The spatial resolution of conventional transcranial direct current stimulation (tDCS) is considered to be relatively diffuse owing to skull dispersion. However, here we show that electric fields may be clustered at distinct gyri/sulci sites due to details in tissue architecture/conductivity notably cerebrospinal fluid (CSF). We calculated the cortical electric field/current density magnitude induced during tDCS using a high spatial resolution (1 mm3) MRI-derived finite element human head model; cortical gyri/sulci were resolved. The spatial focality of conventional rectangular-pad (7 × 5 cm2) and the ring (4 × 1) electrode configurations were compared. The rectangular-pad configuration resulted in diffuse (un-focal) modulation, with discrete clusters of electric field magnitude maxima. Peak induced electric field magnitude was not observed directly underneath the pads, but at an intermediate lobe. The 4 × 1 ring resulted in enhanced spatial focality, with peak induced electric field magnitude at the sulcus and adjacent gyri directly underneath the active electrode. Cortical structures may be focally targeted using ring configurations. Anatomically accurate high resolution MRI-based forward-models may guide the ‘rational’ clinical design and optimization of tDCS.

Transcutaneous electrical stimulation is applied in a range of biomedical applications including Transcranial Direct Current Stimulation (tDCS). tDCS is a non-invasive procedure where a weak direct current (<2 mA) is applied across the scalp to modulate brain function. High-Definition tDCS (HD-tDCS) is a technique used to increase the spatial focality of tDCS by passing current across the scalp using <12 mm diameter electrodes.The purpose of this study was to design and optimize "high-definition" electrode-gel parameters for electrode durability, skin safety, and subjective pain. Anode and cathode electrode potential, temperature, pH, and subjective sensation over time were assessed during application of 2 mA direct current, for up to 22 minutes on agar gel or subject forearms. A selection of 5 types of solidconductors (Ag pellet, Ag/AgCl pellet, Rubber pellet, Ag/AgCl ring, and Ag/AgCl disc) and 7 conductive gels (Signa, Spectra, Tensive, Redux, BioGel, Lectron, and CCNY-4) were investigated.The Ag/AgCl ring in combination with CCNY-4 gel resulted in the most favorable outcomes. Under anode stimulations, electrode potential and temperature rises were generally observed in all electrode-gel combinations except for Ag/AgCl ring and disc electrodes. pH remained constant for all solid-conductors except for both Ag and Rubber pellet electrodes with Signa and CCNY-4 gels. Sensation ratings were independent of stimulation polarity. Ag/AgCl ring electrodes were found to be the most comfortable followed by Ag, Rubber, and Ag/AgCl pellet electrodes across all gels.

Transcranial Direct Current Stimulation (tDCS) is a non-invasive procedure where a weak electrical current (260 μA to 2 mA) is applied across the scalp to modulate brain function. tDCS has been applied for therapeutic purposes (e.g., addiction, depression, mood and sleep disorders) as well as cognitive performance enhancement (e.g., memory consolidation, motor learning, language recall). Despite safety and cost advantages, the developments of tDCS therapies have been restricted by spatial targeting concerns using existing two-channel systems. We have developed novel technology for High-Density tDCS (HD-tDCS) that improves spatial focality. To determine optimal stimulation electrode configurations, based on application specific constraints, we developed a HD-tDCS targeting software. High resolution (gyri/sulci precise) MRI derived finite element (FE) human head models are generated by segmenting grey matter, white matter, CSF, skull, muscle, fatty tissue, eyes, blood vessels, scalp, etc. The models comprised >10 million elements with >15 million degrees of freedom. The induced cortical electric field/current density values are calculated; activation of either radially and tangentially oriented neuronal structures are considered. Our HD-tDCS hardware (4×1-C1, 4×4-S1) currently supports the ‘4×1-Ring’ and the ‘4×4-Strip’ electrode configurations. The peak cortical electric field was matched to ‘conventional’ large rectangular-pad tDCS stimulation; however, the spatial focality was significantly enhanced by 4×1 configuration. Using patient specific head models, our software interface allows simple and rapid screening of stimulation electrode configurations. After selecting a target region, clinicians can customize the electrode configuration to balance: 1) cortical surface and brain depth stimulation focality; 2) total applied current/voltage; and 3) electrode/scalp current density. Our HD-tDCS system allows non-invasive, safe, and targeted modulation of selected cortical structures for electrotherapies that are individualized as well as optimized for a range of therapeutic applications.

