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Biodegradable microrobots for targeting cell delivery
Accepted Manuscript Biodegradable microrobots for targeting cell delivery Pouria TirgarBahnamiri, Shadab Bagheri-Khoulenjani PII: DOI: Reference:
S0306-9877(16)30521-7 http://dx.doi.org/10.1016/j.mehy.2017.02.015 YMEHY 8484
To appear in:
Medical Hypotheses
Received Date: Accepted Date:
28 August 2016 27 February 2017
Please cite this article as: P. TirgarBahnamiri, S. Bagheri-Khoulenjani, Biodegradable microrobots for targeting cell delivery, Medical Hypotheses (2017), doi: http://dx.doi.org/10.1016/j.mehy.2017.02.015
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Biodegradable microrobots for targeting cell delivery Pouria TirgarBahnamiria, Shadab Bagheri-Khoulenjanib,c a- Mechanical Engineering Department, Amirkabir University of Technology, Tehran, Iran. b- Polymer and Color Engineering Department, Amirkabir University of Technology, Tehran, Iran c- Cancer Biology Research Center, Iran Institute of cancer, Tehran University of Medical Sciences, Tehran, Iran
Abstract These days, cell delivery is considered a potential method for treatment of many genetic diseases or tissue regeneration applications. In conventional cell delivery methods, cells are encapsulated in or cultured on biocompatible polymers. However, the main problem with these carriers is their lack of targeting ability. For tissue regeneration or many cell treatments, it is needed to deliver cells to a specific site of action. Magnetic microrobots based onindustrial photoresistshave been studied in literature for magnetically controllable carriers. However, there are some issues about biodegradation and removal of these microrobots from the body. In this paper, we hypothesis fabrication of new generation of biodegradable magnetic microrobots based on additive manufacturing methods to overcome this problem and to bring this evolving field to a new level.
Introduction and background Cell-delivery and therapy Over last years, cell-based therapy has been considered as a potential method for treatment of various diseases including, hirschsprung’s diseases[1], heart problems[2], brain disorders[3], central nervous system problems[4] and cancer[5]. Formerly, cells were administrated directly to the desired site of regeneration like infracted part of the heart[6].However, it is reported that about 90% of cells die after direct injection[7].Alternativeapproaches such as encapsulation of cells with biocompatible polymers were also studied [8]. Although above-mentioned methods reduce the probability of cell removal form the body, there is still aproblem with conventional cell delivery approaches, which is lack of targeting. For many applications like cancer[9], spinal cord injury therapy[10] andcell transplantation to the heartwe need targeting carriers to reach a specific site in the body and trigger the treatment process. However, these conventional cell
carriers do not possess the ability to carry drugs or cells to the desired site. In order to overcome this problem, recently, microrobot based cell delivery is proposed. Macro/microrobots in medicine Nowadays robots are receiving increasing interest for minimally invasive medicine[11]. Significant advances are achieved particularly in centimeter-scale applications like gastrointestinal tract (GI) endoscopy[12]and robotic colonoscopy[13]. However, it is believed that robots with sub-millimeter dimensions (microrobots) can have a myriad of new biomedical applications in diagnosis and therapeutic[14]. Actuation of microrobots is one of their major limitations in design and fabrication. Giventheir submillimeter size, large-scale powering solutions are not applicable[15]. As a result, researchers have explored various off-board actuation methods like piezoelectric actuators[16], bacterial actuators[17,18], swimming tail actuators[19]and so on. In particular, electromagnetic driving method is among most interesting methods due to its unique advantages. Iteliminates challenges like requirement of conductivity or transparency properties. Further more, electromagnetic field enables precise control of magnetic object by controlling a current and does not harm tissues even at relatively high strengths[20]. Consequently, many magnetically driven microrobots have been studied for in-vitro and in-vivo applications, such as microgrippers[21], microparticles[22] and helical swimmers [23]. Sub-millimeter devices can access hard-to-reach locations in the human body like urinary system[24]or inside the eye[25] while being minimally invasive. However, this scale at the same time places a strong constraint on their development. As we scale down, motion is governed by viscous forces and surface effects like electrostatics, which outweigh volumetric effects like weight and inertia[26]. Most micrometer-scale objects like bacteria move at a low Reynolds number situation. A microrobot should posses a non-reciprocal motion in this situation to overcome viscous forces and move forward[27]. Helical propulsion and travelling wave propulsion are among envisioned approaches to mimic the behavior of bacterial and eukaryotic flagella in low Reynolds regimes[28]. The fundamental concept is to form the microrobot in an asymmetrical geometry like helical shape to enable generating the required non-reciprocal motion[29]. Fabrication of microrobots (materials and techniques) for cell delivery applications Researchers have reported fabrication of such structures using different techniques including rollingup[30], two-photon polymerization[31], Direct Laser Writing (DLW) [32] and 3D-printing methods[33].
