Effects of tissue processing on bioactivity of cartilage matrix-based hydrogels encapsulating osteoconductive particles

Progress in microsphere-based scaffolds in bone/cartilage tissue engineering

Bone/cartilage repair and regeneration have been popular and difficult issues in medical research. Tissue engineering is rapidly evolving to provide new solutions to this problem, and the key point is to design the appropriate scaffold biomaterial. In recent years, microsphere-based scaffolds have been considered suitable scaffold materials for bone/cartilage injury repair because microporous structures can form more internal space for better cell proliferation and other cellular activities, and these composite scaffolds can provide physical/chemical signals for neotissue formation with higher efficiency. This paper reviews the research progress of microsphere-based scaffolds in bone/chondral tissue engineering, briefly introduces types of microspheres made from polymer, inorganic and composite materials, discusses the preparation methods of microspheres and the exploration of suitable microsphere pore size in bone and cartilage tissue engineering, and finally details the application of microsphere-based scaffolds in biomimetic scaffolds, cell proliferation and drug delivery systems.

Comparison of a Thiolated Demineralized Bone Matrix Hydrogel to a Clinical Product Control for Regeneration of Large Sheep Cranial Defects

Regeneration of calvarial bone remains a major challenge in the clinic as available options do not sufficiently regenerate bone in larger defect sizes. Calvarial bone regeneration cases involving secondary medical conditions, such as brain herniation during traumatic brain injury (TBI) treatment, further exacerbate treatment options. Hydrogels are well-positioned for severe TBI treatment, given their innate flexibility and potential for bone regeneration to treat TBI in a single-stage surgery. The current study evaluated a photocrosslinking pentenoate-modified hyaluronic acid polymer with thiolated demineralized bone matrix (i.e., TDBM hydrogel) capable of forming a completely interconnected hydrogel matrix for calvarial bone regeneration. The TDBM hydrogel demonstrated a setting time of 120 s, working time of 3 to 7 days, negligible change in setting temperature, physiological setting pH, and negligible cytotoxicity, illustrating suitable performance for in vivo application. Side-by-side ovine calvarial bone defects (19 mm diameter) were employed to compare the TDBM hydrogel to the standard-of-care control material DBX®. After 16 weeks, the TDBM hydrogel had comparable healing to DBX® as demonstrated by mechanical push-out testing (~800 N) and histology. Although DBX® had 59% greater new bone volume compared to the TDBM hydrogel via micro-computed tomography, both demonstrated minimal bone regeneration overall (15 to 25% of defect volume). The current work presents a method for comparing the regenerative potential of new materials to clinical products using a side-by-side cranial bone defect model. Comparison of novel biomaterials to a clinical product control (i.e., standard-of-care) provides an important baseline for successful regeneration and potential for clinical translation.

Temporal changes in the physical and mechanical properties of beetle elytra during maturation

Changes in physical properties of Tenebrio molitor and Tribolium castaneum elytra (hardened forewings) were studied to understand how the development of microstructure and chemical interactions determine cuticle mechanical properties. Analysis of these properties supports a model in which cuticular material is continuously secreted from epidermal cells to produce an extracellular matrix so that the outermost layers mature first. It is hypothesized that enzymatic crosslinking and pigmentation reactions along with dehydration help to stabilize the protein-chitin network within the initial layers of cuticle shortly after eclosion. Mature layers are proposed to bear most of the mechanical loads. The frequency dependence of the storage modulus and the tan δ values decreased during the beginning of maturation, reaching constant values after 48 h post-eclosion. A decrease of tan δ indicates an increase in crosslinking of the material. The water content declined from 75% to 31%, with a significant portion lost from within the open spaces between the dorsal and ventral cuticular layers. Dehydration had a less significant influence than protein crosslinking on the mechanical properties of the elytron during maturation. When Tribolium cuticular protein TcCP30 expression was decreased by RNAi, the tan δ and frequency dependence of E' of the elytron did not change during maturation. This indicates that TcCP30 plays a role in the crosslinking process of the beetle's exoskeleton. This study was inspired by previous work on biomimetic multicomponent materials and helps inform future work on creating robust lightweight materials derived from natural sources. STATEMENT OF SIGNIFICANCE: Examination of changes in the physical properties of the elytra (hardened forewings) of two beetle species advanced understanding of how the molecular interactions influence the mechanical properties of the elytra. Physical characterization, including dynamic mechanical analysis, determined that the outer portion of the elytra matured first, while epidermal cells continued to secrete reactive components until the entire structure reached maturation. RNA interference was used to identify the role of a key protein in the elytra. Suppression of its expression reduced the formation of crosslinked polymeric components in the elytra. Identifying the molecular interactions in the matrix of proteins and polysaccharides in the elytra together with their hierarchical architecture provides important design concepts in the development of biomimetic materials.

