Patent Publication Number: US-2023134282-A1

Title: Residual material withdrawal control method and bio-printer

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a U.S. National Stage Application under 35 U.S.C. § 371 of International Patent Application No. PCT/CN2021/077795, filed on Feb. 25, 2021, which is based on and claims priority to China Patent Application No. 202010215720.5 filed on Mar. 25, 2020, the disclosure of both which are incorporated by reference herein in its entirety. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to the field of bio-printing technology, and in particular, to a residual material withdrawal control method and a bio-printer. 
     BACKGROUND 
     The bio-printer refers to a device that prints bio-ink into a designed three-dimensional structure by using the principles and methods of 3D printing. Biomaterials (for example, cells) and bio-compatible materials (for example, cell solutions) are made into bio-ink. By controlling the movement of the spray head of the bio-printer and ejecting the bio-ink, the bio-ink is printed and molded according to a preset three-dimensional digital model of an object to be printed. 
     Before bio-printing, the spray head needs to be prefilled with bio-ink. In order to ensure that the bio-ink can fill up the spray head during pre-filling, the pre-filled volume of the bio-ink is generally greater than or equal to the volume of the inner cavity of the spray head. When the pre-filled volume of the bio-ink is larger than the volume of the inner cavity of the spray head, the bio-ink may be attached to the nozzle position of the spray head. Due to a high viscosity of the bio-ink, the bio-ink does not drip from the nozzle by itself, so that droplets may be formed at the nozzle position. If the droplets on the nozzle are not cleaned in time, the droplets may affect subsequent printouts, or react with other printing materials to clog the nozzle. 
     In order to clean the droplets attached to the nozzle, cleaning members are used in some related technologies to scrape the droplets on the nozzles. 
     SUMMARY 
     In one aspect of the present disclosure, a residual material withdrawal control method is provided. The method includes: obtaining a length of a printing material attached to the exterior of a spray head of a printer; and performing multiple withdrawals on the printing material attached to the exterior of the spray head by a material driving mechanism until the length of the printing material attached to the exterior of the spray head is not higher than an allowable value, according to the length of the printing material. 
     In some embodiments, a step of obtaining a length of a printing material includes: calculating the length of the printing material according to a pixel value corresponding to the printing material within an image obtained by photographing the spray head and the printing material attached to the exterior of the spray head. 
     In some embodiments, the residual material withdrawal control method further includes: causing the spray head to eject the printing material outwards by the material driving mechanism before the material driving mechanism withdraws the printing material; and causing the material driving mechanism to stop driving the printing material and hold on for a preset period of time when the length of the printing material ejected by the spray head reaches a first preset value, so as to stabilize the printing material attached to the exterior of the spray head. 
     In some embodiments, a step of performing multiple withdrawals on the printing material attached to the exterior of the spray head by the material driving mechanism according to the length of the printing material includes: setting the length of the printing material after holding on for the preset period of time to be an initial length value L0, and setting a withdrawal length D of the printing material according to the initial length value L0; withdrawing the printing material attached to the exterior of the spray head by the material driving mechanism in multiple cycles according to the withdrawal length D; in each cycle, after the printing material attached to the exterior of the spray head is withdrawn according to the withdrawal length D, obtaining a current length L of the printing material attached to the exterior of the spray head after this withdrawal, and calculating a ratio of the current length L to the initial length value L0; if the ratio is in a first preset range R1, calculating a reverse position error δ b  generated when a driving direction of the printing material by the material driving mechanism changes, and compensating a withdrawal length D of next withdrawal by the reverse position error δ b ; and if the ratio is in a second preset range R2, wherein a maximum value of the second preset range R2 is smaller than a minimum value of the first preset range R1, calculating a position error δ p  when the material driving mechanism withdraws the printing material, and compensating a withdrawal length D of the next withdrawal by the position error δ p . 
     In some embodiments, in each cycle, if the ratio is in a third preset range R3, wherein a maximum value of the third preset range R3 is smaller than a minimum value of the second preset range R2, the withdrawal length D is set to a current length L after this withdrawal. 
     In some embodiments, a step of setting the withdrawal length D of the printing material according to the initial length value L0 includes: causing the withdrawal length D to be equal to ⅓˜⅕ times of the initial length value L0. 
     In some embodiments, a maximum value of the first preset range R1 is 1, a minimum value of the first preset range R1 is ⅞˜ 9/10; and a minimum value of the second preset range R2 is ⅓˜⅕. 
     In some embodiments, the reverse position error δ b  is calculated by a formula δ p =D−(L0−L), and the position error δ p  is calculated by a formula δ p =D−(L n-1 −L n ), where L n-1  and L n  are values of the current length L in a previous cycle and this cycle, respectively, and n is a positive integer greater than 1. 