The ideal microrobot should reach a targeted site, execute a predefined operation for a certain period of time and then be removed or degrade without side effects[34]. However, till now researches managed tomostly address the first two issues. In fact, considering fabrication limitations, widely established semiconductor/MEMS procedures and materials were used for microrobots and the biocompatibility was achieved using a covering layer like Titanium[35]. Although such microfabrication techniques provide a precise control over geometries, materials like photoresistsare not designed to be biocompatible or biodegradable. As a result, using them in body can cause foreign body response (FBR), which in turn can lead to achronic inflammatory for such non-degradable materials[36]. In addition, for microrobots to be an ideal cell-delivery scaffolds, they have to possess further biological properties
including
adequate
cell-material
interactions,
architecture
and
biodegradation
properties[37].However, most industrial photoresists are epoxy-based materials without required biological properties. As an example, biological evaluation of SU-8 as a famous negative-tone photoresist showed inflammatory responses when used as an in-vivo implant [38]. Such reactions can reverse the therapeutic goal of microrobots and adversely affect the already damaged tissue. Consequently, providing biocompatible and biodegradable materials for microrobots that are at the same time compatible with microfabrication techniques can ease future applications of this evolving field of study. Precisely defined chiral architecture, biocompatibility, tunable degradation time and cell-adherence all at the same time are characteristics of a promising solution for ideal bio-microrobots. Hypothesis In this paper, we propose using biodegradable and biocompatible microrobots for targeting cell delivery applications. These days, cell delivery has been considered a promising method for treatment of wide range of diseases such as cancer, maternal defects, genetic abnormalities and etc. In many cases, the modified cells should be delivered into a specific site of action, however, most of the conducted researches are based on non-specific cell delivery approaches which diminishes efficacy of this method. In order to overcome this obstacle, recently, some researchers focused on using microrobots for targeting delivery of cells, using electromagnetic fields to control the pathway of cells attached to microrobots in the blood stream. However, the applied microrobots are not biodegradable and removal of such microrobots from the patient body creates new concerns. Thus, we propose using biodegradable and more biocompatible microrobots for targeting cell delivery. Such biocompatible andbiodegradable microrobots can be fabricated using polymeric nanocomposites containing magnetic nanoparticles in well-defined structures by 3D printing method. By choosing
appropriate materials and design, this new generation of magnetically controllable microrobots would provide a novel approach in targeted cell-therapy. Figure 1 illustrates the main idea of using biodegradable cell carrier with the ability to deliver cells to a specific site and its removal from the body.
Evaluation of the hypothesis Magnetic microrobots are shown to be capable of delivering a loaded drug or chemical[39]. However most of these microrobots do not posses basic requirements of cell carrier scaffolds like biocompatibility and biodegradability, which is a major drawback in their use for cell delivery applications. To provide the required biocompatibility, some researchers have coated their microrobots with a layer of Titanium dioxide or gold[40]. Although this method may solve the solution in the first sight, it has considerable disadvantages. First, these materials make the degradation process even harder and fabricated microrobots last longer in the body. Second, the covering layer should completely cover the microrobot during its whole life not to cause any immune response. Thus if during the fabrication or performance any part of the microrobot get exposed to body fluids, biocompatibility problems may occur. Despite of all these problems, the idea of having precisely controllable carriers for drug and cell delivery is so important that many researchers are working on its development. Recently, kim et al proposed a method for targeting cell delivery using magnetic micro-robots. They employed two-photon lithography for fabrication of microrobots with lengths ranging from 154 to 160 µm. Using SU-8 negative tone photoresist, fabricated microrobots had pore sizes in the range of 10 to 21 µm to provide an appropriate architecture for culture of desired cells. Furthermore, microrobots were coated in 150 nm Ni as the magnetic material; followed by deposition of 20 nm of Ti to increase their biocompatibility. The adhesion, migration and proliferation of Human Embryonic Kidney (HEK) 293 cells on fabricated structures and their controllable behavior in presence of an external magnetic field; demonstrate the possible application of these microfabricated structures for in-vivo cell delivery [41]. However, these microrobots lack the ability of self-removal from body via biodegradation. As mentioned above, fabrication of the ideal microrobot especially for cell delivery applications has two main aspects: 1) Selection of the right material, 2) using a precise, fast and cost-effective fabrication technique. Here we evaluate our hypothesis from both points of views.
