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3D printed hydrogels
3D printing of high-strength chitosan hydrogel scaffolds without any organic solvents
Luyu Zhou, ‡ab Hamed Ramezani,‡ab Miao Sun,c Mingjun Xie,ab Jing Nie,ab Shang Lv,ab Jie Cai, d Jianzhong Fu*ab and Yong He *ab
Author affiliations
* Corresponding authors
a
State Key Laboratory of Fluid Power and Mechatronic Systems, School of Mechanical Engineering, Zhejiang University, Hangzhou 310027, China
E-mail:
fjz@zju. edu.cn, [email protected]
b Key Laboratory of 3D Printing Process and Equipment of Zhejiang Province, School of Mechanical Engineering, Zhejiang University, Hangzhou 310027, China
c Department of Oral and Maxillofacial Surgery, Affiliated Stomatology Hospital, School of Medicine, Zhejiang University, Hangzhou 310000, China
d College of Chemistry & Molecular Sciences, Wuhan University, Wuhan 430072, China
Abstract
3D printing of chitosan hydrogels has attracted wide interest because of their excellent biocompatibility, antibacterial activities, biodegradability, zero toxicity and low cost. However, chitosan inks are often involved in toxic and organic solvents. Moreover, the recently reported 3D-printed chitosan scaffolds lack enough strength, thus limiting their use in tissue engineering. Here, we reported a chitosan ink obtained by dissolving chitosan into an alkali aqueous solution. This chitosan ink is a stable solution at low temperature (5 °C), but once heated, the chitosan chains self-assemble to lead to gelation. Based on this principle, a corresponding direct ink writing (DIW) method was developed to print high-strength chitosan hydrogels. Specifically, the chitosan ink was extruded into heated deionized water to complete the in situ gelation. The temperature of the nozzle and hot water was well controlled to keep the printing process stable. The rheological behavior of the chitosan ink was investigated and the printing parameters were systematically studied to print chitosan hydrogel scaffolds with high quality and high strength.
Based on these, high-strength (2.31 MPa for compressive strength) and complex chitosan hydrogel structures can be directly printed. The cell culture and the wound healing results further show that the printed chitosan scaffolds with this method have great potential in tissue engineering.
3D printing of hydrogel composite systems: Recent advances in technology for tissue engineering
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3D printed hydrogel walks in water
Researchers in the US and South Korea have learned how to 3D print complex hydrogel actuators.
Based on them, the authors created several devices that are controlled by an external electric field, can grab and move objects, and also walk, according to ACS Applied Materials & Interfaces magazine .
Since it is undesirable to use rigid elements in contact with the body or internal organs in medical devices, engineers develop soft devices. As a source of movement, soft robots use motors, as well as pneumatic or hydraulic actuators, which take up a lot of space and are not very reliable due to their complexity. As an alternative, the researchers suggest using artificial muscles that can contract, expand, or flex and are not made of massive structures. nine0005
A team of researchers led by Howon Lee at Rutgers University have developed a method for 3D printing complex shape hydrogel actuators. As a material, they chose an electroactivated hydrogel, which significantly changes its shape under the influence of an electric field. After entering the electrolyte solution, its carboxyl groups are ionized and many free cations appear inside the material, and the material itself becomes negatively charged.
Since the concentrations of cations in the hydrogel and the surrounding electrolyte differ, osmotic pressure arises, which is compensated by water molecules from the electrolyte solution. After an electric field appears around the hydrogel, the cations are attracted to the cathode and the osmotic pressure on different sides of the hydrogel becomes different, as a result of which one of the sides begins to absorb more water and the whole structure bends. nine0005
In order to give the hydrogel a complex shape and turn it into a functional actuator, the researchers used projection microstereolithography. During such 3D printing, the polymer precursor is in a tray with a moving substrate. An ultraviolet emitter shines on a dynamic pattern that displays the scheme of the printed layer, and reflects radiation onto the precursor. Thus, the light only hits the desired areas, which harden.
In this way, the researchers created several actuators, including a gripper for small objects, a device capable of gripping and moving objects, and a small human model that can walk in different directions.