Transcranial Direct Current Stimulation (tDCS) is a non-invasive procedure where a weak electrical current (260 μA to 2 mA) is applied across the scalp to modulate brain function. tDCS has been applied for therapeutic purposes (e.g., addiction, depression, mood and sleep disorders) as well as cognitive performance enhancement (e.g., memory consolidation, motor learning and language recall). Despite safety and cost advantages, the developments of tDCS therapies have been restricted by spatial targeting concerns using existing two-channel systems. We have developed novel technology for High-Density tDCS (HD-tDCS) that improves spatial focality. Integral to the system are specialized HD-tDCS electrodes (<12 mm diameter) which allow safe and comfortable passage of current across the scalp. Here we evaluate a range of HD-tDCS electrode designs for comfort as well as test electrode over-potential, pH, and temperature. Passing 2 mA current for 22 minutes, both anodal and cathodal stimulations were evaluated independently. Subjective sensation during forearm stimulation was evaluated in 8 subjects. The benefits of skin electrical or chemical pre-conditioning were tested. Conductive Rubber, Ag, AgCl, pellet electrodes and AgCl ring electrodes were evaluated in combination with salty gels (Signa and CCNY4) and nominally electrolyte free gel (Lectron). The use of AgCl ring electrodes in combination with CCNY4 gel resulted in no significant pH, temperature, or over-potential changes under either polarity stimulation and was well tolerated by subjects. HD-tDCS may thus be applied with 2 mA per electrode for up to 22 minutes without skin irritation. Moreover, skin pre-conditioning can eliminate sensation such that HD-tDCS can be applied in a blinded fashion and under a broad range of therapeutic and performance enhancement applications. Our HD-tDCS system allows non-invasive, safe, and targeted modulation of selected cortical structures for electrotherapies that are individualized as well as optimized for a range of therapeutic applications.

Transcranial Direct Current Stimulation (tDCS) is a non-invasive procedure where a weak electrical current (260 μA to 2 mA) is applied across the scalp to modulate brain function. tDCS has been applied for therapeutic purposes (e.g. addiction, depression, mood and sleep disorders) as well as cognitive performance enhancement (e.g. memory consolidation, motor learning and language recall). Despite safety and cost advantages, the developments of tDCS therapies have been restricted by spatial targeting concerns using existing two-channel systems. We have developed novel technology for High-Density tDCS (HD-tDCS) that improves spatial focality. Integral to the system are specialized HD-tDCS electrodes (<12 mm diameter) which allow safe and comfortable passage of current across the scalp. Here we evaluate a range of HD-tDCS electrode designs for comfort as well as test electrode over-potential, pH, and temperature. Passing 2 mA current for 22 minutes, both anodal and cathodal stimulations were evaluated independently. Subjective sensation during forearm stimulation was evaluated in 8 subjects. The benefits of skin electrical or chemical pre-conditioning were tested. Conductive Rubber, Ag, AgCl, pellet electrodes and AgCl ring electrodes were evaluated in combination with salty gels (Signa and CCNY4) and nominally electrolyte free gel (Lectron). The use of AgCl ring electrodes in combination with CCNY4 gel resulted in no significant pH, temperature, or over-potential changes under either polarity stimulation and was well tolerated by subjects. HD-tDCS may thus be applied with 2 mA per electrode for up to 22 minutes without skin irritation. Moreover, skin pre-conditioning can eliminate sensation such that HD-tDCS can be applied in a blinded fashion and under a broad range of therapeutic and performance enhancement applications. Our HD-tDCS system allows non-invasive, safe, and targeted modulation of selected cortical structures for electrotherapies that are individualized as well as optimized for a range of therapeutic applications.

Transcranial Direct Current Stimulation (tDCS) is a non-invasive procedure where a weak electrical current (260 μA to 2 mA) is applied across the scalp to modulate brain function. tDCS has been applied for therapeutic purposes (e.g., addiction, depression, mood and sleep disorders) as well as cognitive performance enhancement (e.g., memory consolidation, motor learning, language recall). Despite safety and cost advantages, the developments of tDCS therapies have been restricted by spatial targeting concerns using existing two-channel systems. We have developed novel technology for High-Density tDCS (HD-tDCS) that improves spatial focality. Our hardware interface integrates a multichannel stimulating guide with existing two channel tDCS stimulators, and can be configured to target specific brain regions using computational models of current flow and multichannel array accessories. The hardware interface provides real time stimulation quality and safety feedback, and is designed to be MRI and TMS compatible. An electrical “tickle” feature enables skin pre-conditioning to minimize sensation. The full system includes the hardware interface, cable assemblies, head gear, tDCS electrodes, tDCS gel, and electrode adaptors. The head gear allows fixing the electrode adaptors over cortical targets using conventional EEG electrode coordinates. The electrode adaptors “fin” design, tDCS gel composition, and electrode shape are optimized to reduce sensation during direct current stimulation with 2 mA for up to 22 minutes. A five electrode system (4×1-C1), for implementing optimally focal “4×1 ring configuration” protocols, and an 8 electrode system (4×4-S1), that can be configured for “4×4 cortical strip stimulation”, are available. The entire system is robust, intuitive, and ultimately adaptable for home use. Our HD-tDCS system allows non-invasive, safe, and targeted modulation of selected cortical structures for electrotherapies that are individualized as well as optimized for a range of therapeutic applications.