Material The chosen material for this hypothesis should possess specific properties. Above all, it should be biocompatible and provide an appropriate substrate for adhesion and proliferation of cells. In addition, it should be biodegradable with a controllable rate of degradation, as it should maintain its structure during guidance to desired site of delivery. Finally, the material should be compatible with microfabrication techniques that are able to create micron-scale geometries in 3D. There are some polymers showingbiodegradable and biocompatible properties desiredfor this application. These polymers include natural polymers such as Gelatin[42], Chitosan[43], Alginate[44] and synthetic polymers such as PLA [45], PGA [46], PLGA[47], PCL[48] and PPF[49]. These materials have been widely investigated for tissue engineering and cell delivery applications alone or in mixtures. These polymers can be cross-linked using physical[50], chemical[51]and enzymatic[52] methods by proper modifications. Photo-polymerization of gelatin methacrylol for various applications including tissue engineering is previously reported[53]. Using this method, a hydrogel can be made in aqueous phase without any need to organic solvents. In addition, synthesized hydrogels have favorable properties of gelatin, making gel-ma a potential candidate for use in lithography-based 3D fabrication of biomaterials. Another important issue that should be addressed is the magnetic characteristic of the fabricated microrobots. Addition of Super Paramagnetic Nanoparticles to the polymeric matrix can induce magnetic properties to the polymer[54]. Magnetic nanoparticles are well studied for applications in detection and treatment of diseases[55]. The idea of using these nanoparticles in polymeric matrices has been reported for different polymeric materials, such as encapsulation of Fe3O4 in alginatematrix [34]. Method of fabrication As mentioned above, precise design and fabrication of microrobots are critical issues. Their maneuverability is a direct function of their shape[56]. In addition, as we are going to use them as cell carriers, they have to be fabricated with well-defined porous structures. To overcome these limitations a controllable and efficient microfabrication technique is required. Microfabrication techniques based onphoto-polymerization have been used for yearsto make functional structures with geometries well below micron-scale[57]. But, they are mostly compatible with 2D patterning and need modifications to be employed in creating 3D structures. Given the concept of
photopolymerization, new microfabrication methods like micro-stereolithography were developed to fabricate 3D structures. Micro-stereolithography is now a well-known method in prototyping and 3D printing. It is based onphoto-patterning of thin layers of polymers on top of each other. In this methoda laser beam is used that scansa 2D plane and polymerize the photo-polymer [58]. However it results in a very slow process, which takes minutes to pattern each layer. On the other hand, projection stereolithography takes advantage of a Digital Light Processor (DLP) to create dynamic patterns for each layer. It is highly beneficial as exposing the whole layer in one shot can substantially reduce the fabrication time, especially for objects of higher dimensions[59]. Projection stereolithography is used in researches to create well-defined structures in micron scale. It is previously used for a wide variety of applications, ranging from ceramics [60]to tissue engineering[61]. Although its use for biomedical applications is in its early stages, it is expanding and gaining attention more and more. Completing the puzzle: A thorough solution Using gelatin-based materials and SPMNPs in projection stereolithography is the base of our hypothesis. However, to justify the practical possibility of this hypothesis, two issues should be considered. First, the possibility of fabricating the 3D structure and second, the way one can add SPMNPs to the polymeric structure. Recently, stereolithography was used in creating complex porous hydrogels for tissue engineering applications. In a research by Khademhosseini et al.[62], gelatin methacrylol was used in combination with a projection stereolithography system to fabricate 3D structures of woodpile and hexagonal structures. Fabricated structures had geometries less than 200 microns. It was followed by culture of Human Embryonic Vein Endothelial Cells (HUVECs), which showed the potential of such gelatin-based matrices for cell delivery applications. Therefore this method with some regulations can be used to fabricate magnetic nanocomposites. In another study, researchers have proven the idea of addition of SPMNPs to a microfabricated structure in projection stereolithography. Zhu et al. fabricated magnetically actuated micro-fishes using SPMNPs of Fe3O4 in PEGDA solution[63]. They achieved geometries in the order of less than 50 micron and evaluated the magnetic behavior of microfabricated fishes. This research proves that using the nature of stereolithography, a combination of polymers with and without SPMNPs can be used to locally induce magnetic properties.