     In some embodiments, the material driving mechanism includes a servo motor and a syringe, a piston of the syringe is driven by the servo motor through a lead screw, a compensation amount C b  corresponding to the reverse position error δ b  is (δ b /P th )*R c , and a compensation amount C p  corresponding to the position error δ p  is (δ p /P th )*R c , where P th  is a pitch of the lead screw, and R c  is an encoder resolution of the servo motor. 
     In some embodiments, after the withdrawal length D is set to be the current length L after this withdrawal, it is determined whether a length of the printing material attached to the exterior of the spray head is no longer higher than an allowable value La, and whether the number N of cycle times of withdrawals has exceeded a preset number N0 of times; if the length of the printing material attached to the exterior of the spray head is no longer higher than the allowable value La, the material driving mechanism ends a withdrawal operation; if the length of the printing material attached to the exterior of the spray head is still higher than the allowable value La, and the number N of cycle times of withdrawals has exceeded the preset number N0 of times, an alarm signal to prompt abnormal emptying of the spray head is issued; and if the length of the printing material attached to the exterior of the spray head is still higher than the allowable value La, and the number N of cycle times of withdrawals does not reach the preset number N0 of times, a position error δ p  when the material driving mechanism withdraws the printing material is calculated, and a withdrawal length D of next withdrawal is compensated by the position error δ p . 
     In some embodiments, the printer is a bio-printer, and the printing material is bio-ink. 
     In one aspect of the present disclosure, a bio-printer is provided. The bio-printer includes: a material driving mechanism having a spray head and configured to drive a printing material to be ejected or withdrawn through the spray head, wherein the printing material is bio-ink; a detecting mechanism configured to obtain a signal representative of a length of the printing material attached to the exterior of the spray head; and a controller signally connected with the detecting mechanism and the material driving mechanism, and configured to perform the preceding residual material withdrawal control method. 
     In some embodiments, the material driving mechanism includes: a power unit and a syringe, the syringe has a syringe barrel, the spray head and a piston, and the power unit is drivingly connected with the piston. 
     In some embodiments, the power unit includes: a servo motor having a power output end that is drivingly connected with the piston. 
     In some embodiments, the detecting mechanism includes: a camera; and a camera adjusting mechanism connected with the camera and configured to adjust a capturing position and a capturing angle of the camera. 
     In some embodiments, the camera adjusting mechanism includes: a support base; a sliding base connected with the support base through a guide structure, and slidable relative to the support base; and a turntable slidably disposed on the sliding base and rotatably connected with the camera. 
     BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS 
     The accompanying drawings which constitute part of this specification, illustrate the exemplary embodiments of the present disclosure, and together with this specification, serve to explain the principles of the present disclosure. 
     The present disclosure may be more explicitly understood from the following detailed description with reference to the accompanying drawings, in which: 
    
    
     
         FIG.  1    is a functional block diagram of some embodiments of the bio-printer according to the present disclosure; 
         FIG.  2    is a schematic structural view of some embodiments of the bio-printer according to the present disclosure; 
         FIG.  3    is a schematic view of a mounting structure of the material driving mechanism in some embodiments of the bio-printer according to the present disclosure; 
         FIG.  4    is a schematic structural view of a detecting mechanism in some embodiments of the bio-printer according to the present disclosure; 
         FIG.  5    is a schematic flowchart of some embodiments of the residual material withdrawal control method according to the present disclosure; 
         FIG.  6    is a schematic flow chart of other embodiments of the residual material withdrawal control method according to the present disclosure; 
         FIG.  7    is a schematic flowchart of still other embodiments of the residual material withdrawal control method according to the present disclosure. 
     
    
    
     It should be understood that the dimensions of various parts shown in the accompanying drawings are not drawn according to actual proportional relations. In addition, the same or similar components are denoted by the same or similar reference signs. 
     DETAILED DESCRIPTION 
     Various exemplary embodiments of the present disclosure will now be described in detail with reference to the accompanying drawings. The description of the exemplary embodiments is merely illustrative and is in no way intended as a limitation to the present disclosure, its application or use. The present disclosure may be implemented in many different forms, which are not limited to the embodiments described herein. These embodiments are provided to make the present disclosure thorough and complete, and fully convey the scope of the present disclosure to those skilled in the art. It should be noticed that: relative arrangement of components and steps, material composition, numerical expressions, and numerical values set forth in these embodiments, unless specifically stated otherwise, should be explained as merely illustrative, and not as a limitation. 
