3D-Bioprinter

Patented Trimed 3D-Printer for Biomaterial

With 3D printing, complex geometries can be tailor-made for the patient without loss of material and without the need to produce a mold.
The printed biomaterial eliminates the usual removal of the body's own bone material. Possible rejection reactions of the body to the implanted foreign body are largely avoided.

Trimed's Biomaterial 3D Printer, which is capable of producing high-precision biomaterial bone replacements, is the only 3D printer in the world to use a proprietary system with a printhead that can move freely and precisely in space over six axes while still pivoting. The system uses eight cables which are all controlled separately via a geared motor.
The printing process is genuinely three-dimensional, unlike previous systems available on the market, where the object to be produced is divided into layers and then printed in layers. For example, an individual computed tomographic scan directly from the patient is used as a template for the bone part to be replicated.

The material is synthesized in the printing process as a porous structure; After a transplantation, the surrounding tissue now connects to the artificial bone. The interstices of the porous tissue are penetrated by blood vessels and filled with the body's own bone cells, so that a new bone has formed after the degradation of the organic plastic.

For reinforcement, titanium rods can be incorporated during the printing process so that the bones can be moved under full pressure immediately after insertion.

In a further step, the machine can, if necessary, by means of appropriate milling tools subsequent machining operations (holes, grooves, etc.) into the finished surface structure of the 3D printing object, so that the print object already demonstrates the necessary connections in a subsequent screwing or mounting a joint.

The decisive advantage of the patented six-axis cable system is the optimization of the smooth operation during surgery: After a printing process has been completed, the system can be easily disassembled, unlike conventional 3D printers working with significantly more mechanical parts, and together with the rest of surgical instruments in the printer can be sterilize together with existing disinfection equipment in the surgical department, so that a completely germ-free medical apparatus is available for the next printing process.

The print object is created in a completely enclosed workspace. This avoids environmental influences such as dust, dirt and bacteria. Work places can be placed under vacuum, overpressure and or inert gas atmosphere to avoid reactions of the biomaterial with the ambient air. In addition, the work space can be heated. Through the viewing window, the printing process can be observed.

The Biomaterial 3D printer uses the principle of the rope robot to position the printhead. This uses eight ropes or straps to position and load the manipulator in all six degrees of operation. On the printhead up to two additional axes of rotation can be attached, if the mobility in the rotational degrees of freedom by the cable robot alone is insufficient.

The positions of the rope exit points of the machine form like the work space a cuboid. The positions of the cable fasteners on the print head can form a flat square, a tetrahedron or a cuboid. In the first two cases, two ends of the rope are merged at the same point. In all three cases, the attachment points on the printhead may be parallel or rotated by 45 ° with respect to the working space in the neutral position. For all cases there are several cable arrangements according to the following table:

Parallel
45°

Square

Tetrahedron

Cuboid

For reasons of hygiene, the eight drives are located outside the working area. The torque is transmitted via a shaft or via a non-contact magnetic coupling in the working space. In the first case, the working space at the entry point of the rotating shaft is sealed with a radial shaft seal.

Inside the pressure room is the cable drum. The rope is wound spirally on a plane so that the ropes do not cross each other. Thus, the rope length over the drum rotation and the cable force on the torque is always clearly defined. The rope drum is in two parts and can be easily disassembled for sterilization.

The cable drum has an outer toothing, where it is driven by a worm shaft. The cable drum is pivotally mounted around the drive shaft so that the print head can get into all corners of the working space. The pivoting movement of the cable drum is not driven, but is aligned by the cable tension itself after the printhead.

The printhead consists of the eight spherical cable suspensions, a nozzle and a valve. With a pump, the liquid or viscous polymer for 3D printing is fed to the print head and dosed. In order to insert prefabricated reinforcing elements, for example made of titanium, the printhead also has a mechanical gripper.