Independent Control of Molecular Weight, Concentration, and Stiffness of Hyaluronic Acid Hydrogels

Hyaluronic acid (HA) hydrogels have been used for a multitude of applications, perhaps most notably for tissue engineering and regenerative medicine, owing to the versatility of the polymer and its tunable nature. Various groups have investigated the impact of hydrogel parameters (e.g., molecular weight, concentration, stiffness, etc.) in vitro and in vivo to achieve desired material performance characteristics. A limitation in the literature to date has been that altering one hydrogel parameter (a 'manipulated variable') to achieve a given hydrogel characteristic (a 'controlled variable') changes two variables at a time (e.g., altering molecular weight and/or concentration to investigate cell response to stiffness). Therefore, if cell responses differ, it may be possible that more than one variable caused the changes in observed responses. In the current study, we leveraged thiol-ene click chemistry with a crosslinker to develop a method that minimizes material performance changes and permitted multiple material properties to be independently held constant to evaluate a single variable at a time. Independent control was accomplished by tuning the concentration of crosslinker to achieve an effectively constant stiffness for different HA hydrogel molecular weights and polymer concentrations. Specific formulations were thereby identified that enabled the molecular weight (76 - 1550 kDa), concentration (2 - 10%), or stiffness (~1 - 350 kPa) to be varied while the other two were held constant, a key technical achievement. The response of rat mesenchymal stem cells to varying molecular weight, concentration, and stiffness demonstrated consistent upregulation of osteocalcin gene expression. The methodology presented to achieve independent control of hydrogel parameters may potentially be adopted by others for alternative hydrogel polymers, cell types, or cell culture medium compositions to minimize confounding variables in experimental hydrogel designs.

The Rheology and Printability of Cartilage Matrix-Only Biomaterials

The potential chondroinductivity from cartilage matrix makes it promising for cartilage repair; however, cartilage matrix-based hydrogels developed thus far have failed to match the mechanical performance of native cartilage or be bioprinted without adding polymers for reinforcement. There is a need for cartilage matrix-based hydrogels with robust mechanical performance and paste-like precursor rheology for bioprinting/enhanced surgical placement. In the current study, our goals were to increase hydrogel stiffness and develop the paste-like precursor/printability of our methacryl-modified solubilized and devitalized cartilage (MeSDVC) hydrogels. We compared two methacryloylating reagents, methacrylic anhydride (MA) and glycidyl methacrylate (GM), and varied the molar excess (ME) of MA from 2 to 20. The MA-modified MeSDVCs had greater methacryloylation than GM-modified MeSDVC (20 ME). While GM and most of the MA hydrogel precursors exhibited paste-like rheology, the 2 ME MA and GM MeSDVCs had the best printability (i.e., shape fidelity, filament collapse). After crosslinking, the 2 ME MA MeSDVC had the highest stiffness (1.55 ± 0.23 MPa), approaching the modulus of native cartilage, and supported the viability/adhesion of seeded cells for 15 days. Overall, the MA (2 ME) improved methacryloylation, hydrogel stiffness, and printability, resulting in a stand-alone MeSDVC printable biomaterial. The MeSDVC has potential as a future bioink and has future clinical relevance for cartilage repair.

