**4. Applications of bio-inspired hydrogels using 3D bioprinting**

#### **4.1 Tissue regeneration**

Tissue regeneration research is aim to develop substitute for damaged or diseased tissues or organs using principles of life science, engineering and medicine synergistically. It is crucial to fabricate the substitute as scaffolds, which is inspired by the natural 3D structure of tissue. The natural ECM regulates essential cellular functions, such as adhesion, migration, proliferation, differentiation and morphogenesis [68]. It is important of mimicking the ECM with dynamic nature using 3D bioprinting techniques, and the bio-inspired hydrogels via such techniques displayed potential applications in tissue regeneration, such as cartilage tissue, vascularized engineered tissue, bone tissue, skin regeneration, heart tissue, aortic valve conduits, muscle-tendon, and so on [69]. For example, Alexander et al. displayed a chemically and mechanically biomimetic filler-free bioink for 3D bioprinting of soft neural tissues, as shown in **Figure 3**. The thiolated Pluronic F-127, dopamine-conjugated (DC) gelatin, and DC hyaluronic acid were used as bioinks via a thiol-catechol reaction and photocuring; the storage modulus of the cured bioinks ranged from 6.7 to 11.7 kPa. The micro-extrusion 3D bioprinting was used to fabricate free-standing cell-laden tissue constructs. The Rodent Schwann

**9**

**Figure 3.**

*Bio-Inspired Hydrogels via 3D Bioprinting DOI: http://dx.doi.org/10.5772/intechopen.94985*

cells, rodent neuronal cells, and human glioma cell-laden tissue constructs were printed and cultured over seven days and exhibited excellent viability, which has implications in micro physiological neural systems for neural tissue regenerative medicine [70]. Several works could be found in a recent study that focuses on the specific properties of bio-inspired hydrogels for tissue regeneration, such as high strength structures [30]. Also, the enhancement of printing resolution and versatility is vital for tissue regeneration. For example, the self-healing hydrogels were used to support the direct 3D bioprinting with high resolution by utilizing shearthinning hydrogels, then the constructs could be printed in any direction [71]. The bio-inspired hydrogels could be accomplished via *in vitro* and *in vivo* 3D bioprinting as for tissue constructs, which are potential and convenient for clinic operation.

*glioma cells (b), and rodent model neuronal cells (c) are shown at day 7 [70].*

*(A) Native ECM components of neural tissue were combined with a synthetic polymer for microextrusion 3D bioprinting of soft, free-standing neural tissues. (B) Two curing pathways, including UV light exposure, and chelation of dopamine groups with iron (III), are shown to the formulation of photocuring containing methacrylated dopamine-conjugated gelatin. With the increase of PF127-SH content, the compressive properties of inks cured through UV exposure or chelation increased.* **(C***) Printed bioinks are shown. (D) Fluorescence micrographs of 3D bioprinted neural and glial tissue bioink containing rodent Schwann cells (a), human* 

*Bio-Inspired Hydrogels via 3D Bioprinting DOI: http://dx.doi.org/10.5772/intechopen.94985*

*Biomimetics*

(HA) interconnected porous scaffold via a melt-wire method; the 3D printing technique avoided thermal degradation of PLA, the porosity and pore size of the scaffold could be well controlled. The porous PLA/HA scaffold with 15% HA has a considerable crack resistance and can work for a long time under the stress of 21 MPa, which was potential for bone tissue engineering applications [61]. Polyvinyl alcohol (PVA) is a synthetic water-soluble polymer, it has good biodegradation, biocompatibility, and no side effects on the human body [62]. PVA has been widely used in ophthalmology, wound dressing, artificial joint, and so on [42, 63]. Shi et al. prepared an injectable dynamic hydrogel using HA grafted with PVA and phenyl boric acid (PBA). The synthesized HA-PBA-PVA dynamic hydrogel has the reactive oxygen species reactivity and the scavenging activity of active oxygen. Furthermore, the hydrogel had good biocompatibility to the encapsulated neural precursor cells (NPC), and its ability to scavenge reactive oxygen species could protect the NPC cells from the damage of reactive oxygen species. The

HA-PBA-PVA hydrogel could be used as bioink for 3D biological printing to prepare multilayer and cell loaded structures. The NPC cells showed good viability (85 ± 2% of living cells) after extrusion and maintained the excellent viability of 81 ± 2% of living cells after 3 days of culture. The results indicated that multifunctional injectable and ROS responsive self-healing HA-PBA-PVA dynamic hydrogels were

Besides, there are also many other synthetic polymers for the fabrication of bio-inspired hydrogels, such as Pluronic and derivatives, PEG or polyethylene oxide (PEO) based block copolymers, poly(L-glutamic acid), poly(propylene fumarate), methoxy polyethylene glycol, and so on. Though, the synthetic polymers can precisely control their gel structure and properties and have better physical and chemical stability and more raw materials to prepare bio-inspired hydrogels. However, it is necessary to pay attention to the possible biocompatibility of unreacted monomers and residual initiators during the preparation of synthetic polymer materials, and the biocompatibility could be greatly improved via compositing or

expected to be candidates for 3D culture and 3D bioprinting [64].