     The use of the terms “first”, “second” and similar words in the present disclosure do not denote any order, quantity or importance, but are merely used to distinguish between different parts. A word such as “comprise”, “include” or variants thereof means that the element before the word covers the element(s) listed after the word without excluding the possibility of also covering other elements. The terms “up”, “down”, “left”, “right”, or the like are used only to represent a relative positional relationship, and the relative positional relationship may be changed correspondingly if the absolute position of the described object changes. 
     In the present disclosure, when it is described that a particular device is located between the first device and the second device, there may be an intermediate device between the particular device and the first device or the second device, and alternatively, there may be no intermediate device. When it is described that a particular device is connected to other devices, the particular device may be directly connected to said other devices without an intermediate device, and alternatively, may not be directly connected to said other devices but with an intermediate device. 
     All the terms (including technical and scientific terms) used in the present disclosure have the same meanings as understood by those skilled in the art of the present disclosure unless otherwise defined. It should also be understood that terms as defined in general dictionaries, unless explicitly defined herein, should be interpreted as having meanings that are consistent with their meanings in the context of the relevant art, and not to be interpreted in an idealized or extremely formalized sense. 
     Techniques, methods, and apparatus known to those of ordinary skill in the relevant art may not be discussed in detail, but where appropriate, these techniques, methods, and apparatuses should be considered as part of this specification. 
     In some related technologies, a cleaning member is used to scrape the droplets on the nozzle. It has been found through studies that, in view of the particularity of the printed biological tissue or organ, the scraping method might contaminate the nozzle or the bio-ink in the nozzle, so that the printed biological tissue or organ cannot be used. 
     In view of this, the embodiments of the present disclosure provide a bio-printer and a residual material withdrawal control method, which can effectively eliminate the printing material attached to the spray head of the printer. 
       FIG.  1    is a functional block diagram of some embodiments of the bio-printer according to the present disclosure.  FIG.  2    is a schematic structural view of some embodiments of the bio-printer according to the present disclosure.  FIG.  3    is a schematic view of a mounting structure of the material driving mechanism in some embodiments of the bio-printer according to the present disclosure. 
     Referring to  FIGS.  1 - 3   , in some embodiments, the bio-printer includes: a material driving mechanism, a detecting mechanism  60  and a controller  70 . The material driving mechanism has a spray head  12 . The material driving mechanism can drive the printing material to be ejected or withdrawn through the spray head. 
     The detecting mechanism  60  is configured to obtain a signal representative of a length of the printing material attached to an exterior of the spray head  12 . The detecting mechanism  60  may perform detection in multiple methods, for example detecting a signal representative of a length of the printing material attached to the exterior of the spray head  12  by means of visual detection or by way of laser or infrared. In some embodiments, the detecting mechanism  60  performs detection by means of visual detection, which may effectively improve the detection efficiency. 
     The controller  70  is signally connected with the detecting mechanism  60  and the material driving mechanism, and configured to obtain a length of the printing material attached to the exterior of the spray head  12 , and withdraw the printing material attached to the exterior of the spray head  12  by the material driving mechanism according to the length of the printing material, so as to shorten or eliminate the printing material attached to the exterior of the spray head  12 . 
     Compared with the related art of scraping the droplets on the nozzle, in this embodiment, the printing material attached to the exterior of the spray head is withdrawn so that less or no printing material is attached to the exterior of the spray head, thereby preventing the printing material attached to the exterior of the spray head from adversely affecting on subsequent prints or reacting with other printing materials to clog the spray head. Since there is no need to use a cleaning member to scrape the nozzle or the printing material in the nozzle, it is possible to correspondingly avoid contamination of the printing material by the cleaning member, thereby ensuring the quality of the printed product (for example, biological tissue or organ). 
     In some embodiments, the printing material attached to the exterior of the spray head refers to the printing material adhered to the nozzle of the spray head and suspended below the nozzle. Correspondingly, the length of the printing material attached to the exterior of the spray head is the distance from the nozzle to the lowermost point of the printing material suspended below the nozzle. In other embodiments, the printing material attached to the exterior of the spray head includes not only the printing material adhered to the nozzle of the spray head and suspended below the nozzle, but also the printing material wrapped around part of the outer wall of the spray head. Correspondingly, the length of the printing material attached to the exterior of the spray head is the distance from the uppermost point of the printing material wrapped outside the spray head to the lowermost point of the printing material suspended below the nozzle. 
     In some embodiments, the controller  70  may withdraw the printing material attached to the exterior of the spray head  12  in multiple cycles by the material driving mechanism according to the length of the printing material, until the length of the printing material attached to the exterior of the spray head  12  is not less than an allowable value. By withdrawing the printing material attached to the exterior of the spray head in multiple cycles, it is possible to more finely control the withdrawing process of the printing material, so as satisfy the residual material withdrawal control requirements under different operational conditions (for example, printing materials with different viscosities, driving mechanisms with different precisions, or different external environmental parameters, and the like). 