Automated Decellularization of Musculoskeletal Tissues with High Extracellular Matrix Retention

Manual tissue decellularization is an onerous process that requires the application of many sequential treatments by an operator and can be prone to user error and result variability. While automated decellularization devices have been previously reported, with advances being made in recent years toward open-source platforms, previous automated decellularization devices have been reliant on hardware or software components that are closed-source and proprietary. The aim of the current work was to develop and validate a full open-source automated decellularization system to be available for others to adopt. The open-source decellularization apparatus is a low-cost (<$2000) device that may easily be adapted to an array of decellularization protocols, with an example parts' list provided herein. The automated decellularization device was used to decellularize hyaline cartilage, knee meniscus, and tendon tissues. Cartilage, meniscus, and tendon tissue demonstrated 97%, 99%, and 96% reduction in DNA content after decellularization, respectively, and with effective decellularization confirmed visually via histology. High retentions of glycosaminoglycans (GAGs), collagen, and other proteins were observed in meniscus and tendon following decellularization. Results with manual decellularization with meniscus tissue were consistent with the automated decellularization process. Decellularized cartilage (DCC) demonstrated a 34% decrease in GAG content, while the protein and collagen content did not significantly change. The current study demonstrated that native-like decellularized tissues were produced reproducibly using the reported open-source automated decellularization platform, providing an adoptable platform for production of decellularized tissues by others. Impact statement Decellularized extracellular matrix (ECM)-based materials are appealing for tissue engineering, but production of these materials is historically time-intensive, tedious, and prone to user error. Adoption of an automated system may be a barrier for many research groups due to cost and complexity. In this article, a low-cost open-source platform for automated decellularization is presented. This method is validated by decellularizing porcine musculoskeletal tissues and demonstrating the native-like compositional properties of these decellularized tissues. The ability to produce decellularized tissue in an automated manner is useful for further research of ECM-based materials and potential clinical applications.

In vivo behavior of bioactive glass-based composites in animal models for bone regeneration

This review presents the recent advances and the current state-of-the-art of bioactive glass-based composite biomaterials intended for bone regeneration. Composite materials comprise two (or more) constituents at the nanometre scale, in which typically, one constituent is organic and functions as the matrix phase and the other constituent is inorganic and behaves as the reinforcing phase. Such materials, thereby, more closely resemble natural bio-nanocomposites such as bone. Various glass compositions in combination with a wide range of natural and synthetic polymers have been evaluated in vivo under experimental conditions ranging from unloaded critical-sized defects to mechanically-loaded, weight-bearing sites with highly favourable outcomes. Additional possibilities include controlled release of anti-osteoporotic drugs, ions, antibiotics, pro-angiogenic substances and pro-osteogenic substances. Histological and morphological evaluations suggest the formation of new, highly vascularised bone that displays signs of remodelling over time. With the possibility to tailor the mechanical and chemical properties through careful selection of individual components, as well as the overall geometry (from mesoporous particles and micro-/nanospheres to 3D scaffolds and coatings) through innovative manufacturing processes, such biomaterials present exciting new avenues for bone repair and regeneration.

An in vivo evaluation of induced chondrogenesis by decellularized extracellular matrix particles