**4. Applications of bio-inspired hydrogels using 3D bioprinting**

Tissue regeneration research is aim to develop substitute for damaged or diseased tissues or organs using principles of life science, engineering and medicine synergistically. It is crucial to fabricate the substitute as scaffolds, which is inspired by the natural 3D structure of tissue. The natural ECM regulates essential cellular functions, such as adhesion, migration, proliferation, differentiation and morphogenesis [68]. It is important of mimicking the ECM with dynamic nature using 3D bioprinting techniques, and the bio-inspired hydrogels via such techniques displayed potential applications in tissue regeneration, such as cartilage tissue, vascularized engineered tissue, bone tissue, skin regeneration, heart tissue, aortic valve conduits, muscle-tendon, and so on [69]. For example, Alexander et al. displayed a chemically and mechanically biomimetic filler-free bioink for 3D bioprinting of soft neural tissues, as shown in **Figure 3**. The thiolated Pluronic F-127, dopamine-conjugated (DC) gelatin, and DC hyaluronic acid were used as bioinks via a thiol-catechol reaction and photocuring; the storage modulus of the cured bioinks ranged from 6.7 to 11.7 kPa. The micro-extrusion 3D bioprinting was used to fabricate free-standing cell-laden tissue constructs. The Rodent Schwann

liking with natural polymers [65–67].

**4.1 Tissue regeneration**

**8**

#### **Figure 3.**

*(A) Native ECM components of neural tissue were combined with a synthetic polymer for microextrusion 3D bioprinting of soft, free-standing neural tissues. (B) Two curing pathways, including UV light exposure, and chelation of dopamine groups with iron (III), are shown to the formulation of photocuring containing methacrylated dopamine-conjugated gelatin. With the increase of PF127-SH content, the compressive properties of inks cured through UV exposure or chelation increased.* **(C***) Printed bioinks are shown. (D) Fluorescence micrographs of 3D bioprinted neural and glial tissue bioink containing rodent Schwann cells (a), human glioma cells (b), and rodent model neuronal cells (c) are shown at day 7 [70].*

cells, rodent neuronal cells, and human glioma cell-laden tissue constructs were printed and cultured over seven days and exhibited excellent viability, which has implications in micro physiological neural systems for neural tissue regenerative medicine [70]. Several works could be found in a recent study that focuses on the specific properties of bio-inspired hydrogels for tissue regeneration, such as high strength structures [30]. Also, the enhancement of printing resolution and versatility is vital for tissue regeneration. For example, the self-healing hydrogels were used to support the direct 3D bioprinting with high resolution by utilizing shearthinning hydrogels, then the constructs could be printed in any direction [71]. The bio-inspired hydrogels could be accomplished via *in vitro* and *in vivo* 3D bioprinting as for tissue constructs, which are potential and convenient for clinic operation.

#### **4.2 Wound dressing and wearable devices**

The bio-inspired hydrogels via 3D bioprinting can be applied for wound dressing and wearable devices, which are considered as important applications, especially in recent years. Skin plays an essential role in protecting the body from external damages, such as abrasions, lacerations, and burns, and so on. The full-thickness defects of the dermis layers are the most challenging wounds to heal because of the limitation of self-repairing capability; thus, the skin regeneration of skin with skin appendages still remains a tough challenge [72]. 3D bioprinting is being applied to fabricate skin constructs using biomaterial scaffolds with or without cells, to address the need for skin tissues suitable for transplantation for wound healing therapy. The natural polymers, including cellulose, collagen and chitin, alginate, and hyaluronic acids are employed to synthesis skin constructs due to the favorable biocompatibility, biodegradation, low-toxicity or nontoxicity, high moisture content, high availability and mechanical stability [73]. Feifei et al. fabricated gelatin methacrylate (GelMA) based bioink to print functional living skin using DLP-based 3D printing (**Figure 2**), while the printed skin could promote skin regeneration and neovascularization via mimicking the physiological structure of natural skin [52].

