Beta Hairpin Peptide Hydrogels as an Injectable Solid Vehicle for Neurotrophic Growth Factor Delivery

Advancements in Regenerative Hydrogels in Skin Wound Treatment: A Comprehensive Review

This state-of-the-art review explores the emerging field of regenerative hydrogels and their profound impact on the treatment of skin wounds. Regenerative hydrogels, composed mainly of water-absorbing polymers, have garnered attention in wound healing, particularly for skin wounds. Their unique properties make them well suited for tissue regeneration. Notable benefits include excellent water retention, creating a crucially moist wound environment for optimal healing, and facilitating cell migration, and proliferation. Biocompatibility is a key feature, minimizing adverse reactions and promoting the natural healing process. Acting as a supportive scaffold for cell growth, hydrogels mimic the extracellular matrix, aiding the attachment and proliferation of cells like fibroblasts and keratinocytes. Engineered for controlled drug release, hydrogels enhance wound healing by promoting angiogenesis, reducing inflammation, and preventing infection. The demonstrated acceleration of the wound healing process, particularly beneficial for chronic or impaired healing wounds, adds to their appeal. Easy application and conformity to various wound shapes make hydrogels practical, including in irregular or challenging areas. Scar minimization through tissue regeneration is crucial, especially in cosmetic and functional regions. Hydrogels contribute to pain management by creating a protective barrier, reducing friction, and fostering a soothing environment. Some hydrogels, with inherent antimicrobial properties, aid in infection prevention, which is a crucial aspect of successful wound healing. Their flexibility and ability to conform to wound contours ensure optimal tissue contact, enhancing overall treatment effectiveness. In summary, regenerative hydrogels present a promising approach for improving skin wound healing outcomes across diverse clinical scenarios. This review provides a comprehensive analysis of the benefits, mechanisms, and challenges associated with the use of regenerative hydrogels in the treatment of skin wounds. In this review, the authors likely delve into the application of rational design principles to enhance the efficacy and performance of hydrogels in promoting wound healing. Through an exploration of various methodologies and approaches, this paper is poised to highlight how these principles have been instrumental in refining the design of hydrogels, potentially revolutionizing their therapeutic potential in addressing skin wounds. By synthesizing current knowledge and highlighting potential avenues for future research, this review aims to contribute to the advancement of regenerative medicine and ultimately improve clinical outcomes for patients with skin wounds.

Calming the Nerves via the Immune Instructive Physiochemical Properties of Self-Assembling Peptide Hydrogels

Current therapies for the devastating damage caused by traumatic brain injuries (TBI) are limited. This is in part due to poor drug efficacy to modulate neuroinflammation, angiogenesis and/or promoting neuroprotection and is the combined result of challenges in getting drugs across the blood brain barrier, in a targeted approach. The negative impact of the injured extracellular matrix (ECM) has been identified as a factor in restricting post-injury plasticity of residual neurons and is shown to reduce the functional integration of grafted cells. Therefore, new strategies are needed to manipulate the extracellular environment at the subacute phase to enhance brain regeneration. In this review, potential strategies are to be discussed for the treatment of TBI by using self-assembling peptide (SAP) hydrogels, fabricated via the rational design of supramolecular peptide scaffolds, as an artificial ECM which under the appropriate conditions yields a supramolecular hydrogel. Sequence selection of the peptides allows the tuning of these hydrogels' physical and biochemical properties such as charge, hydrophobicity, cell adhesiveness, stiffness, factor presentation, degradation profile and responsiveness to (external) stimuli. This review aims to facilitate the development of more intelligent biomaterials in the future to satisfy the parameters, requirements, and opportunities for the effective treatment of TBI.

Visualization of Self-Assembly and Hydration of a β-Hairpin through Integrated Small and Wide-Angle Neutron Scattering

Fundamental understanding of the structure and assembly of nanoscale building blocks is crucial for the development of novel biomaterials with defined architectures and function. However, accessing self-consistent structural information across multiple length scales is challenging. This limits opportunities to exploit atomic scale interactions to achieve emergent macroscale properties. In this work we present an integrative small- and wide-angle neutron scattering approach coupled with computational modeling to reveal the multiscale structure of hierarchically self-assembled β hairpins in aqueous solution across 4 orders of magnitude in length scale from 0.1 Å to 300 nm. Our results demonstrate the power of this self-consistent cross-length scale approach and allows us to model both the large-scale self-assembly and small-scale hairpin hydration of the model β hairpin CLN025. Using this combination of techniques, we map the hydrophobic/hydrophilic character of this model self-assembled biomolecular surface with atomic resolution. These results have important implications for the multiscale investigation of aqueous peptides and proteins, for the prediction of ligand binding and molecular associations for drug design, and for understanding the self-assembly of peptides and proteins for functional biomaterials.