Bioengineered scaffolds composed of synthetic materials and extracellular matrix (ECM) components can offer a tissue-specific microenvironment capable of regulating cells to regenerate the structure and function of the native cartilage. Here, given the potential preservation of biomechanical and biochemical cues found in the native cartilage, particulate decellularized ECM (DC-ECM) was utilized for immobilization on the surface of nanofibrous scaffolds. Afterward, the chondro-inductive potential and ectopic cartilage formation after subcutaneous implantation of bioengineered DC-ECM scaffolds were investigated in mice model. Eight weeks post-implantation, no growth of considerable inflammatory response and neovascularization was observed in histological images of bioengineered DC-ECM scaffolds. Pre-seeded bioengineered scaffolds with human adipose-derived stem cells exhibited high levels of chondro-induction capability, indicated with immunohistochemical and gene expression results. In both interval times, we also observed chondrogenesis and tissue formation after implanting unseeded bioengineered scaffolds, which denote that the presence of DC-ECM particles can even enhance attachment and migration of the host cells and induce chondrogenesis to them. To sum up, the incorporation of DC-ECM materials to tissue engineered constructs is a promising avenue to mimic the native tissue environment for regulation of cartilage regeneration in both in vivo and in vitro settings.

Natural hydrogels R&D process: technical and regulatory aspects for industrial implementation

Since hydrogel therapies have been introduced into clinic treatment procedures, the biomedical industry has to face the technology transfer and the scale-up of the processes. This will be key in the roadmap of the new technology implementation. Transfer technology and scale-up are already known for some applications but other applications, such as 3D printing, are still challenging. Decellularized tissues offer a lot of advantages when compared to other natural gels, for example they display enhanced biological properties, due to their ability to preserve natural molecules. For this reason, even though their use as a source for bioinks represents a challenge for the scale-up process, it is very important to consider the advantages that originate with overcoming this challenge. Therefore, many aspects that influence the scaling of the industrial process should be considered, like the addition of drugs or cells to the hydrogel, also, the gelling process is important to determine the chemical and physical parameters that must be controlled in order to guarantee a successful process. Legal aspects are also crucial when carrying out the scale-up of the process since they determine the industrial implementation success from the regulatory point of view. In this context, the new law Regulation (EU) 2017/745 on biomedical devices will be considered. This review summarizes the different aspects, including the legal ones, that should be considered when scaling up hydrogels of natural origin, in order to balance these different aspects and to optimize the costs in terms of raw materials and engine.

Pancreatic extracellular matrix and platelet-rich plasma constructing injectable hydrogel for pancreas tissue engineering

The development of pancreatic extracellular matrices enriched with insulin-secreting β-cells is a promising tissue engineering approach to treat type 1 diabetes. However, its long-term therapeutic efficacy is restricted by the defensive mechanism of host immune response and the lack of developed vascularization as appropriate after transplantation. Platelet-rich plasma (PRP), as an autologous platelet concentrate, contains a large number of active factors that are essential for the cell viability, vascularization, and immune regulation. In this study, we have incorporated pancreatic extracellular matrix (PEM) with PRP to develop a three-dimensional (3D) injectable PEM-PRP hydrogel to coculture and transplant rat insulinoma cells (INS-1) and human umbilical vein endothelial cells (HUVECs). Results from this study demonstrated that PEM-PRP hydrogel mimicked the biochemical compositions of native extracellular matrices, and possessed the enhanced elastic modulus and resistance to enzymatic degradation that enabled biomaterials to maintain its volume and slowly degrade. Additionally, PEM-PRP hydrogel could release growth factors in a sustained manner. In vitro, PEM-PRP hydrogel significantly promoted the viability, insulin-secreting function, and insulin gene expression of gel encapsulated INS-1 cells. Moreover, HUVECs encapsulated in PEM-PRP hydrogel were found to constitute many lumen-like structures and exhibited high expression of proangiogenic genes. In vivo transplantation of PEM-PRP hydrogel encapsulated with INS-1 cells and HUVECs improved the viability of INS-1 cells, promoted vascularization, inhibited the host inflammatory response, and reversed hyperglycemia of diabetic rats. Our study suggests that the PEM-PRP hydrogel offers excellent biocompatibility and proangiogenic property, and may serve as an effective biomaterial platform to maintain the long-term survival and function of insulin-secreting β cells.

