A simple semi-quantitative approach studying the in vivo degradation of regenerated silk fibroin scaffolds with different pore sizes

Collagen type I-based recombinant peptide promotes bone regeneration in rat critical-size calvarial defects by enhancing osteoclast activity at late stages of healing

Introduction

We recently demonstrated the bone-forming potential of medium-cross-linked recombinant collagen peptide (mRCP) in animal models of bone defects. However, these studies were limited to a 4-week observation period; therefore, in the present study, we aimed to further evaluate mRCP as a suitable bone graft material for the alveolar cleft by analyzing its bone-forming potential, osteogenic-inducing ability, and biodegradation over an extended period of 12 weeks, using a rat critical-size calvarial defect model.

Methods

Using Sprague-Dawley rats, we created critical-size calvarial defects through a surgical procedure. The defects were then filled with 3 mg of mRCP (mRCP group) or 18 mg of Cytrans® (CA) granules, which has a carbonate apatite-based composition resembling natural bone, was used as a reference material (CA group). For negative control, the defects were left untreated. Bone volume, total bone volume (bone volume including CA granules), and bone mineral density (BMD) in the defect were assessed using micro-computed tomography (μ-CT) at 0, 4, 8, and 12 weeks after implantation. Using histomorphometric analyses of hematoxylin and eosin (H&E)-stained sections, we measured the amount of newly formed bone and total newly formed bone (new bone including CA granules) in the entire defect site, as well as the amount of newly formed bone in the central side, two peripheral sides (left and right), periosteal (top) side, and dura mater (bottom) side. In addition, we measured the amount of residual bone graft material in the defect. Osteoclasts and osteoblasts in the newly formed bone were detected using tartrate-resistant acid phosphatase (TRAP) and alkaline phosphatase (ALP) staining, respectively.

Results

Bone volume in the mRCP group increased over time and was significantly larger at 8 and 12 weeks after surgery than at 4 weeks. The bone volume in the mRCP group was greater than that of the CA and control groups at 4, 8, and 12 weeks after implantation, and while the total bone volume was greater in the CA group after 4 and 8 weeks, the mRCP group had comparable levels of total bone volume to that of the CA group at 12 weeks after implantation. The BMD of the mRCP group reached similar levels to native calvaria bone at the same time point. H&E-stained sections revealed a larger amount of newly formed bone 12 weeks after implantation in the mRCP group compared to that of the CA and control groups. The total newly formed bone at 12 weeks after implantation was on par with that in the CA group. Furthermore, at the defect site, the area of newly formed bone was larger on the peripheral and dura mater sides. Notably, the number of osteoclasts in the mRCP group was higher than in the CA and control groups and peaked 8 weeks after implantation, which coincided with the timing of the greatest resorption of mRCP. Although the ALP-positive area was greater in the mRCP group compared to other groups, we did not detect any significant changes in the number of osteoblasts over time.

Conclusion

This study demonstrated the bone-forming potential of mRCP over an extended period of 12 weeks, suggesting that mRCP sufficiently resists resorption to promote bone formation through induction of osteoclast activation in the late stages of the healing period.

Hierarchically patterned protein scaffolds with nano-fibrillar and micro-lamellar structures modulate neural stem cell homing and promote neuronal differentiation

Biophysical factors are essential in cell survival and behaviors, but constructing a suitable 3D microenvironment for the recruitment of stem cells and exerting their physiological functions remain a daunting challenge. Here, we present a novel silk fibroin (SF)-based fabrication strategy to develop hierarchical microchannel scaffolds for biomimetic nerve microenvironments in vitro. We first modulated the formation of SF nanofibers (SFNFs) that mimic the nanostructures of the native extracellular matrix (ECM) by using graphene oxide (GO) nanosheets as templates. Then, SFNF-GO systems were shaped into 3D porous scaffolds with aligned micro-lamellar structures by freeze-casting. The interconnected microchannels successfully induced cell infiltration and migration to the SFNF-GO scaffolds' interior. Meanwhile, the nano-fibrillar structures and the GO component significantly induced neural stem cells (NSCs) to differentiate into neurons within a short timeframe of 14 d. Importantly, these 3D hierarchical scaffolds induced a mild inflammatory response, extensive cell recruitment, and effective stimulation of NSC neuronal differentiation when implanted in vivo. Therefore, these SFNF-GO lamellar scaffolds with distinctive nano-/micro-topographies hold promise in the fields of nerve injury repair and regenerative medicine.

