other precious metallic elements like Pd and Ag could also be enriched on the PyC electrode surface using the same methodology. This paper demonstrates a remarkable advance-ment in the recovery of trace amounts of platinum from real industrial solutions that are not currently considered as a source of Pt metal.

Carbon-based materials have attracted much attention in biological applications like interfacing electrodes with neurons and cell growth platforms due to their natural biocompatibility and tailorable material properties. Here we have fabricated sputtered carbon thin film electrodes for bioelectrical measurements. Reactive ion etching (RIE) recipes were optimized with Taguchi method to etch the close field unbalanced magnetron sputtered carbon thin film (nanocarbon, nC) consisting of nanoscale crystalline sp²-domains in amorphous sp³-bonded backbone. Plasma etching processes used gas mixtures of Ar/O2/SF6/CHF3 for RIE and O2/SF6 for ICP-RIE. The highest achieved etch rate for nanocarbon was ≫389 nm/min and best chromium etch mask selectivity was 135:1. Biocompatibility of the material was tested with rat neuronal cultures. Next, we fabricated multielectrode arrays (MEA) with carbon recording electrodes and metal wiring. Organotypic brain slices grown on the MEAs were viable and showed characteristic spontaneous electrical network activity. The results demonstrate that interactions with nanocarbon substrate support neuronal survival and maturation of functional neuronal networks. Thus the material can have wide applications in biomedical research.

Here we investigated electrochemical properties and dopamine (DA) detection capability of SU-8 photoresist based pyrolytic carbon (PyC) as well as its biocompatibility with neural cells. This approach is compatible with microfabrication techniques which is crucial for device development. X-ray photoelectron spectroscopy shows that PyC consists 98.5% of carbon, while oxygen plasma treatment (PyC-O2) increases the amount of oxygen up to 27.1%. PyC showed nearly reversible (∆Ep 63 mV) electron transfer kinetics towards outer sphere redox probe (Ru(NH3)6 2+/3+), while the reaction on PyC-O2 was quasi-reversible (∆Ep >75 mV). DA showed both diffusion and adsorption-defined reaction kinetics with fast electron transfer with ∆Ep of 50 mV and 30 mV, for PyC and PyC-O2 respectively. The strong interaction between the hydroxyl groups on the surface and DA, as confirmed by simulations, facilitates the redox reactions of DA. DA showed linear response in the measured physiologically relevant range (50 nM – 1 µM) and sensitivities were 1.2 A M-1 cm-2 for PyC and 2.7 A M-1 cm-2 for PyC-O2. Plasma oxidation (PyC-O2) improved cell adhesion even more than poly-l-lysine (PLL) coating on PyC, but best adhesion was achieved on PLL coated PyC-O2. Glial cells, neuroblastoma cells and neural stem cells all showed similar behavior.

Kainate type of glutamate receptors (KARs) are highly expressed during early brain development and may influence refinement of the circuitry, via modulating synaptic transmission and plasticity. KARs are also localized to axons, however, their exact roles in regulating presynaptic processes remain controversial. Here, we have used a microfluidic chamber system allowing specific manipulation of KARs in presynaptic neurons to study their functions in synaptic development and function in vitro. Silencing expression of endogenous KARs resulted in lower density of synaptophysin immunopositive puncta in microfluidically isolated axons. Various recombinant KAR subunits and pharmacological compounds were used to dissect the mechanisms behind this effect. The calcium permeable (Q) variants of the low-affinity (GluK1–3) subunits robustly increased synaptophysin puncta in axons in a manner that was dependent on receptor activity and PKA and PKC dependent signaling. Further, an associated increase in the mean active zone length was observed in electron micrographs. Selective presynaptic expression of these subunits resulted in higher success rate of evoked EPSCs consistent with higher probability of glutamate release. In contrast, the calcium-impermeable (R) variant of GluK1 or the high-affinity subunits (GluK4,5) had no effect on synaptic density or transmission efficacy. These data suggest that calcium permeable axonal KARs promote efferent connectivity by increasing the density of functional presynaptic release sites.