To sum up, for fabrication of biodegradable magnetic cell-carrier microrobots we propose nanocomposites based on super paramagnetic nanoparticles (SPMNPs) in gelatin matrices.
Consequences of the hypothesis and discussion Using such micro-robots, there is no need for surgery to implant the tissue engineering devices. In addition the main problem in stem cell therapy is the fact that when cells are injected into the body, there is no control on their final site of action and their faith. It has been suggested that if an engineered cell used for therapy or tissue regeneration, settles in a wrong site, it can cause neu-genetic based problems. Using these microrobots can help the physicians and biomedical engineers to design more specific cell therapy systems. More importantly, being biodegradable, these systems can be replaced with natural ECM and after a while there is no external object in the body and it can assist the generation of the target tissues.
Conflict of interests Authors confirm that there are no known conflicts of interest associated with this publication and there has been no significant financial support for this work that could have influenced its outcome.
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Figures:
a
b
c
d
Figure 1- the main idea of using biodegradable microrobots for targeting cell delivery. (a) Cells attached onto the microrobots with no targeting capacity: the cells will circulate in blood stream a very low portion of cells can reach the targeting tissues. (b) The cells attached on the magnetically targeting nonbiodegradable microrobots: these microrobots can be guided to reach the site of action; however, not being biodegradable, the cells cannot leave microrobots. Although the efficiency of this approach is higher than non-targeting, still it is not satisfactory, in addition, the removal of microrobots from the blood stream is a challenge, (c): the proposed magnetically – targeting biodegradable microrobots: they can reach the targeting defect by proper magnetic field and after a while microrobots degrade and cells can be released and (d) the releases cells can reach their site of action and perform their effect.
S0306-9877(16)30521-7 http://dx.doi.org/10.1016/j.mehy.2017.02.015 YMEHY 8484
To appear in:
Medical Hypotheses
Received Date: Accepted Date:
28 August 2016 27 February 2017
Please cite this article as: P. TirgarBahnamiri, S. Bagheri-Khoulenjani, Biodegradable microrobots for targeting cell delivery, Medical Hypotheses (2017), doi: http://dx.doi.org/10.1016/j.mehy.2017.02.015
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Biodegradable microrobots for targeting cell delivery Pouria TirgarBahnamiria, Shadab Bagheri-Khoulenjanib,c a- Mechanical Engineering Department, Amirkabir University of Technology, Tehran, Iran. b- Polymer and Color Engineering Department, Amirkabir University of Technology, Tehran, Iran c- Cancer Biology Research Center, Iran Institute of cancer, Tehran University of Medical Sciences, Tehran, Iran
Abstract These days, cell delivery is considered a potential method for treatment of many genetic diseases or tissue regeneration applications. In conventional cell delivery methods, cells are encapsulated in or cultured on biocompatible polymers. However, the main problem with these carriers is their lack of targeting ability. For tissue regeneration or many cell treatments, it is needed to deliver cells to a specific site of action. Magnetic microrobots based onindustrial photoresistshave been studied in literature for magnetically controllable carriers. However, there are some issues about biodegradation and removal of these microrobots from the body. In this paper, we hypothesis fabrication of new generation of biodegradable magnetic microrobots based on additive manufacturing methods to overcome this problem and to bring this evolving field to a new level.