Irrigating degradation properties of silk fibroin-collagen type II composite cartilage scaffold in vitro and in vivo

Silk fibroin-collagen type II scaffolds are promising in cartilage tissue engineering due to their suitable biological functionality to promote proliferation of chondrocytes in vitro. However, their degradation properties, which are of crucial importance as scaffold degradation should consistent with the new tissue formation process, are still unknown. In this study, degradability of silk fibroin-collagen type II cartilage scaffolds was probed both in vitro and in vivo. In vitro degradation experiments show that the scaffolds decreased 32.25 % ± 0.62 %, 34.27 % ± 0.96 %, 36.27 % ± 2.39 % in weight after 8 weeks of degradation at the irrigation velocity of 0 mL/min, 7.89 mL/min and 15.79 mL/min. The degradation ratio, which increases with time and increasing irrigation velocity, is described by combining the built mathematic model and finite element modeling method. The scaffolds after 8 weeks of degradation in vitro keep their mechanical structural integrity to support new tissues. In vivo degradation experiments conducted in rabbits further show that the scaffolds degrade gradually, be absorbed with time and finally collapse in structure. The degradation process is accompanied by the growth of fibrous tissues and the scaffold is filled by fibrous tissues after 12 weeks of implantation. Immunohistology analysis shows that the inflammation caused by scaffolds is controllable and gradually alleviates with time. To sum up, silk fibroin-collagen type II cartilage scaffolds, which show suitable mechanical properties and biocompatibility during degradation in vitro and in vivo, have great potential in cartilage repair. The novelty of the study is that it not only introduces a mathematical model to predict the irrigation degradation ratio, but also provides experimental degradation data support for clinical application of silk fibroin-collagen type II cartilage scaffolds.

Evaluating the Degradation Process of Collagen Sponge and Acellular Matrix Implants In Vivo Using the Standardized HPLC-MS/MS Method

The purpose of this study was to establish a collagen determination method based on an isotope-labeled collagen peptide as an internal reference via high-performance liquid chromatography–tandem mass spectrometry (HPLC–MS/MS), and using the established method to evaluate the degradation process of collagen-based implants in vivo. The specific peptide (GPAGPQGPR) of bovine type I collagen was identified with an Orbitrap mass spectrometer. Then, the quantification method based on the peptide detection with HPLC-MS/MS was established and validated, and then further used to analyze the degradation trend of the collagen sponge and acellular matrix (ACM) in vivo at 2, 4, 6, 8, 12, 16, and 18 weeks after implantation. The results indicate that the relative standard deviation (RSD) of the detection precision and repeatability of the peptide-based HPLC-MS/MS quantification method were 3.55% and 0.63%, respectively. The limitations of quantification and detection were 2.05 × 10−3 μg/mL and 1.12 × 10−3 μg/mL, respectively. The collagen sponge and ACM were completely degraded at 10 weeks and 18 weeks, respectively. Conclusion: A specific peptide (GPAGPQGPR) of bovine type I collagen was identified with an Orbitrap mass spectrometer, and a standardized HPLC-MS/MS-based internal reference method for the quantification of bovine type I collagen was established. The method can be used for the analysis of the degradation of collagen-based implants in vivo.

Appropriate pore size for bone formation potential of porous collagen type I-based recombinant peptide

Introduction

In this study, we developed porous medium cross-linked recombinant collagen peptide (mRCP) with two different ranges of interconnected pore sizes, Small-mRCP (S-mRCP) with a range of 100–300 μm and Large-mRCP (L-mRCP) with a range of 200–500 μm, to compare the effect of pore size on bone regeneration in a calvarial bone defect.

Methods

Calvarial bone defects were created in Sprague–Dawley rats through a surgical procedure. The rats were divided into 2 groups: S-mRCP implanted group and L-mRCP implanted group. The newly formed bone volume and bone mineral density (BMD) was evaluated by micro-computed tomography (micro-CT) immediately after implantation and at 1, 2, 3, and 4 weeks after implantation. In addition, histological analyses were carried out with hematoxylin and eosin (H&E) staining at 4 weeks after implantation to measure the newly formed bone area between each group in the entire defect, as well as the central side, the two peripheral sides (right and left), the periosteal (top) side and the dura matter (bottom) side of the defect.

Results

Micro-CT analysis showed no significant differences in the amount of bone volume between the S-mRCP and L-mRCP implanted groups at 1, 2, 3 and 4 weeks after implantation. BMD was equivalent to that of the adjacent native calvaria bone at 4 weeks after implantation. H&E images showed that the newly formed bone area in the entire defect was significantly larger in the S-mRCP implanted group than in the L-mRCP implanted group. Furthermore, the amount of newly formed bone area in all sides of the defect was significantly more in the S-mRCP implanted group than in the L-mRCP implanted group.

