Experimental setup for insertion of needles and microneedles into soft solids including biological tissue

CAD files for experimental setup to insert needles and microneedles into soft solids including biological tissue. Main assembly file is "A_AssemblyWithoutOCT". CAD files created in SolidWorks, including assemblies and parts. Experimental setup used for dynamic insertion (up to 4-5 m/s) of needles and microneedles into soft solids. The different components of the experimental setup are: - Imaging system: The imaging system includes a high-speed camera, a telecentric lens and a ring light illuminator. The high-speed camera (Phantom Miro 4, Vision Research Inc.) with a 12 bit 800x600 SR-CMOS sensor has a pixel size of 22 µm. For the dynamic insertions, the frame rate is set to 7800 frames/second with a field of view of 208 pixels by 320 pixels (4.576 mm by 7.040 mm). A telecentric lens (1.0X - 3.0X VariMagTL™ Telecentric Lens, Edmund Optics Inc.) is attached to the high-speed camera to eliminate parallax/magnification errors. A ring light, powered by a halogen lamp (Steromaster Illuminator 12-562-6, Fisher Scientific), is attached to the telecentric lens to illuminate the soft solid sample and the needle assembly. - Translational stages: A vertical (Z) translational stage (Compact Lab Jack, Thorlabs Inc.) under the camera adjusts the field of view (FOV) for the high-speed camera and another stage under the soft solid adjusts the relative position between the needle and the soft solid. A horizontal (Y) translational stage (DTS50, 2" Dovetail Translation Stage, Thorlabs Inc.) adjusts the FOV of the high-speed camera in that direction. Horizontal (X) translation to adjust the focus of the high-speed camera is implemented using two rail carriages (XT95P13 and XT95RC3, Thorlabs Inc.) that slide on a horizontal rail (XT95SP-500, Thorlabs Inc.). - Force sensor: A piezoelectric force sensor (208C01, ICP Force sensor, PCB Piezotronics) is mounted, through a 3D printed adapter, to the vertical translational stage under the soft solid. The top part of the sensor includes an impact cap (084A03, PCB Piezotronics), on which a 2 mm thick stiff PDMS sample (made with 10:1 mixing ratio) rests to prevent damage to the sensor and the 4 mm PDMS sample (made with 30:1 mixing ratio) used for insertion. By setting the analog gain of the data acquisition system (DAQ) to 10, the measurement range for the force was set to ±8.811 N and the force resolution measured by the DAQ was around 0.3 mN.- Vertical motion: Two linear sleeve bearings (6673K11, High-Speed Linear Sleeve Bearing, McMaster-Carr) are mounted on a 3D printed housing that is fixed to the frame, while two linear motion shafts (1031K62, Linear Motion Shaft Ceramic-Coated 6061 Aluminum, McMaster-Carr) interface with the bearings. The shafts are cut to lengths of 45 mm and are attached to the moving assembly that includes the needle. Prior to using linear bearings, compliant mechanisms or flexures were also tested to prevent lateral motion (Fig. S5 in Supplementary information), but linear bearings were chosen for the final design because of their superior positional accuracy. - Moving assembly: The moving assembly has a mass of around 20 grams and consists of a ferromagnetic disk (made of mild steel) at the top, two aluminum linear motion shafts, the needle-Luer assembly and 3-D printed connectors between these components. - Spring: Compression of a 1 inch long spring (9002T467, McMaster-Carr) with a spring constant of 1677 N/m provides the impact energy of the needle and moving assembly. This spring provides impact velocities up to 4-5 m/s, and it could also be swapped for other stiffer springs if higher impact velocities are required. - Vertical positioning system: An electromagnet (BDE-1212-12, Bunting Magnetics Co.) holds the ferromagnetic plate and the moving assembly in place after spring compression, and can release the moving assembly during the experiment. The electromagnet is mounted on a vertical position stand and can be positioned vertically in 1 mm increments (which corresponds to force increments of around 1.7 N and impact velocity increments of around 0.3 m/s). - Connections: The piezoelectric force sensor is connected to a signal conditioner (482C05, PCB Piezotronics) and a data acquisition (DAQ) system (NI USB 6211, National Instruments Co.) that is connected to a laptop (Latitude 7490, Dell). The high-speed camera is connected to the laptop via an ethernet cable, and is also connected to the DAQ to enable synchronization of the high-speed imaging and the force measurements through software (Phantom Camera Control, Vision Research Inc.). The electromagnet is connected to a power supply via a double pole double throw (DPDT) switch, which enables reversing the polarity of the electromagnet to release the moving assembly. The following procedure is followed for dynamic insertion and re-insertion experiments: 1. The soft solid sample (coated with carbon black and cut to a size of 10 mm by 10 mm) is placed on top of the piezoelectric force sensor and a conservative wait time of at least 5 minutes before insertion allows the force signal to decay and stabilize completely (discharge time constant of the force sensor is ≥50 sec). 2. The relative position of the needle tip and the soft solid surface is adjusted such that they are just in contact at the equilibrium position of the spring. Live imaging from the high-speed camera helps identify the position of contact. 3. With the electromagnet turned on, the moving assembly is moved upwards to contact the ferromagnetic disk with the electromagnet, which holds the moving assembly in place with the spring compressed at the preset compression length. 4. The high-speed camera is triggered and simultaneously, the DPDT switch is flipped to either turn the current off or to reverse the current polarity in the electromagnet, which releases the moving assembly and facilitates dynamic insertion of the needle into the soft solid. 5. For the re-insertion experiment, the moving assembly is moved upwards to remove the needle from the soft solid while the soft solid remains in the same position, and steps 3 and 4 are repeated after a 5 minute wait for the force signal to decay/stabilize.

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