• hand biomechanics
• surface electromyography
• tattoo electronics

Surface electromyography (sEMG) is widely used in several applications, including the diagnosis of neurological and neuromuscular diseases, rehabilitation of motor disabilities, and prosthetic control. sEMG is a non-invasive recording of electrical signals generated by muscle activity, from the skin surface.
Muscle contraction is initiated by neural impulses delivered by motor neurons. Each motor neuron innervates a specific group of muscle fibers, collectively forming a motor unit (MU), which is the fundamental functional element of the neuromuscular system. Upon activation, the motor neuron induces action potentials in all the fibers it controls, and the combined electrical response constitutes a motor unit action potential (MUAP). The EMG signals reflects the summation of MUAPs from multiple MUs. Therefore, by analyzing EMG signal, particularly through decomposition techniques that isolate individual MUAPs, it is possible to extract information not only about muscle activation, but also about the underlying central nervous system activity, such as motor unit recruitment strategies and neural drive.
Over time, sEMG has evolved from single-channel to high-density surface EMG (HD-sEMG) systems. In HD-sEMG, multiple electrodes (32, 64, or more) are used to record activity from various regions of the same muscle. This configuration enables higher spatial resolution, and for this reason, HD-sEMG is used for analysis of neuromuscular activity, as it allows the spatial mapping of muscle activation and the decomposition of signals into individual MUAPs.

Traditional electrodes are stiff and bulky; they do not match the compliance of the skin. This is limiting the applications of sEMG on curved body parts, signal stability (i.e., movement artefacts, missing data due to sensors detachment), and user comfort and mobility. The inherent stiffness and bulkiness of traditional electrodes can affect skin displacement during muscle activation, resulting in movement artifacts due to relative motion between the skin and the electrodes. Moreover, traditional electrodes require an electrolytic gel to reduce skin-electrode impedance, but the gel dries over time, leading to signal instability and potential short circuits in high-density arrays.
Arrays made with such conventional electrodes are rigid, preventing their application on curved surfaces, such as the hand.
As a result, only a few studies have employed HD-sEMG to investigate the small intrinsic muscles of the hand, often using a limited number of electrodes. This has hindered a comprehensive understanding of hand biomechanics.
Intrinsic hand muscles, including the lumbricals, interossei (palmar and dorsal), thenar and hypothenar groups, play a crucial role in refining the actions of the extrinsic muscles by enabling complex movements, such as simultaneous flexion of the metacarpophalangeal joints and extension of the interphalangeal joints. Alongside extrinsic muscles, they contribute to coordinated movement and contribute to hand strength. Despite their key role in fine motor control, the activation patterns of intrinsic muscles during tasks like pinching and gripping are not yet fully understood.

To address these limitations, in my thesis I explored the use of tattoo electronics to enable HD-sEMG recordings from intrinsic hand muscles.
In the field of biosignal monitoring, epidermal electronics have emerged as a promising solution to overcome the key limitations of traditional skin-mounted interfaces. Especially, tattoo electronics (TE) have been developed for the fabrication of ultrathin (thickness of less than 1 μm) and ultraconformable electrodes. Thanks to their low footprint such sensors can conform to any substrate, acting as a second skin, exhibiting good adhesion, and thus limiting relative movements between the skin and the electrodes which cause movement artifacts. These devices were fabricated by printing conductive polymers, such as PEDOT:PSS, onto temporary tattoo paper. This substrate is low-cost, biocompatible, available in large-area formats, and ensures ease of application. It consists of multiple layers, and the conductive ink Poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) is printed onto the outermost layer. PEDOT:PSS is a commercially available conductive polymer known for its high electrical performance, processability, stability, biocompatibility, and optical transparency.
The fabrication process involves screen printing, a versatile and cost-effective technique that enables the controlled deposition of inks through a patterned mask, making it well-suited for fabricating precise and repeatable electrode patterns.
Once printed, the arrays are transferred to the skin by dissolving a water-soluble sacrificial layer, enabling conformal adhesion to complex anatomical surfaces such as the hand.

In this thesis, I have developed HD tattoo electrodes for surface hand EMG. The primary objective was to optimize the design and fabrication of tattoo electrode arrays specifically for this application. I have studied and developed tailored matrices of electrodes, able to tackle the main intrinsic muscles of the hand, from the palm and the dorsal.
Anthropometric data were considered to design electrode arrays adaptable to various hand shapes and sizes. The electrode arrays were designed to target specific muscles as the Thenar and Hypothenar groups, the Lumbricals in the palmar region, and the Interossei muscles on the dorsal side. The palmar matrices were developed to enable a comprehensive investigation of all intrinsic hand muscles, ensuring full coverage of the three muscle groups within the palm, whereas the dorsal matrix was specifically designed to target the first dorsal interosseous (FDI) muscle. Two matrices for the palm were fabricated: one with 64 electrodes (3 mm diameter) and another with 48 electrodes (5 mm diameter). Additionally, one dorsal matrix with 32 electrodes (3 mm diameter) was created. The inter-electrode distance (IED) was uniformly set to 5 mm across all designs. The matrices were produced via screen printing onto tattoo paper, using custom-designed masks.


A challenging aspect was the fabrication of stable interconnections and external connectors, while maintaining an easy release on the skin. A key requirement was to ensure a reliable electrical interface between the PEDOT:PSS electrodes and the connector of the acquisition system. Simultaneously, it was necessary to bridge the dimensional gap between the ultrathin electrode layer and the macroscopic dimensions of the acquisition hardware.
To address this, the interconnections involved silver tracks on a thin polyamide substrate, providing mechanical flexibility and electrical conductivity.

Finally, the connector, compatible with the benchtop acquisition systems (Myolink and Quattrocento, OT Bioelettronica) was fabricated using additive manufacturing techniques, including fused deposition modeling (FDM) and stereolithography (SLA). These custom-designed connectors were optimized for wearability, ease of insertion, and to provide a smooth transition from the tattoo electrode arrays to the acquisition system.

I’ve fabricated and tested the electrodes array on 17 healthy volunteer, at the Imperial College London. The protocol was made by static and dynamic exercises and two electrode arrays were used for each subject: a 32-electrode array placed on the dorsal side of the hand, and either a 64-electrode or a 48-electrode array positioned on the palmar side.
The acquisition protocol consisted of two phases: an initial isometric EMG and force recording session, followed by dynamic data collection combining EMG acquisition with motion capture for kinematic analysis. The acquisition system used in this study was Myolink, a custom-designed system for HD-sEMG recordings.

In conclusion, this work focused on the design and fabrication of tattoo electrode arrays for HD-sEMG recordings of the hand. This technology overcomes several limitations of traditional EMG systems by offering skin conformability, wearability, and enhanced spatial resolution.
Thanks to their properties, the tattoo electrode arrays allowed for simultaneous recording from intrinsic hand muscles, enabling the investigation of synergistic muscle coordination during hand movements. This represents the first implementation of a high-density electrode array with 32 or more channels that is both conformable to the skin and stable throughout the entire experimental protocol.
Their implementation in preliminary data collection on healthy subjects demonstrates their potential for comprehensive muscle activity monitoring, a deeper understanding of hand biomechanics, and the generation of data for hand gesture recognition.