Conclusion

These results indicate that the smaller pore size range of 100–300 μm is appropriate for mRCP in bone regeneration.

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This study confirmed the regenerative potential of mRCP as novel bone substitute.

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mRCP with 2 different interconnected pores sizes have been developed.

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The smaller pore size range of 100–300 μm was optimal for calvarial bone regeneration.

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The slower absorption rate of smaller pore size mRCP influenced its effectiveness.

Reconstruction of Rabbit Anterior Cruciate Ligament by Bone Marrow-Derived Mesenchymal Stem Cell Implantation Through a Weft-Knitted Silk Mesh Scaffold Covering a Whip-Shaped Core

To investigate the feasibility of using whip core wrapped by silk weft knitted mesh sheath as a scaffold and bone marrow-derived mesenchymal stem cells (BMSCs) to reconstruct the rabbit anterior cruciate ligament (ACL), BMSC implantation using the mesh-whip scaffold was performed to construct a BMSC-scaffold complex. Then, the BMSC-scaffold complex was implanted into an animal model of an ACL deficient rabbit. Regenerated ACLs were then taken from the animal model three and six months after implantation, followed by hematoxylin-eosin and Masson staining, quantitative RT-PCR detection, as well as mechanical performance evaluation. The results showed that many Sharpey’s fibers had arranged regularly between the neo-ACL and the bone three months after surgery, and an interface structure formed six months after surgery. Regenerated ligaments contained silk fibers and suficient collagen. Type I collagen, type III collagen, and tenascin-C were all highly expressed in the experimental group compared to the control group (no BMSC implantation) in the regenerated ligaments. In addition, the maximum pullout force values of neo-ACL in the three- and six-month experimental groups were 70.6±17.8 N and 122.8±25.7 N, respectively. The findings suggest that BMSC implantation using the mesh-whip scaffold is a promising method to reconstruct rabbit ACL.

In Vitro and In Vivo Degradation, Mechanical Properties, and Histocompatibility of Weft-Knitted Silk Mesh-Like Grafts

To investigate the degradation, mechanical properties, and histocompatibility of weft-knitted silk mesh-like grafts, we carried out the In Vitro and In Vivo silk grafts degradation assay. The In Vitro degradation experiment was performed by immersing the silk grafts in simulated body fluid for 1 year, and the results showed that the degradation rate of the silk mesh-like grafts was very slow, and there were few changes in the mechanical properties and quality of the silk mesh-like graft. In Vivo degradation assay was taken by implantation of the silk mesh-like grafts into the subcutaneous muscles of rabbits. At 3, 6, and 12 months postoperation, the rate of mass loss was 19.36%, 31.84%, and 58.77%, respectively, and the maximum load was 63.85%, 34.63%, and 10.76%, respectively of that prior to degradation. The results showed that the degradation rate of the silk graft and the loss of mechanical properties In Vivo were faster than the results obtained in the In Vitro experiments. In addition, there were no significant differences in secretion of serum IL-6 and TNF-α between the experimental and normal rabbits (P >0.05), suggesting no obvious inflammatory reaction. The findings suggest that the weft-knitted silk mesh-like grafts have good mechanical properties, histocompatibility, and In Vivo degradation rate, and therefore represent a candidate material for artificial ligament

Application of an ultrasound semi-quantitative assessment in the degradation of silk fibroin scaffolds in vivo

BACKGROUND

Research on the degradation of silk fibroin (SF) scaffolds in vivo lacks uniform and effective standards and experimental evaluation methods. This study aims to evaluate the application of ultrasound in assessing the degradation of SF scaffolds.

METHODS

Two groups of three-dimensional regenerated SF scaffolds (3D RSFs) were implanted subcutaneously into the backs of Sprague-Dawley rats. B-mode ultrasound and hematoxylin and eosin (HE) staining were performed on days 3, 7, 14, 28, 56, 84, 112, 140, and 196. The cross-sectional areas for two groups of 3D RSFs that were obtained using these methods were semi-quantitatively analyzed and compared to evaluate the biodegradation of the implanted RSFs.