     Referring to  FIGS.  2  and  3   , the material driving mechanism includes a power unit  20  and a syringe  10 . The syringe  10  has a syringe barrel  11 , a spray head  12  and a piston  13 . The piston  13  is inserted inside the syringe barrel  11 , and the spray head  12  is located at one end of the syringe barrel  11  and communicates with an interior of the syringe barrel  11 . The power unit  20  outputs power to drive the piston  13  to move relative to the syringe barrel  11 , so that the internal volume of the syringe barrel  11  can be changed, thereby implementing withdrawing or ejecting the printing material. In some embodiments, the printing material is bio-ink. The bio-ink within the syringe barrel  11  may be formed of a biological material (for example, a cell) and a bio-compatible material (for example, a cell solution). 
     Referring to  FIG.  3   , in some embodiments, the power unit  20  includes a servo motor  21  having a power output end drivingly connected to the piston  13 . The servo motor  21  has a high control precision, which may effectively improve the precision of the bio-printer to withdraw residual material of the printing material. In other embodiments, the power unit  20  may also use other driving elements, for example a stepper motor, an air cylinder, and the like. 
     In  FIG.  3   , the power unit  20  may further include a sliding block  22 , a lead screw, a chute  24  and a bracket  23 . The chute  24  and the syringe barrel  11  are disposed on the bracket  23 , and the lead screw is disposed on the bracket  23  and connected to the power output end. The sliding block  22  is connected with the piston  13  and threadedly mated with the lead screw. When the syringe  10  containing the bio-ink is provided on the bracket  23 , the sliding block  22  may be clamped to the piston  13 . 
     The sliding block  22  is slidably disposed on the chute  24 . When the servo motor  21  outputs torque through the power output end, the rotation of the lead screw can be converted into a linear motion of the sliding block  22  along the chute  24 . The sliding block  22  drives the piston  13  to move relative to the syringe barrel  11  so as to change the internal volume of the syringe barrel  11 . 
     In order to enable the printing material to be printed according to a preset three-dimensional digital model of a printed object as target, referring to  FIGS.  1  and  2   , in some embodiments, the bio-printer further includes: a pedestal  40  and a print position adjusting mechanism  30 . The pedestal  40  is a work bench of the bio-printer, which may be fixed on the ground or movably provided on the ground by wheels. A printing platform  50  may be fixedly or movably provided on the pedestal  40 . The print position adjusting mechanism  30  is also disposed on the pedestal  40 , and may be drivingly connected with the material driving mechanism for driving the material driving mechanism to move, so as to adjust a spatial position of the spray head  12  relative to the pedestal  40 . 
     In  FIG.  2   , the print position adjusting mechanism includes a first motion unit moving along a first direction (for example, the X-axis direction in  FIG.  2   ) and a second motion unit mounted on the first motion unit and moving along a second direction (for example, the Z-axis in  FIG.  2   ). The second direction may be parallel to the movement direction of the piston of the syringe  10 . The second motion unit can move relative to the first motion unit. The first motion unit includes a first guide rail for guiding the movement of the second motion unit and a first driving portion for driving the second motion unit to move along the first guide rail. The first driving portion includes a lead screw, a swivel nut threadedly mated with the lead screw, and an electric motor for driving the lead screw to rotate. The extending direction of the lead screw is consistent with the extending direction of the first guide rail, and the swivel nut is connected with the second motion unit. When the motor drives the lead screw to rotate, the swivel nut moves along the lead screw, and the swivel nut drives the second motion unit to move along the first guide rail. 
     The second motion unit includes a second guide rail for guiding the movement of the material driving mechanism and a second driving portion for driving the material driving mechanism to move along the second guide rail. The second driving portion may also include a lead screw, a swivel nut threadedly mated with the lead screw, and an electric motor for driving the lead screw to rotate. The extending direction of the lead screw is consistent with the extending direction of the second guide rail, and the swivel nut is connected with the material driving mechanism. When the motor drives the lead screw to rotate, the swivel nut moves along the lead screw, and the swivel nut drives the material driving mechanism to move along the second guide rail. 
       FIG.  4    is a schematic structural view of a detecting mechanism in some embodiments of the bio-printer according to the present disclosure. 
     Referring to  FIG.  4   , in some embodiments, the detecting mechanism  60  detects the signal representative of a length of the printing material attached to the exterior of the spray head by means of visual detection. In  FIG.  4   , the detecting mechanism  60  includes: a camera  61  and a camera adjusting mechanism. The camera adjusting mechanism may be provided on the pedestal  40  and connected with the camera  61 , and configured to adjust a capturing position and a capturing angle of the camera  61 . 