Introduction and background Cell-delivery and therapy Over last years, cell-based therapy has been considered as a potential method for treatment of various diseases including, hirschsprung’s diseases[1], heart problems[2], brain disorders[3], central nervous system problems[4] and cancer[5]. Formerly, cells were administrated directly to the desired site of regeneration like infracted part of the heart[6].However, it is reported that about 90% of cells die after direct injection[7].Alternativeapproaches such as encapsulation of cells with biocompatible polymers were also studied [8]. Although above-mentioned methods reduce the probability of cell removal form the body, there is still aproblem with conventional cell delivery approaches, which is lack of targeting. For many applications like cancer[9], spinal cord injury therapy[10] andcell transplantation to the heartwe need targeting carriers to reach a specific site in the body and trigger the treatment process. However, these conventional cell
carriers do not possess the ability to carry drugs or cells to the desired site. In order to overcome this problem, recently, microrobot based cell delivery is proposed. Macro/microrobots in medicine Nowadays robots are receiving increasing interest for minimally invasive medicine[11]. Significant advances are achieved particularly in centimeter-scale applications like gastrointestinal tract (GI) endoscopy[12]and robotic colonoscopy[13]. However, it is believed that robots with sub-millimeter dimensions (microrobots) can have a myriad of new biomedical applications in diagnosis and therapeutic[14]. Actuation of microrobots is one of their major limitations in design and fabrication. Giventheir submillimeter size, large-scale powering solutions are not applicable[15]. As a result, researchers have explored various off-board actuation methods like piezoelectric actuators[16], bacterial actuators[17,18], swimming tail actuators[19]and so on. In particular, electromagnetic driving method is among most interesting methods due to its unique advantages. Iteliminates challenges like requirement of conductivity or transparency properties. Further more, electromagnetic field enables precise control of magnetic object by controlling a current and does not harm tissues even at relatively high strengths[20]. Consequently, many magnetically driven microrobots have been studied for in-vitro and in-vivo applications, such as microgrippers[21], microparticles[22] and helical swimmers [23]. Sub-millimeter devices can access hard-to-reach locations in the human body like urinary system[24]or inside the eye[25] while being minimally invasive. However, this scale at the same time places a strong constraint on their development. As we scale down, motion is governed by viscous forces and surface effects like electrostatics, which outweigh volumetric effects like weight and inertia[26]. Most micrometer-scale objects like bacteria move at a low Reynolds number situation. A microrobot should posses a non-reciprocal motion in this situation to overcome viscous forces and move forward[27]. Helical propulsion and travelling wave propulsion are among envisioned approaches to mimic the behavior of bacterial and eukaryotic flagella in low Reynolds regimes[28]. The fundamental concept is to form the microrobot in an asymmetrical geometry like helical shape to enable generating the required non-reciprocal motion[29]. Fabrication of microrobots (materials and techniques) for cell delivery applications Researchers have reported fabrication of such structures using different techniques including rollingup[30], two-photon polymerization[31], Direct Laser Writing (DLW) [32] and 3D-printing methods[33].
The ideal microrobot should reach a targeted site, execute a predefined operation for a certain period of time and then be removed or degrade without side effects[34]. However, till now researches managed tomostly address the first two issues. In fact, considering fabrication limitations, widely established semiconductor/MEMS procedures and materials were used for microrobots and the biocompatibility was achieved using a covering layer like Titanium[35]. Although such microfabrication techniques provide a precise control over geometries, materials like photoresistsare not designed to be biocompatible or biodegradable. As a result, using them in body can cause foreign body response (FBR), which in turn can lead to achronic inflammatory for such non-degradable materials[36]. In addition, for microrobots to be an ideal cell-delivery scaffolds, they have to possess further biological properties
including
adequate
cell-material
interactions,
architecture
and
biodegradation
properties[37].However, most industrial photoresists are epoxy-based materials without required biological properties. As an example, biological evaluation of SU-8 as a famous negative-tone photoresist showed inflammatory responses when used as an in-vivo implant [38]. Such reactions can reverse the therapeutic goal of microrobots and adversely affect the already damaged tissue. Consequently, providing biocompatible and biodegradable materials for microrobots that are at the same time compatible with microfabrication techniques can ease future applications of this evolving field of study. Precisely defined chiral architecture, biocompatibility, tunable degradation time and cell-adherence all at the same time are characteristics of a promising solution for ideal bio-microrobots. Hypothesis In this paper, we propose using biodegradable and biocompatible microrobots for targeting cell delivery applications. These days, cell delivery has been considered a promising method for treatment of wide range of diseases such as cancer, maternal defects, genetic abnormalities and etc. In many cases, the modified cells should be delivered into a specific site of action, however, most of the conducted researches are based on non-specific cell delivery approaches which diminishes efficacy of this method. In order to overcome this obstacle, recently, some researchers focused on using microrobots for targeting delivery of cells, using electromagnetic fields to control the pathway of cells attached to microrobots in the blood stream. However, the applied microrobots are not biodegradable and removal of such microrobots from the patient body creates new concerns. Thus, we propose using biodegradable and more biocompatible microrobots for targeting cell delivery. Such biocompatible andbiodegradable microrobots can be fabricated using polymeric nanocomposites containing magnetic nanoparticles in well-defined structures by 3D printing method. By choosing
appropriate materials and design, this new generation of magnetically controllable microrobots would provide a novel approach in targeted cell-therapy. Figure 1 illustrates the main idea of using biodegradable cell carrier with the ability to deliver cells to a specific site and its removal from the body.