RESULTS

The 3D RSFs in the SF-A group were wholly degraded at the 28th week after implantation. In contrast, the 3D RSFs in the SF-B group were completely degraded at the 16th week. Ultrasonic examination showed that the echoes of 3D RSFs in both groups gradually decreased with the increase of the implantation time. In the early stages of degradation, the echoes of the samples were higher than the echo of the muscle. In the middle of degeneration, the echoes were equal to the echo of the muscle. In the later stage, the echoes of the samples were lower than that of the muscle. The above changes in the SF-B group were earlier than those in the SF-A group. Semi-quantitative analysis of the cross-sectional areas detected using B-mode ultrasound revealed that the degradations of the two 3D RSF groups were significantly different. The degradation rate of the SF-B group was found to be higher than that of the SF-A group. This was consistent with the semi-quantitative detection results for HE staining. Regression analysis showed that the results of the B-mode ultrasound and HE staining were correlated in both groups, indicating that B-mode ultrasound is a reliable method to evaluate the SF scaffold degradation in vivo.

CONCLUSIONS

This study suggests that B-mode ultrasound can clearly display the implanted SF scaffolds non-invasively and monitor the degradation of the different SF scaffolds after implantation in living organisms in real-time.

Enzymatic Degradation of Bombyx mori Silk Materials: A Review

As a biomaterial, silk presents unique features with a combination of excellent mechanical properties, biocompatibility, and biodegradability. The biodegradability aspects of silk biomaterials, especially with options to control the rate from short (days) to long (years) time frames in vivo, make this protein-based biopolymer a good candidate for developing biodegradable devices used for tissue repairs and tissue engineering, as well as medical device implants. Silk materials, including native silk fibers and a broad spectrum of regenerated silk materials, have been investigated in vitro and in vivo to demonstrate degradation by proteolytic enzymes. In this Review, we summarize the findings on these studies on the enzymatic degradation of Bombyx mori (B. mori) silk materials. We also present a discussion on the factors that dictate the degradation properties of silk materials. Finally, in future perspectives, we highlight some key challenges and potential directions toward the future study of the degradation of silk materials.

Porous Three-Dimensional Silk Fibroin Scaffolds for Tracheal Epithelial Regeneration in Vitro and in Vivo

The regeneration of functional epithelial lining is critical for artificial grafts to repair tracheal defects. Although silk fibroin (SF) scaffolds have been widely studied for biomedical application (e.g., artificial skin), its potential for tracheal substitute and epithelial regeneration is still unknown. In this study, we fabricated porous three-dimensional (3D) silk fibroin scaffolds and cocultured them with primary human tracheobronchial epithelial cells (HBECs) for 21 days in vitro. Examined by scanning electronic microscopy (SEM) and calcein-AM staining with inverted phase contrast microscopy, the SF scaffolds showed excellent properties of promoting cell growth and proliferation for at least 21 days with good viability. In vivo, the porous 3D SF scaffolds (n = 18) were applied to repair a rabbit anterior tracheal defect. In the control group (n = 18), rabbit autologous pedicled trachea wall without epithelium, an ideal tracheal substitute, was implanted in situ. Observing by endoscopy and computed tomography (CT) scan, the repaired airway segment showed no wall collapse, granuloma formation, or stenosis during an 8-week interval in both groups. SEM and histological examination confirmed the airway epithelial growth on the surface of porous SF scaffolds. Both the epithelium repair speed and the epithelial cell differentiation degree in the SF scaffold group were comparable to those in the control group. Neither severe inflammation nor excessive fibrosis occurred in both groups. In summary, the porous 3D SF scaffold is a promising biomaterial for tracheal repair by successfully supporting tracheal wall contour and promoting tracheal epithelial regeneration.

Transparent biocompatible wool keratin film prepared by mechanical compression of porous keratin hydrogel

We could prepare a transparent wool keratin film by mechanical compression of the keratin hydrogel, which was prepared by our method previously reported. Optical transmittance of the keratin film was approximately 70% at 400 nm and 80% at 550 nm. The keratin film had higher mechanical strength than the keratin hydrogel estimated from the tensile test. Young's modulus of the keratin film and that of keratin hydrogel were 0.582 ± 0.294 MPa and 0.041 ± 0.008 MPa, respectively. We evaluated degradability of keratin film by tryptic digestion in vitro and that also by implantation test in vivo. The keratin film showed slower degradation rate in the presence of trypsin in vitro, and also that as a subcutaneous implant in mouse in vivo. Biocompatibility is also a key factor for application of keratin as biomaterials. Within several days after subcutaneous implantation of the sample in mouse, an apparent symptom of acute inflammation of tissues, such as swelling of the reddish skin, was not observed. Keratin film remained in the original morphology of sheet-like structure while keratin hydrogel was degraded with many cracks and gaps after implantation for several weeks. We concluded from those results that keratin film was mostly biocompatible without provoking inflammation nor encapsulation, mechanically stronger than the keratin hydrogel, and was more resistant to degradation than the keratin hydrogel.