All of them work in a sodium phosphate buffer and are driven by the field generated by the electrodes. nine0005
Last year, American engineers created soft and transparent hydraulic actuators from hydrogel. And in 2016, researchers from Harvard created a moving octopus robot, in all elements of which only soft materials are used.
Grigory Kopiev
Found a typo? Select the fragment and press Ctrl+Enter.
hybrid hydrogel and extrusion 3D printing / Sudo Null IT News71 years. For a long time, 3D printers were used exclusively for the production of functional or aesthetic prototypes, and the technology itself was called "rapid prototyping". The rapid development of computing technology has led to the emergence of various methods for implementing additive technologies: from laser stereolithography (SLA) to the more famous 3D printing (3DP).
Another term, which appeared as early as 1894, is hydrogel - a polymer capable of absorbing water (if very exaggerated). Hydrogels, like additive technologies, have many applications: medicine, pharmacology, and even energy. So scientists from the University of North Carolina decided to combine 3D printing and hydrogel to create hydrogel structures with the desired properties. There was news about this development on Habré, but we will try to dig deeper. What does the hydrogel under study consist of, what properties can it be endowed with, and what can be made of it? We will find answers to these questions in the report of scientists. Go. nine0005 Study basis
To begin with, it is worth briefly explaining what a hydrogel is. This is a network of intersecting polymer chains capable of equilibrium and reversible swelling in water and aqueous solutions. The basis of the hydrogel are hydrophilic molecules.
The problem with classic hydrogels made from polymer networks in water is that they are soft and brittle; they lack elasticity and strength.
Because of this, the use of hydrogels in various industries (robotics, tissue engineering, etc.) is severely limited. nine0005
The elasticity of hydrogel materials can be improved by combining interpenetrating covalent and ionic polymer networks to form highly extensible and strong structures. Another method for improving the mechanical properties of hydrogels is based on the use of fillers with a high aspect ratio (high ratio of the length of the filler to the diameter of its cross section). This allows the gel matrix to be mechanically strengthened. However, the use of a filler material other than the matrix material results in stresses on the interface surfaces that cause cracking when deformed or heated. nine0005
The opposite method is based on the use of single-polymer composites or so-called homocomposites. The mesoscale amplifying network of homocomposites is made from a material that is chemically identical to the primary matrix material. Networks of homocomposite reinforcement make it possible to modulate the mechanical properties of the primary (main) matrix without stress, delamination points, etc.
It sounds very promising, but there is also a problem - the manufacture of HHG (from homocomposite hydrogel , i.e. homocomposite hydrogel) is an extremely difficult process due to the lack of methods for creating reinforcing networks with the same chemical composition as the hydrogel matrix. nine0005
In the work we are considering today, scientists describe a new type of HHG, in which both the primary gel matrix and the reinforcing network consist of sodium alginate (SA from sodium alginate ; C 6 H 9 NaO 7 ). These HHGs are reinforced with a fibrillar network of alginate soft dendritic colloids (SDC from soft dendritic colloid ). SDC is a hierarchically structured class of soft matter synthesized through a shear-induced deposition process in a turbulent medium. nine0005
The high degree of branching around the cores of SDC particles makes them morphologically similar to polymeric molecular dendrimers. However, SDCs are much larger than these dendrimers.
Scientists believe that hierarchically branched SDCs could be an excellent option for effective reinforcement of composite materials. Significantly, the SDC branches cover a larger surface area, which can increase the stability of the composite by distributing the load more evenly.
Study results
The first step was to prepare soft alginate dendritic colloids (i.e. SDC) to serve as reinforcing meshes. For this, turbulent settling was used. To prepare the SDC hydrogel, an alginate solution (120–190 kDa) was introduced into an aqueous solution of Ca 2+ ions, which effectively bind two -COO- side groups on the alginate backbone ( 1а ).
Image #1
The precipitation process leads to the formation of SDC with a characteristic hierarchical morphology ( 1b ) with branching of different scale and generations (secondary branches) of fibers. In fact, SDCs are made up of micron-sized fibers that fork repeatedly into ever finer fibers.