     Referring to  FIGS.  2  and  4   , in some embodiments, the camera adjusting mechanism may include: a support base  62 , a sliding base  63  and a turntable  64 . The support base  62  may be provided on the pedestal  40 . A connecting plate  65  is provided at one end of the support base  62 , and the connecting plate  65  may be fixed to the pedestal  40  by a connecting piece (for example, a bolt or the like). The sliding base  63  is connected to the support base  62  through a guide structure, and slidable relative to the support base  62 . In  FIG.  4   , the guide structure is a structure in which a guide rail is mated with a guide groove. For example, a guide groove is provided on the sliding base  63 , and a guide rail mated with the guide groove is provided on the surface of one side of the support base  62  adjacent to the sliding base  63 , so as to realize a stable sliding pair between the sliding base  63  with the support base  62 . 
     In  FIG.  4   , the turntable  64  is slidably disposed on the sliding base  63  and rotatably connected to the camera  61 . The turntable  64  may be partially embedded within a long groove of the sliding base  63 , wherein the long groove may extend along the Y-axis direction, so that the turntable  64  can drive the camera  61  to adjust a position along the Y-axis. The turntable  64  is rotatably connected to the camera  61 , so that the camera  61  can rotate relative to the turntable  64  around the Z axis. By way of position adjustment on the Y axis and rotation adjustment around the Z axis of the camera  61 , and cooperating with driving the movement of the material driving mechanism in the Z axis and the X axis by the print position adjusting mechanism  30 , the capturing position and the capturing angle of the camera  61  relative to the spray head  12  is adjusted so as to realize a high cost performance whilst satisfying the image acquiring needs. 
     When the camera  61  photographs the spray head  12  and the printing material attached to the exterior of the spray head  12  to obtain an image, the controller  70  may calculate a length of the printing material according to a pixel value corresponding to the printing material within this image. For example, a preset ratio of the pixel value within the image to an actual length is determined by calculation or multiple experiments, and the actual length of this object in this direction is calculated according to the preset ratio and the pixel value occupied by the object within the image obtained by photographing in one direction (for example, the Z-axis direction). 
     In the various embodiments of the bio-printer described above, the controller may employ a general purpose processor, a digital signal processor (DSP), an application specific integrated circuit (ASIC), a field programmable gate array (FPGA) or other programmable logic devices, discrete gate or transistor logic, discrete hardware assembly, or any combination thereof designed to perform the functions described herein. The general purpose processor may be a microprocessor, but in an alternative solution, the processor may be any conventional processor, microcontroller, or state machine. The processor may also be implemented as a combination of computing devices, for example a combination of DSP and a microprocessor, a plurality of microprocessors, one or more microprocessors cooperating with a DSP core, or any other such configuration. 
       FIG.  5    is a schematic flowchart of some embodiments of the residual material withdrawal control method according to the present disclosure. 
     Referring to  FIG.  5   , in some embodiments, the residual material withdrawal control method includes: 
     Step  200 : obtaining a length of the printing material attached to an exterior of the spray head of the printer; 
     Step  400 : performing multiple withdrawals of the printing material attached to the exterior of the spray head by the material driving mechanism until the length of the printing material attached to the exterior of the spray head is not higher than an allowable value, according to the length of the printing material. 
     The residual material withdrawal control method may be performed by the controller in various embodiments of the bio-printer of the present disclosure, and may also be performed by the controller in other printers capable of realizing the printing function by driving the printing material. The printing material includes but is not limited to bio-ink. 
     In step  200 , the step of obtaining a length of the printing material may include: calculating the length of the printing material according to the pixel value corresponding to the printing material within the image obtained by photographing the spray head and the printing material attached to the exterior of the spray head. Referring to  FIGS.  2  and  4   , in some embodiments, the camera  61  may photograph the spray head  12  and the printing material attached to the exterior of the spray head  12  at a certain frequency so as to obtain at least one image. The controller  70  may convert the pixel value of the printing material within the image in the vertical direction into the actual length of the printing material attached to the exterior of the spray head  12 . 
     In step  400 , after determining the actual length of the printing material attached to the exterior of the spray head  12 , the controller may send a control instruction to the material driving mechanism according to the length, so that the servo motor  21  in the material driving mechanism drives the piston  13  to perform withdrawal operation on the printing material in multiple cycles by the lead screw and the sliding block, thereby allowing that the length of the printing material attached to the exterior of the spray head  12  is not higher than an allowable value. 
     In this embodiment, by withdrawing the printing material attached to the exterior of the spray head in multiple cycles, it is possible to more finely control the withdrawal process of the printing material so as to satisfy the residual material withdrawal control requirements under different operational conditions (for example, printing materials with different viscosities, driving mechanisms with different precisions or different parameters of external environment and the like). In addition, the risk of contamination increased by manual operation is avoided by the detection of the length of the printing material by the detecting mechanism, and the withdrawal control of the material driving mechanism by the controller. 