Thiolated bone and tendon tissue particles covalently bound in hydrogels for in vivo calvarial bone regeneration

Bone regeneration of large cranial defects, potentially including traumatic brain injury (TBI) treatment, presents a major problem with non-crosslinking, clinically available products due to material migration outside the defect. Commercial products such as bone cements are permanent and thus not conducive to bone regeneration, and typical commercial bioactive materials for bone regeneration do not crosslink. Our previous work demonstrated that non-crosslinking materials may be prone to material migration following surgical placement, and the current study attempted to address these problems by introducing a new hydrogel system where tissue particles are themselves the crosslinker. Specifically, a pentenoate-modified hyaluronic acid (PHA) polymer was covalently linked to thiolated tissue particles of demineralized bone matrix (TDBM) or devitalized tendon (TDVT), thereby forming an interconnected hydrogel matrix for calvarial bone regeneration. All hydrogel precursor solutions exhibited sufficient yield stress for surgical placement and an adequate compressive modulus post-crosslinking. Critical-size calvarial defects were filled with a 4% PHA hydrogel containing 10 or 20% TDBM or TDVT, with the clinical product DBX^Ⓡ being employed as the standard of care control for the in vivo study. At 12 weeks, micro-computed tomography analysis demonstrated similar bone regeneration among the experimental groups, TDBM and TDVT, and the standard of care control DBX^Ⓡ. The group with 10% TDBM was therefore identified as an attractive material for potential calvarial defect repair, as it additionally exhibited a sufficient initial recovery after shearing (i.e., > 80% recovery). Future studies will focus on applying a hydrogel in a rat model for treatment of TBI. STATEMENT OF SIGNIFICANCE: Non-crosslinking materials may be prone to material migration from a calvarial bone defect following surgical placement, which is problematic for materials intended for bone regeneration. Unfortunately, typical crosslinking materials such as bone cements are permanent and thus not conducive to bone regeneration, and typical bioactive materials for bone regeneration such as tissue matrix are not crosslinked in commercial products. The current study addressed these problems by introducing a new biomaterial where tissue particles are themselves the crosslinker in a hydrogel system. The current study successfully demonstrated a new material based on pentenoate-modified hyaluronic acid with thiolated demineralized bone matrix that is capable of rapid crosslinking, with desirable paste-like rheology of the precursor material for surgical placement, and with bone regeneration comparable to a commercially available standard-of-care product. Such a material may hold promise for a single-surgery treatment of severe traumatic brain injury (TBI) following hemicraniectomy.

Nanohydroxyapatite Reinforced Chitosan Composite Hydrogel with Tunable Mechanical and Biological Properties for Cartilage Regeneration

With the continuous quest of developing hydrogel for cartilage regeneration with superior mechanobiological properties are still becoming a challenge. Chitosan (CS) hydrogels are the promising implant materials due to an analogous character of the soft tissue; however, their low mechanical strength and durability together with its lack of integrity with surrounding tissues hinder the load-bearing application. This can be solved by developing a composite chitosan hydrogel reinforced with Hydroxyapatite Nanorods (HANr). The objective of this work is to develop and characterize (physically, chemically, mechanically and biologically) the composite hydrogels loaded with different concentration of hydroxyapatite nanorod. The concentration of hydroxyapatite in the composite hydrogel was optimized and it was found that, reinforcement modifies the hydrogel network by promoting the secondary crosslinking. The compression strength could reach 1.62 ± 0.02 MPa with a significant deformation of 32% and exhibits time-dependent, rapid self-recoverable and fatigue resistant behavior based on the cyclic loading-unloading compression test. The storage modulus value can reach nearly 10 kPa which is needed for the proposed application. Besides, composite hydrogels show an excellent antimicrobial activity against Escherichia coli, Staphylococcus aureus bacteria's and Candida albicans fungi and their cytocompatibility towards L929 mouse fibroblasts provide a potential pathway to developing a composite hydrogel for cartilage regeneration.