The outermost layer ("crown") surrounding each SDC is made up of flexible nanofibers that can be up to 10 nm thick ( 1b ). The nanofibers in the crowns endow them with physical adhesion, which is a major factor in the ability of dendricolloids to create the structural strength of the colloidal network. The final size of conventional SDCs, including their corona, is in the range of 100–500 µm. nine0005
Next, the viscoelastic properties of aqueous SDC suspensions had to be evaluated. First, the scientists tested whether alginate SDC hydrogels form a colloidal network at low volume fractions in water. In theory, in aqueous suspensions, contacting SDCs will adhere strongly due to van der Waals forces to form a percolation* network of branched fiber subcontacts ( 2a ).
Percolation* - in chemistry, the phenomenon of the flow or non-flow of liquids through porous materials. nine0080Image #2
Evaluation of the storage moduli (G') and loss (G″) of SDC suspensions in the linear viscoelastic region showed that SDCs have a strong tendency to form colloidal networks.
Dynamic modulus* : The dynamic modulus G set can be used to represent the relationship between vibration voltage and load: G' is the accumulation module; G″ is the loss modulus. nine0005Yield strength was observed in aqueous suspensions of 0.25 wt% SDC, i.e. at a lower concentration than most types of conventional colloidal gels.
A suspension of 1.0 wt.% SDC showed a G' value of 200 Pa, while the reported value for 1.0 wt.% alginate microgels should be 10–100 Pa. Those. SDC slurries have more pronounced solid-like characteristics than conventional alginate particle slurries.
Continuous phase HHG consists of molecular alginate gel combined with Ca 9 ions0057 2+ . First of all, the scientists analyzed the properties of molecular hydrogels containing 1.0 wt.% bound alginate, but without SDC. The hydrogel was prepared by adding CaCO 3 nanoparticles and D-gluconic acid δ-lactone (GDL) to the SA solution.
As GDL undergoes hydrolysis and lowers pH, CaCO 3 slowly releases Ca 2+ ions. After 2 hours of equilibration, data were obtained regarding the viscoelastic properties of the hydrogel ( 2c ). At a concentration of CaCO 3 above 0.05 wt.% the hydrogel behaved like a solid. With further introduction of Ca 2+ into the hydrogel, its rigidity continued to increase. But at CaCO 3 above 0.2 wt.%, syneresis (structure aging) of the hydrogel was observed, followed by water release. As a result, it was found that to maintain the stability of the hydrogel, it should contain 1.0 wt.% SA and 0.1 wt.% CaCO 3 .
As a result, the researchers had two components on hand that required combining - SA SDC (alginate soft dendritic colloids) and SA CMH molecular matrix (alginate gel bound by Ca 9 ions0057 2+ ).
A variety of hybrid HHGs have been synthesized where the total SA concentration is kept constant at 1 wt% and the ratio of SDC to CMH is varied.
Image #3All samples obtained in this way exhibited solids characteristics ( 3a ). The characteristic stress-strain curves ( 3b ) obtained from the mechanical tensile test also demonstrate that hybrid HHGs are more stiff than SDC or CMH-only gels. On chart 3c shows tensile-strain and rheometry measurements for all specimens. Analysis of these data shows that homocomposite systems containing mixed SDC and CMH exhibit a strong synergistic effect. The complex modulus (G) and Young's modulus (E) values for the homocomposite gels showed a threefold increase with maxima at low SDC to CMH ratios.
However, this cannot be attributed solely to the increase in the concentration of Ca 2+ in the homocomposite system. So the maximum shear modulus in HHG (G = 950 Pa at 0.125 wt. % SDC / 0.875 wt.
Therefore, a strong synergistic effect leading to an increase in the mechanical strength of HHG can be directly related to the physical intertwining of molecular SA and colloidal networks of SDC ( 3d ).
The resulting structure is stable in most environments, but can be easily destroyed by placing it in solutions of strong chelating agents such as EDTA (ethylenediaminetetraacetic acid; C 10 H 16 N 2 O 8 ).
Scientists note that another important feature of the developed hybrid hydrogel is the ability to change the kinetics of its gelation depending on.