       FIG.  6    is a schematic flow chart of other embodiments of the residual material withdrawal control method according to the present disclosure. 
     Referring to  FIG.  6   , in some embodiments, the residual material withdrawal control method further includes: 
     Step  110 : causing the spray head  12  to eject the printing material outward by the material driving mechanism before the material driving mechanism withdraws the printing material; 
     Step  120 : causing the material driving mechanism to stop driving the printing material, and hold on for a preset period of time when the length of the printing material ejected by the spray head  12  reaches a first preset value, so as to stabilize the printing material attached to the exterior of the spray head  12 . 
     Before step  110 , the syringe  10  filled with the printing material may be first mounted on the bracket  23  of the power unit  20 , and the piston  13  of the syringe  10  is fixedly connected with the sliding block  22 . 
     In step  110 , after the spray head  12  moves to the pre-filling position (within the capturing range of the camera  61 ), the controller sends a control instruction to the servo motor  21  in the power unit  20  so as to drive the piston  13  to move towards one side where the spray head  12  is situated, so that the spray head  12  ejects the printing material outward. In this process, considering that some printing materials have a large flow inertia, it is difficult to control the length of the printing material ejected from the spray head if the speed is too fast. Therefore, the material driving mechanism may drive the printing material to be ejected from the spray head at a slow speed (for example, 0.5-2 mm/s). 
     In step  120 , during the process of ejecting the printing material, the spray head may be photographed by the camera at a certain frequency, and the length of the printing material in the image obtained by photographing may be determined. When the length of the printing material ejected by the spray head  12  reaches a first preset value, the driving motor  21  in the power unit  20  stops driving the piston  13 . The determination of the first preset value here mainly depends on the viscosity of the material. In some embodiments, for some types of printing materials, the first preset value may be selected to be 2-3 mm, so that the withdrawal control process of length of the printing material satisfies the precision requirements with a high efficiency. 
     Considering the flow inertia of the printing material, the controller holds on for a preset period of time (for example, 2-4 seconds) after the controller causes the material driving mechanism to stop driving the printing material, so as to stabilize the printing material attached to the exterior of the spray head  12 . 
     The above-described steps  110  and  120  are performed before step  400 , and may be performed before or after step  200 , or simultaneously with step  200 . 
       FIG.  7    is a schematic flowchart of still other embodiments of the residual material withdrawal control method according to the present disclosure. 
     Referring to  FIG.  7   , in some embodiments, step  400  includes step  410  and step  420 . In step  410 , the length of the printing material after holding on for a preset time period is set to be an initial length value L0, and the withdrawal length D of the printing material is set according to the initial length value L0. In some embodiments, the withdrawal length D may be set to be ⅓˜⅕ times of the initial length L0. For a controller using PID control, considering the characteristics of PID control, the control process is subject to overshoot or oscillation within a certain range in the early stage of turning, and tends to be stable after about 1-3 cycles. Therefore, in some embodiments, the withdrawal length D is set to be ¼ times of the initial length L0, that is, D=L0*¼, so as to ensure that the control curve is as stable as possible. 
     In step  420 , according to the withdrawal length D, the material driving mechanism withdraws the printing material attached to the exterior of the spray head  12  in multiple cycles. The controller may control the servo motor  21  to rotate reversely so as to drive the sliding block  22  and the piston  13  to retreat, thereby realizing the withdrawal operation of the printing material attached to the exterior of the spray head. 
     Each cycle of step  420  may specifically include steps  421  to  424 . 
     In step  421 , after the printing material attached to the exterior of the spray head  12  is withdrawn according to the withdrawal length D, it is possible to hold on for a preset period of time (for example, 2 to 4 seconds) so as to stabilize the printing material attached to the exterior of the spray head  12 . Referring to the length obtaining method in step  200 , the controller obtains a current length L of the printing material attached to the exterior of the spray head  12  after this withdrawal (that is, the remaining length after this withdrawal), and calculates a ratio L/L0 of the current length L to the initial length L0. 
     Taking the piston of the syringe driven by the servo motor as an example, during the process of pre-filling the spray head, the extrusion action force of the bio-ink on the syringe can cause the deformation of the syringe barrel to expand outward. When the bio-ink is withdrawn, the bio-ink can in turn cause the deformation of the syringe to contract inward. Due to the viscosity of the bio-ink, it is very difficult for the deformation produced by the syringe to restore to an initial shape, thereby indirectly affecting the change of the volume of the syringe and changing the amount of the bio-ink actually ejected by the spray head. In addition, the difference in the viscosity of the bio-ink and the inherent characteristics of the mechanical system also cause that the amount of the bio-ink actually ejected/withdrawn is different from a theoretical value when the servo motor executes a position control instruction, thereby generating a position error δ p . 