Focal adhesion kinase promotes BMP2-induced osteogenic differentiation of human urinary stem cells via AMPK and Wnt signaling pathways

Human urine-derived stem cells (hUSCs) serve as favorable candidates for bone transplants due to their efficient proliferative and multipotent differentiation abilities, as well as the capacity to secrete a variety of vasoactive agents to facilitate tissue engineering. The present study aimed to explore the role of focal adhesion kinase (FAK) in bone morphogenetic protein 2 (BMP2)-induced osteogenic differentiation of hUSCs and to investigate the underlying mechanism. The degree of osteogenic differentiation and the correlated signals, following BMP2 overexpression and siRNA-mediated silencing of FAK, were determined in vitro. Moreover, hUSCs induced bone formation in a rat model with cranial defects, in vivo. Our findings revealed that alkaline phosphatase production, calcium deposits, osteocalcin and osteopontin expression, and bone formation were upregulated in vitro and in vivo following BMP2-induced osteogenic differentiation, and AMPK and Wnt signaling pathway activation by FAK could effectively regulate BMP2-enhanced osteogenic differentiation of hUSCs. Taken together, these findings indicated that FAK could mediate BMP2-enhanced osteogenic differentiation of hUSCs through activating adenosine 5'-monophosphate-activated protein kinase and Wnt signaling pathways.

Development and quantitative characterization of the precursor rheology of hyaluronic acid hydrogels for bioprinting

Bioprinting technologies have tremendous potential for advancing regenerative medicine due to the precise spatial control over depositing a printable biomaterial, or bioink. Despite the growing interest in bioprinting, the field is challenged with developing biomaterials for extrusion-based bioprinting. The paradigm of contemporary bioink studies relies on trial-and-error methods for discovering printable biomaterials, which has little practical use for others who endeavor to develop bioinks. There is pressing need to follow the precedent set by a few pioneering studies that have attempted to standardize bioink characterizations for determining the properties that define printability. Here, we developed a pentenoate-functionalized hyaluronic acid hydrogel (PHA) into a printable bioink and used three recommended, quantitative rheological assessments to characterize the printability: 1) yield stress, 2) viscosity, and 3) storage modulus recovery. The most important characteristic is the yield stress; we found a yield stress upper limit of ∼1000 Pa for PHA. Measuring the viscosity was advantageous for determining shear-thinning behavior, which aided in extruding highly viscous PHA through a nozzle. Post-printing recovery is required to maintain shape fidelity and we found storage modulus recoveries above ∼85% were sufficient for PHA. Two formulations had superior printability (i.e., 1.5 MDa PHA - 4 wt%, and 1 MDa PHA - 8 wt%), and increasing cell concentrations in PHA up to 9 × 106 cells/mL had minimal effects on the printability. Even so, other factors such as sterilization and peptide modifications to enhance bioactivity may influence printability, highlighting the need for investigators to consider such factors when developing new bioinks. STATEMENT OF SIGNIFICANCE: Bioprinting has potential for regenerating damaged tissues; however, there are a limited number of printable biomaterials, and developing new bioinks is challenging because the required material physical properties for extrusion-based printing are not yet known. Most new bioinks are developed by trial-and-error, which is neither efficient nor comparable across materials. There is a need for the field to begin utilizing standard methods proposed by a few pioneering studies to characterize new bioinks. Therefore, we have developed the printability of a hyaluronic acid based-hydrogel and characterized the material with three quantitative rheological tests. The current work impacts the bioprinting field by demonstrating and encouraging the use of universal bioink characterizations and by providing printability windows to advance new bioink development.