Furthermore, the bio-inspired hydrogels could not only be functionalized on skin regeneration but also as medical wearable devices. The conductive hydrogels could be designed and fabricated to acquire electronic devices with conductive, capacitive, switching properties, image displaying, and motion sensing [74]. Meihong et al. developed conductive, healable, and self-adhesive hybrid network hydrogels based on conductive functionalized single-wall carbon nanotube (FSWCNT), PVA and polydopamine. The prepared hydrogel exhibits fast selfhealing ability around 2 s, high self-healing efficiency of about 99%, and robust adhesiveness, which could be used for healable, adhesive, and soft human-motion sensors [75]. Zijian et al. synthesized a stretchable, self-healing and conductive hydrogel based on gelatin-enhanced hydrophobic association poly(acrylamide-*co*dopamine) with lithium chloride via physical crosslinking including hydrogen bonding, hydrophobic association, and complexation effect. The hydrogels displayed the stretchability of 1150%, tensile strength of 112 kPa, flexibility and puncture resistance. Also, the hydrogels possess extraordinary conductive property and stable changes in resistance signals [76]. Furthermore, the organogel-hydrogel hybrids have been limelight due to that such kind of hybrids could mimic biological organisms with exceptional freezing tolerance, and thus could provide an advantageous skill to fabricate robust ionic skins [77]. Zhixing developed a series of lauryl acrylate-based polymeric organogels with high transparency, mechanical adaptability, adhesive capability, and self-healing properties; the prepared organogels were expected to provide insights to design the artificial human-like skins with unprecedented functionalities [78]. Due to the delicate structure can be accomplished using 3D bioprinting, bio-inspired hydrogel shows potential applications in medical wearable devices.

#### **4.3 Pharmaceutical applications**

The bio-inspired hydrogels could also be used in drug delivery system, such as protein carriers, anti-inflammatory drug carriers, in the pharmaceutical industry [79]. Rana et al. designed a magnetic natural hydrogel based on alginate, gelatin, and iron oxide magnetic nanoparticles as an efficient drug delivery system, the drug doxorubicin hydrochloride (DOX) was loaded, the anticancer activity against Hela cells could be regulated by the release of DOX from hydrogels [80]. Maling et al. provided a proof-of-concept of detoxification using a 3D-printed biomimetic

**11**

**Figure 4.**

*Bio-Inspired Hydrogels via 3D Bioprinting DOI: http://dx.doi.org/10.5772/intechopen.94985*

**5. Future outlook**

nanocomposite construct in the hydrogel. A bio-inspired 3D detoxification device by installing polydiacetylene (PDA) nanoparticles in a 3D matrix was fabricated using dynamic optical projection stereolithography (DOPsL) technology; the nanoparticles could attract, capture and sense toxins, while the 3D matrix with a modified liver lobule microstructure allows toxins to be trapped efficiently [36]. The bio-inspired hydrogels via multi-materials 3D bioprinting can easy regulate the loading and release profiles of drugs, which show potentials as biomedicines.

The design paradigms shift from 2D to 3D has revolutionized the way of bioinspired hydrogels for materials components, engineered constructs, *in vitro* disease modeling, medical wearable devices, and precision medicine. 3D bioprinting technology realizes to fabricate the delicate bio-inspired hydrogels with excellent properties and necessary signals to promote healing, tissue regeneration, therapeutics delivery, and health monitor in real-time. However, there are still some issues that need to be addressed in the near future (**Figure 4**). As the researchers begin to scale-up the production of bio-inspired hydrogels, new parameters during the fabrication need be met, such as the bioprinting speeds and resolutions, such parameters need to be simultaneously be increased to create constructs of clinic size. In the near future, it will be essential to develop microscale organ-on-a-chip, such as liver- and heart-on-a-chip, tumor-on-a-chip, etc., that integrate bioinspired microenvironments with fluid flow inside hydrogels, also other dynamic physiological processes were well regulated by controlling the 3D bioprinting process. For example, the bio-inspired 3D culture in hydrogels could be employed to produce an *in vitro* model of Alzheimer's disease, providing a useful tool for the development of new therapeutics [82]. Future fabrication of bio-inspired hydrogels would be involved with multi-material 3D bioprinting, which provides the ability

*The future outlook of 3D bioprinting for fabrication of bio-inspired tissues for tissue engineering applications [81].*

*Bio-Inspired Hydrogels via 3D Bioprinting DOI: http://dx.doi.org/10.5772/intechopen.94985*

nanocomposite construct in the hydrogel. A bio-inspired 3D detoxification device by installing polydiacetylene (PDA) nanoparticles in a 3D matrix was fabricated using dynamic optical projection stereolithography (DOPsL) technology; the nanoparticles could attract, capture and sense toxins, while the 3D matrix with a modified liver lobule microstructure allows toxins to be trapped efficiently [36]. The bio-inspired hydrogels via multi-materials 3D bioprinting can easy regulate the loading and release profiles of drugs, which show potentials as biomedicines.