     The mechanical transmission system generally has a back clearance, which causes that a reverse position error δ b , that is, back clearance is generated when the driving direction of the printing material by the material driving mechanism is changed (for example, when the servo motor commutates). When the printing material is withdrawn for the first time, the system is present with a reverse position error δ b  and a position error δ p  at the same time. After the first withdrawal, there may be a circumstance in which there is no change or a slight change in the length of the printing material attached to the exterior of the spray head  12 , and at this time, the reverse position error δ b  is much larger than the position error δ p . At this time, the position error δ p  may be ignored, and the withdrawal length D of the next withdrawal is compensated only according to the reverse position error δ b . 
     After the withdrawal length D is compensated according to the reverse position error δ b  during the next withdrawal, since the reverse position error δ b  has been compensated, the back clearance may be ignored, and the withdrawal length D of the next withdrawal is compensated according to the position error δ p . 
     In step  422 , the preset range in which the ratio falls may be determined. If the ratio is in the first preset range R1 (i.e., L/L0∈R1), step  423  is performed, and if the ratio is in the second preset range R2 (i.e., L/L0∈R2), step  424  is performed. The maximum value of the second preset range R2 is smaller than the minimum value of the first preset range R1. 
     In step  423 , the reverse position error δ b  generated when the driving direction of the printing material by the material driving mechanism is changed is calculated, and the withdrawal length D of the next withdrawal is compensated by the reverse position error δ b . In step  424 , the position error δ p  when the printing material is withdrawn by the material driving mechanism is calculated, and the withdrawal length D of the next withdrawal is compensated by the position error δ p . After steps  423  and  424  are executed, it returns to step  421  for the next withdrawal. 
     In this embodiment, the back clearance and the position error are calculated during each withdrawal cycle process, and the withdrawal length of the next withdrawal cycle is correspondingly compensated, so that it is possible to improve the withdrawal precision, and avoid a phenomenon of inward recess of the printing material at the spray head resulting from excessive withdrawals of the printing material due to a low withdrawal precision. 
     In some embodiments, the maximum value of the first preset range R1 is 1, and the minimum value is ⅞˜ 9/10, for example, ⅞. For example, after this withdrawal (for example, the first withdrawal), if 1≥L/L0≥⅞, the position error δ p  is ignored. In step  423 , the reverse position error δ b  is calculated, and the withdrawal length D of the next withdrawal is compensated by the reverse position error δ b . The reverse position error δ b  may be calculated by the formula δ b =D−(L0−L). 
     In some embodiments, the material driving mechanism includes the servo motor  21  and the syringe  10 . The piston  13  of the syringe  10  is driven by the servo motor  21  through the lead screw. Correspondingly, according to the reverse position error δ b , the compensation amount C b  of the servo motor corresponding to the reverse position error δ b  may be further calculated as (δ b /P th )*R c . P th  is the pitch of the lead screw, and R c  is the encoder resolution of the servo motor  21 . For example, for a material driving mechanism using a servo motor with a 22-bit encoder and the lead screw having a pitch of 5 mm, the calculated compensation amount C b =[(D−(L0−L))/5]*2{circumflex over ( )}22 count is compensated to the controller. 
     In some embodiments, the minimum value of the second preset range R2 is ⅓˜⅕, for example ¼. The maximum value of the second preset range R2 is smaller than the minimum value of the first preset range R1. For example, after this withdrawal (for example, the second withdrawal), if ⅞&gt;L/L0≥¼, the back clearance is ignored. In step  424 , the position error δ p  is calculated, and the withdrawal length D of the next withdrawal is compensated by the position error δ p . The position error δ p  may be calculated by the formula δ p =D−(L n-1 −L n ), where L n-1  and L n  are the values of the current length L in the previous cycle and this cycle respectively, and n is a positive integer greater than 1. 
     In some embodiments, the material driving mechanism includes the servo motor  21  and the syringe  10 . The piston  13  of the syringe  10  is driven by the servo motor  21  through the lead screw. Correspondingly, according to the position error δ p , the compensation amount C p  of the servo motor corresponding to the position error δ p  may be further calculated as (δ p /P th )*R c . P th  is the pitch of the lead screw, and R c  is the encoder resolution of the servo motor  21 . For example, for a material driving mechanism using a servo motor with a 22-bit encoder and a lead screw having a pitch of 5 mm, the calculated compensation amount C p =[(D−(L n-1 −L n ))/5]*2{circumflex over ( )}22 count is compensated to the controller. 