First, the dependence of gelation on time was tested for three variants of samples: SDC, CMH and composite HHG (the SA concentration was the same for all - 1 wt.%).
Picture #4Chart 4a presents the results of the analysis of a pure suspension of SDC. It can be seen that the SDC immediately exhibits a solid behavior without the addition of CaCO 3 or GDL. This is explained by the fact that the formation of this network occurs due to contact splitting and interweaving of fibrillar dendricolloids.
On the other hand, pure CMH exhibits liquid behavior at first and gradually solidifies as the bonding agent Ca 2+ is released by hydrolysis.
The CMH becomes a fully connected structure after 120 minutes ( 4b ).
It is important to note that the time-dependent evolution of HHG is directly dependent on the kinetics with which SDC and CMH (the main building blocks of HHG) are assembled in the network. HHG initially solidifies due to gelation of the mechanically rigid SDC network. A stronger hydrogel then forms as the interpenetrating CMH molecular network becomes bound by Ca 2+ ions ( 4c and 4d ).
These material properties show controlled initial yield stresses and a slow increase in hydrogel elasticity over time. Therefore, such a material can be used in 3D printing, which the scientists decided to test at the next stage of the study. nine0005
The fact that the created hydrogel is a homocomposite system allows precise control of its properties. Due to this, such a hydrogel can be used in 3D printing using extrusion, which was previously an extremely difficult task. For example, both SDC and CMH are not extrudable in their pure form, unlike hybrid HHG.
The ability to control the properties of the hydrogel allows the creation of an extrusion "ink" in which the time-independent yield strength and setting time can be adjusted to suit the individual. nine0005
Synergistic effect in mixed SA-SDC composites.
Because the 3D printer applies a pressure drop that exceeds the HHG yield strength, the extruded shape is maintained by the rapid gelation of the SDC network ( 4c , video above).
Picture #5It is also important that the developed hydrogel could be used for printing under normal conditions without additional processing or material preparation ( 5a ). It is noteworthy that the G' of a pure SDC suspension (1500 Pa) is almost four orders of magnitude greater than that of the CMH mixture before the addition of GDL (0.5 Pa; 5b ).
Despite maximum HHG gel stiffness occurring at lower SDC/CMH ratios ( 4c ), HHG with higher SDC ratios produced more filaments with improved layering (video below).
3D printing of a multilayer structure by extrusion.
for 5a and 5c show the process of 3D printing a homocomposite hydrogel by direct extrusion. HHG is extruded through a nozzle (25 G, inner diameter 0.26 mm) at 140 kPa and the gel retains its shape due to its yield strength (≈80 Pa). Additional structuring in the z-direction can be achieved by stacking successive layers that have been found to adhere well to the underlying ones. The scientists were able to additively print more than 10 layers of hydrogel in the vertical direction without reducing the extrusion speed. After curing (60 minutes), the finished printed structure could be easily removed from the substrate ( 5d ). If there is a need for a structure of large dimensions, then staged extrusion is necessary, giving additional time for the gel to solidify, it is also necessary to increase the yield strength of the material by changing the composition of HHG.
For a more detailed acquaintance with the nuances of the study, I recommend looking at the report of scientists and additional materials to it.
Epilogue
In this work, scientists have demonstrated their amazing creation - a composite hydrogel, the properties of which can be manipulated depending on the needs of the end user. The hydrogel they created is excellent for 3D printing via extrusion, something that previous hydrogels could not boast of. nine0005
Scientists say that water-based materials are not very strong, they are brittle and soft, which is quite expected. However, if alginate soft dendritic colloids and alginate gel bound by Ca 2+ ions are combined, a hydrogel with increased strength can be obtained. In other words, they have combined two different hydrogels into one with properties that are superior to those of its constituents.
Applications for the new hydrogel include medicine, the food industry, and soft robotics.
But full use is still far away, since the hydrogel needs to be improved. In particular, the scientists want to modify the hydrogel so that it can be used in 3D printing of biomedical injection materials. nine0005
Thank you for your attention, stay curious and have a great week everyone. :)
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