Evaluation of the hypothesis Magnetic microrobots are shown to be capable of delivering a loaded drug or chemical[39]. However most of these microrobots do not posses basic requirements of cell carrier scaffolds like biocompatibility and biodegradability, which is a major drawback in their use for cell delivery applications. To provide the required biocompatibility, some researchers have coated their microrobots with a layer of Titanium dioxide or gold[40]. Although this method may solve the solution in the first sight, it has considerable disadvantages. First, these materials make the degradation process even harder and fabricated microrobots last longer in the body. Second, the covering layer should completely cover the microrobot during its whole life not to cause any immune response. Thus if during the fabrication or performance any part of the microrobot get exposed to body fluids, biocompatibility problems may occur. Despite of all these problems, the idea of having precisely controllable carriers for drug and cell delivery is so important that many researchers are working on its development. Recently, kim et al proposed a method for targeting cell delivery using magnetic micro-robots. They employed two-photon lithography for fabrication of microrobots with lengths ranging from 154 to 160 µm. Using SU-8 negative tone photoresist, fabricated microrobots had pore sizes in the range of 10 to 21 µm to provide an appropriate architecture for culture of desired cells. Furthermore, microrobots were coated in 150 nm Ni as the magnetic material; followed by deposition of 20 nm of Ti to increase their biocompatibility. The adhesion, migration and proliferation of Human Embryonic Kidney (HEK) 293 cells on fabricated structures and their controllable behavior in presence of an external magnetic field; demonstrate the possible application of these microfabricated structures for in-vivo cell delivery [41]. However, these microrobots lack the ability of self-removal from body via biodegradation. As mentioned above, fabrication of the ideal microrobot especially for cell delivery applications has two main aspects: 1) Selection of the right material, 2) using a precise, fast and cost-effective fabrication technique. Here we evaluate our hypothesis from both points of views.
Material The chosen material for this hypothesis should possess specific properties. Above all, it should be biocompatible and provide an appropriate substrate for adhesion and proliferation of cells. In addition, it should be biodegradable with a controllable rate of degradation, as it should maintain its structure during guidance to desired site of delivery. Finally, the material should be compatible with microfabrication techniques that are able to create micron-scale geometries in 3D. There are some polymers showingbiodegradable and biocompatible properties desiredfor this application. These polymers include natural polymers such as Gelatin[42], Chitosan[43], Alginate[44] and synthetic polymers such as PLA [45], PGA [46], PLGA[47], PCL[48] and PPF[49]. These materials have been widely investigated for tissue engineering and cell delivery applications alone or in mixtures. These polymers can be cross-linked using physical[50], chemical[51]and enzymatic[52] methods by proper modifications. Photo-polymerization of gelatin methacrylol for various applications including tissue engineering is previously reported[53]. Using this method, a hydrogel can be made in aqueous phase without any need to organic solvents. In addition, synthesized hydrogels have favorable properties of gelatin, making gel-ma a potential candidate for use in lithography-based 3D fabrication of biomaterials. Another important issue that should be addressed is the magnetic characteristic of the fabricated microrobots. Addition of Super Paramagnetic Nanoparticles to the polymeric matrix can induce magnetic properties to the polymer[54]. Magnetic nanoparticles are well studied for applications in detection and treatment of diseases[55]. The idea of using these nanoparticles in polymeric matrices has been reported for different polymeric materials, such as encapsulation of Fe3O4 in alginatematrix [34]. Method of fabrication As mentioned above, precise design and fabrication of microrobots are critical issues. Their maneuverability is a direct function of their shape[56]. In addition, as we are going to use them as cell carriers, they have to be fabricated with well-defined porous structures. To overcome these limitations a controllable and efficient microfabrication technique is required. Microfabrication techniques based onphoto-polymerization have been used for yearsto make functional structures with geometries well below micron-scale[57]. But, they are mostly compatible with 2D patterning and need modifications to be employed in creating 3D structures. Given the concept of