     Due to factors such as the viscosity of the printing material and the ambient temperature, when the printing material is withdrawn by using the same withdrawal length D for each batch of printing material, the actual withdrawal length of the printing material may be different. By calculating the position error δ p , this difference can be gradually eliminated. 
     When the ratio L/L0 is gradually reduced to within the third preset range R3 after multiple withdrawal cycles, the printing material attached to the exterior of the spray head  12  may be reduced as much as possible by adjusting the next withdrawal length D. Referring to  FIG.  7   , in each cycle of step  420 , if the ratio L/L0 is in the third preset range R3, and the maximum value of the third preset range R3 is smaller than the minimum value of the second preset range R2, step  425  may be performed. In step  425 , the withdrawal length D is set as the current length L, that is, the current length L serves as the withdrawal length D to perform the next withdrawal. 
     Taking a value of ¼ for the minimum value of the second preset range R2 as an example, after this withdrawal (for example, the first withdrawal), if L/L0&lt;¼, the withdrawal length D is set to be the current length L. For the initial value of the withdrawal length D which is ¼ times of L0, the withdrawal length D is set to be the current length L, so that it is possible to perform more accurate withdrawal of the printing material attached to the exterior of the spray head in the next cycle, thereby avoiding a phenomenon of an inward recess of the printing material at the spray head resulting from excessive withdrawal of the printing material for the initial value of the withdrawal length D. 
     In some embodiments, after the current length L serves as the withdrawal length D to perform withdrawal, it is determined whether the length of the printing material attached to the exterior of the spray head  12  is not higher than an allowable value La, and whether the number N of cycle times of withdrawals has exceeded the preset number N0 of times. If the length L of the printing material attached to the exterior of the spray head  12  is no longer higher than the allowable value La, the material driving mechanism will end the withdrawal operation of the piston  13 . 
     If the length of the printing material attached to the exterior of the spray head  12  is still higher than the allowable value La, and the number N of cycle times of withdrawal has exceeded the preset number N0 of times, an alarm signal to prompt abnormal emptying of the spray head  12  is issued. If the length of the printing material attached to the exterior of the spray head  12  is still higher than the allowable value La, and the number N of cycle times of withdrawal does not reach the preset number N0 of times, the position error δ p  when the spray head  12  is driven to move is calculated, and the withdrawal length D of the next withdrawal is compensated by the position error δ p . 
     Referring to  FIG.  7   , step  420  may further include step  426  and step  427 . After step  425 , the controller determines whether the length of the printing material attached to the exterior of the spray head  12  is no longer higher than an allowable value La in step  426 . The allowable value La may vary according to the material of the printing material, and La may be set to be 0.01-0.5 mm, for example, 0.05 mm. If the current length L is no longer higher than the allowable value La, the material driving mechanism may end the withdrawal operation, for example, a control instruction is sent to the servo motor to shut it down, thereby ending the withdrawal operation. 
     If the current length L is still higher than the allowable value La, step  427  is performed to determine whether the number N of cycle times of withdrawal has exceeded the preset number N0 of times. The preset number N0 of times may be determined according to the usual value of the number of cycle times for completing the withdrawal in multiple experiments, for example, NO is set to be 3, that is, 3 cycles. If the number N of cycle times of withdrawal exceeds the preset number N0 of times, it is indicated that there may be fault conditions such as abnormal light source and mechanical malfunction, thereby sending an alarm signal to an operator to prompt abnormal emptying of the spray head  12  by means of sound, light or electricity. At this time, a control instruction may be sent to the material driving mechanism to shut the servo motor down so that the operator performs inspection and troubleshooting. 
     If the number N of cycle times of withdrawals does not exceed the preset number N0 of times, it is possible to return to step  424  to calculate the position error δ p  when the material driving mechanism withdraws the printing material, and compensate the withdrawal length D of the next withdrawal by the position error δ p . 
     In some embodiments, each step of the above-described residual material withdrawal control method is included in a pre-filling process before printing, for example a pre-filling process of a blood vessel printer. In other embodiments, each step of the above-described residual material withdrawal control method is included in the printing process after at least one layer is printed, for example, the printing process of a bone printer. 
     Hereto, various embodiments of the present disclosure have been described in detail. Some details well known in the art are not described in order to avoid obscuring the concept of the present disclosure. According to the above description, those skilled in the art would fully understand how to implement the technical solutions disclosed here. 
     Although some specific embodiments of the present disclosure have been described in detail by way of examples, those skilled in the art should understand that the above examples are only for the purpose of illustration but not for limiting the scope of the present disclosure. It should be understood by those skilled in the art that modifications to the above embodiments and equivalently substitution of part of the technical features may be made without departing from the scope and spirit of the present disclosure. The scope of the present disclosure is defined by the appended claims.