The supply of the printing material to the print head is effected by a flexible spiral hose or a container on the print head is filled directly with the printing material. The energy supply is made by a spiral cable or on the printhead is a battery. The signal transmission also takes place via spiral cable or via radio.

The purpose of the control loop is to track as closely as possible the target position of the print head interpolated from the G code and constantly changing. The control loop contains the following digital function blocks: geometry model, cable force control, position determination, position control, linearization and adaptive LQ control.

The measured variables for the control loop are all eight cable lengths, which are determined by encoders on the drives. In addition, an inertial system of accelerometers and gyroscopes, a biaxial gravity pendulum, time-of-flight signal transmitters or digital cameras can be used to perform the Increase measurement accuracy. The control variables of the control loop are the rope forces, respectively the torques of the eight rope drums.


Overall scheme of the control loop

The blocks linearization and LQ controller can be clocked about 10 times more slowly according to simulations, without any problems in order to save computing power.

From the nominal position and target angle of the print head, the geometry model calculates the cable vectors from the cable end on the print head to the exit point at the cable drum. From these, the rope lengths can be derived directly. The normalized rope vectors times the rope forces give the vectorial forces generated by the ropes on the printhead. The cross product of rope endpoints and rope forces gives the vectorial torques generated by the ropes on the printhead. The input matrix has one column with 6 rows for every 8 ropes. Each column contains in the top three lines the normalized cable vector and in the lower three lines the cross product of cable end points and normalized cable vector of the respective cable. The matrix multiplication of the input matrix with all the cable forces thus gives the total force generated by the ropes and the total moment on the printhead.


Schema geometry model

The rotation matrix calculates the 3x3 matrix that can be used to convert any point from the printhead coordinate system to the auxiliary coordinate system that is parallel to the machine coordinate system but of the same origin as the printhead coordinate system.

The cable force control calculates the ideal rope forces from the input matrix to balance the print head at that position. For this purpose, the eighth-order linear system of equations is reduced to a sixth-order resolvable system by fixing the two lowest cable forces to the fixed minimum preload force. In order to reduce the computational load, this calculation can be preceded and the cable forces either in accordance with the movement sequence in G-code given or deposited depending on the printhead position in a multi-dimensional table and interpolated at runtime.


Cable force curve

The picture shows the calculated theoretical rope forces of a curved track at the minimum preload force of 20 Newton. At 9 and 24.5 seconds, a transition occurs where a minimum-preload rope is replaced by another. This process takes place as soon as the third lowest cable force threatens to drop below the minimum preload force and is due to the change in the cable vectors due to the movement of the printhead.

The position determination determines the position error of the print head from the mentioned quantities. The linearization of the geometry relationships shows that the transposed negative input matrix multiplied by the displacement of the printhead in all six degrees of freedom gives exactly the rope length difference. Reversing this equation shows that the position error can be determined by the negative transposed pseudoinverse of the input matrix multiplied by the measurement deviations. Applying the Moore-Penrose pseudoniverse ensures that the algorithm provides the position that most closely matches the measured pitches.


Scheme item determination

The algorithm can be extended to take account of the rope elongations. This is then calculated according to Hookes’s law set and with the entire rope length.

The position control calculates the necessary cable force corrections from the position error. The controller is a PID state controller with manipulated variable limiting and anti windup. The real rope forces which the drives are supposed to produce result from the sum of the ideal rope forces and the rope force corrections.

The linearization calculates the system matrix. This tells the current printhead position how the balance changes when the cable vectors change due to a slight shift in the position of the printhead. This matrix describes the inherent dynamics of the system and is crucial for stability and control.

The adaptive LQ controller specifies the controller setting for the position control. From the aforementioned linearization matrix, the LQ method supplies the regulator settings with optimum settling time. The system matrix A and the input matrix B are known from the previous blocks.

To reduce the computational load, the calculation of the linearization and the LQ controller can either be performed at a lower frequency and / or upstream and the controller setting either matched to the movement in G-code given or deposited depending on the printhead position in a multi-dimensional table and Runtime be interpolated.