Needles are advanced tools commonly used in minimally invasive medical procedures. The accurate manoeuvrability of flexible needles through soft tissues is strongly determined by variations in tissue stiffness, which affects the needle-tissue interaction and thus causes needle deflection. This work presents a variable stiffness mechanism for percutaneous needles capable of compensating for variations in tissue stiffness and undesirable trajectory changes. It is composed of compliant segments and rigid plates alternately connected in series and longitudinally crossed by four cables. The tensioning of the cables allows the omnidirectional steering of the tip and the stiffness tuning of the needle. The mechanism was tested separately under different working conditions, demonstrating a capability to exert up to 3.6 N. Afterwards, the mechanism was integrated into a needle, and the overall device was tested in gelatine phantoms simulating the stiffness of biological tissues. The needle demonstrated the capability to vary deflection (from 11.6 to 4.4 mm) and adapt to the inhomogeneity of the phantoms (from 21 to 80 kPa) depending on the activation of the variable stiffness mechanism. [Figure not available: see fulltext.]

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Needles are instruments frequently employed in Minimally Invasive Surgery (MIS), and are especially requested in percutaneous diagnosis and in local therapies [1]. During procedures, tissue inhomogeneity, tissue deformation, and thus needle deflection can cause tip misplacement resulting in inaccurate anesthesia, repuncturing with additional damages of the tissue or false negative detection [2]. Previous studies focused on needle-tissue interaction and possibilities to improve the needle steerability [3]. The proposed work is aimed to design a new concept of needle based on variable stiffness able both to balance tissue inhomogeneity and to guarantee needle steering during the insertion. The structure of the Variable Stiffness Mechanism (VSM) allows the needle to adjust the deflection when passing through tissues, while the omnidirectional steering of the tip facilitates the achievement of the target.@en