Microporous methacrylated glycol chitosan-montmorillonite nanocomposite hydrogel for bone tissue engineering

Injectable hydrogels can fill irregular defects and promote in situ tissue regrowth and regeneration. The ability of directing stem cell differentiation in a three-dimensional microenvironment for bone regeneration remains a challenge. In this study, we successfully nanoengineer an interconnected microporous networked photocrosslinkable chitosan in situ-forming hydrogel by introducing two-dimensional nanoclay particles with intercalation chemistry. The presence of the nanosilicates increases the Young’s modulus and stalls the degradation rate of the resulting hydrogels. We demonstrate that the reinforced hydrogels promote the proliferation as well as the attachment and induced the differentiation of encapsulated mesenchymal stem cells in vitro. Furthermore, we explore the effects of nanoengineered hydrogels in vivo with the critical-sized mouse calvarial defect model. Our results confirm that chitosan-montmorillonite hydrogels are able to recruit native cells and promote calvarial healing without delivery of additional therapeutic agents or stem cells, indicating their tissue engineering potential.

Injectable hydrogels could be used to repair bone defects. Here the authors incorporate nanoclay particles into chitosan creating an interconnected microporous hydrogel and show that this hydrogel can support MSC proliferation and differentiation in vitro, and support the recruitment of native cells and bone regeneration in a mouse calvarial defect model.

Scaffolding Strategies for Tissue Engineering and Regenerative Medicine Applications

During the past two decades, tissue engineering and the regenerative medicine field have invested in the regeneration and reconstruction of pathologically altered tissues, such as cartilage, bone, skin, heart valves, nerves and tendons, and many others. The 3D structured scaffolds and hydrogels alone or combined with bioactive molecules or genes and cells are able to guide the development of functional engineered tissues, and provide mechanical support during in vivo implantation. Naturally derived and synthetic polymers, bioresorbable inorganic materials, and respective hybrids, and decellularized tissue have been considered as scaffolding biomaterials, owing to their boosted structural, mechanical, and biological properties. A diversity of biomaterials, current treatment strategies, and emergent technologies used for 3D scaffolds and hydrogel processing, and the tissue-specific considerations for scaffolding for Tissue engineering (TE) purposes are herein highlighted and discussed in depth. The newest procedures focusing on the 3D behavior and multi-cellular interactions of native tissues for further use for in vitro model processing are also outlined. Completed and ongoing preclinical research trials for TE applications using scaffolds and hydrogels, challenges, and future prospects of research in the regenerative medicine field are also presented.

Flow Behavior Prior to Crosslinking: The Need for Precursor Rheology for Placement of Hydrogels in Medical Applications and for 3D Bioprinting

Hydrogels - water swollen cross-linked networks - have demonstrated considerable promise in tissue engineering and regenerative medicine applications. However, ambiguity over which rheological properties are needed to characterize these gels before crosslinking still exists. Most hydrogel research focuses on the performance of the hydrogel construct after implantation, but for clinical practice, and for related applications such as bioinks for 3D bioprinting, the behavior of the pre-gelled state is also critical. Therefore, the goal of this review is to emphasize the need for better rheological characterization of hydrogel precursor formulations, and standardized testing for surgical placement or 3D bioprinting. In particular, we consider engineering paste or putty precursor solutions (i.e., suspensions with a yield stress), and distinguish between these differences to ease the path to clinical translation. The connection between rheology and surgical application as well as how the use of paste and putty nomenclature can help to qualitatively identify material properties are explained. Quantitative rheological properties for defining materials as either pastes or putties are proposed to enable easier adoption to current methods. Specifically, the three-parameter Herschel-Bulkley model is proposed as a suitable model to correlate experimental data and provide a basis for meaningful comparison between different materials. This model combines a yield stress, the critical parameter distinguishing solutions from pastes (100-2000 Pa) and from putties (>2000 Pa), with power law fluid behavior once the yield stress is exceeded. Overall, successful implementation of paste or putty handling properties to the hydrogel precursor may minimize the surgeon-technology learning time and ultimately ease incorporation into current practice. Furthermore, improved understanding and reporting of rheological properties will lead to better theoretical explanations of how materials affect rheological performances, to better predict and design the next generation of biomaterials.