photopolymerization, new microfabrication methods like micro-stereolithography were developed to fabricate 3D structures. Micro-stereolithography is now a well-known method in prototyping and 3D printing. It is based onphoto-patterning of thin layers of polymers on top of each other. In this methoda laser beam is used that scansa 2D plane and polymerize the photo-polymer [58]. However it results in a very slow process, which takes minutes to pattern each layer. On the other hand, projection stereolithography takes advantage of a Digital Light Processor (DLP) to create dynamic patterns for each layer. It is highly beneficial as exposing the whole layer in one shot can substantially reduce the fabrication time, especially for objects of higher dimensions[59]. Projection stereolithography is used in researches to create well-defined structures in micron scale. It is previously used for a wide variety of applications, ranging from ceramics [60]to tissue engineering[61]. Although its use for biomedical applications is in its early stages, it is expanding and gaining attention more and more. Completing the puzzle: A thorough solution Using gelatin-based materials and SPMNPs in projection stereolithography is the base of our hypothesis. However, to justify the practical possibility of this hypothesis, two issues should be considered. First, the possibility of fabricating the 3D structure and second, the way one can add SPMNPs to the polymeric structure. Recently, stereolithography was used in creating complex porous hydrogels for tissue engineering applications. In a research by Khademhosseini et al.[62], gelatin methacrylol was used in combination with a projection stereolithography system to fabricate 3D structures of woodpile and hexagonal structures. Fabricated structures had geometries less than 200 microns. It was followed by culture of Human Embryonic Vein Endothelial Cells (HUVECs), which showed the potential of such gelatin-based matrices for cell delivery applications. Therefore this method with some regulations can be used to fabricate magnetic nanocomposites. In another study, researchers have proven the idea of addition of SPMNPs to a microfabricated structure in projection stereolithography. Zhu et al. fabricated magnetically actuated micro-fishes using SPMNPs of Fe3O4 in PEGDA solution[63]. They achieved geometries in the order of less than 50 micron and evaluated the magnetic behavior of microfabricated fishes. This research proves that using the nature of stereolithography, a combination of polymers with and without SPMNPs can be used to locally induce magnetic properties.
To sum up, for fabrication of biodegradable magnetic cell-carrier microrobots we propose nanocomposites based on super paramagnetic nanoparticles (SPMNPs) in gelatin matrices.
Consequences of the hypothesis and discussion Using such micro-robots, there is no need for surgery to implant the tissue engineering devices. In addition the main problem in stem cell therapy is the fact that when cells are injected into the body, there is no control on their final site of action and their faith. It has been suggested that if an engineered cell used for therapy or tissue regeneration, settles in a wrong site, it can cause neu-genetic based problems. Using these microrobots can help the physicians and biomedical engineers to design more specific cell therapy systems. More importantly, being biodegradable, these systems can be replaced with natural ECM and after a while there is no external object in the body and it can assist the generation of the target tissues.
Conflict of interests Authors confirm that there are no known conflicts of interest associated with this publication and there has been no significant financial support for this work that could have influenced its outcome.
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Figures:
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Figure 1- the main idea of using biodegradable microrobots for targeting cell delivery. (a) Cells attached onto the microrobots with no targeting capacity: the cells will circulate in blood stream a very low portion of cells can reach the targeting tissues. (b) The cells attached on the magnetically targeting nonbiodegradable microrobots: these microrobots can be guided to reach the site of action; however, not being biodegradable, the cells cannot leave microrobots. Although the efficiency of this approach is higher than non-targeting, still it is not satisfactory, in addition, the removal of microrobots from the blood stream is a challenge, (c): the proposed magnetically – targeting biodegradable microrobots: they can reach the targeting defect by proper magnetic field and after a while microrobots degrade and cells can be released and (d) the releases cells can reach their site of action and perform their effect.