Constructing Antiretroviral Supramolecular Polymers as Long-Acting Injectables through Rational Design of Drug Amphiphiles with Alternating Antiretroviral-Based and Hydrophobic Residues

One of the main challenges in the development of long-acting injectables for HIV treatment is the limited duration of drug release, which results in the need for frequent dosing and reduced patient adherence. In this context, we leverage the intrinsic reversible features of supramolecular polymers and their unique ability to form a three-dimensional network under physiological conditions to design a class of self-assembling drug amphiphiles (DAs) based upon lamivudine, a water-soluble antiretroviral (ARV) agent and nucleoside reverse transcriptase inhibitor. The designed ARV DAs contain three pairs of alternating hydrophobic valine (V) and hydrophilic lamivudine-modified lysine (K3TC) residues with a varying number of glutamic acids (E) placed on the C-terminus. Upon dissolution in deionized water, all three ARV DAs were found to spontaneously associate into supramolecular filaments of several micrometers in length, with varying levels of lateral stacking. Addition of 1× PBS triggered immediate gelation of the two ARV DAs with 2 or 3 E residues, and upon dilution in an in vitro setting, the dissociation from the supramolecular state to the monomeric state enabled a long-acting linear release of the ARV DAs. In vivo studies further confirmed their injectability, rapid in situ hydrogel formation, enhanced local retention, and long-acting therapeutic release over a month. Importantly, our pharmacokinetic studies suggest that the injected ARV supramolecular polymeric hydrogel was able to maintain a plasma concentration of lamivudine above its IC50 value for more than 40 days in mice and showed minimal systemic immunogenicity. We believe that these results shed important light on the rational design of long-acting injectables using the drug-based molecular assembly strategy, and the reported ARV supramolecular hydrogels hold great promise for improving HIV treatment outcomes.

Simple Complexity: Incorporating Bioinspired Delivery Machinery within Self-Assembled Peptide Biogels

Bioinspired self-assembly is a bottom-up strategy enabling biologically sophisticated nanostructured biogels that can mimic natural tissue. Self-assembling peptides (SAPs), carefully designed, form signal-rich supramolecular nanostructures that intertwine to form a hydrogel material that can be used for a range of cell and tissue engineering scaffolds. Using the tools of nature, they are a versatile framework for the supply and presentation of important biological factors. Recent developments have shown promise for many applications such as therapeutic gene, drug and cell delivery and yet are stable enough for large-scale tissue engineering. This is due to their excellent programmability—features can be incorporated for innate biocompatibility, biodegradability, synthetic feasibility, biological functionality and responsiveness to external stimuli. SAPs can be used independently or combined with other (macro)molecules to recapitulate surprisingly complex biological functions in a simple framework. It is easy to accomplish localized delivery, since they can be injected and can deliver targeted and sustained effects. In this review, we discuss the categories of SAPs, applications for gene and drug delivery, and their inherent design challenges. We highlight selected applications from the literature and make suggestions to advance the field with SAPs as a simple, yet smart delivery platform for emerging BioMedTech applications.

3D Hydrogel Cultures for High-Throughput Drug Discovery

Our increased understanding of how a cell's microenvironment influences its behavior has fueled an interest in three-dimensional (3D) cell cultures for drug discovery. Particularly, scaffold-based 3D cultures are expected to recapitulate in vivo tissue stiffness and extracellular matrix composition more accurately than standard two-dimensional (2D) monolayer cultures. Here we present a 3D hydrogel cell culture setup suitable for automated screening with standard high-throughput screening (HTS) liquid handling equipment commonly found in a drug discovery laboratory. Further, we describe the steps required to validate the assay system for compound screening.

Advancements in Hydrogel Application for Ischemic Stroke Therapy

Ischemic stroke is a major cause of death and disability worldwide. There is almost no effective treatment for this disease. Therefore, developing effective treatment for ischemic stroke is urgently needed. Efficient delivery of therapeutic drugs to ischemic sites remained a great challenge for improved treatment of strokes. In recent years, hydrogel-based strategies have been widely investigated for new and improved therapies. They have the advantage of delivering therapeutics in a controlled manner to the poststroke sites, aiming to enhance the intrinsic repair and regeneration. In this review, we discuss the pathophysiology of stroke and the development of injectable hydrogels in the application of both stroke treatment and neural tissue engineering. We also discuss the prospect and the challenges of hydrogels in the treatment of ischemic strokes.

Self-Healing Injectable Hydrogels for Tissue Regeneration

Biomaterials with the ability to self-heal and recover their structural integrity offer many advantages for applications in biomedicine. The past decade has witnessed the rapid emergence of a new class of self-healing biomaterials commonly termed injectable, or printable in the context of 3D printing. These self-healing injectable biomaterials, mostly hydrogels and other soft condensed matter based on reversible chemistry, are able to temporarily fluidize under shear stress and subsequently recover their original mechanical properties. Self-healing injectable hydrogels offer distinct advantages compared to traditional biomaterials. Most notably, they can be administered in a locally targeted and minimally invasive manner through a narrow syringe without the need for invasive surgery. Their moldability allows for a patient-specific intervention and shows great prospects for personalized medicine. Injected hydrogels can facilitate tissue regeneration in multiple ways owing to their viscoelastic and diffusive nature, ranging from simple mechanical support, spatiotemporally controlled delivery of cells or therapeutics, to local recruitment and modulation of host cells to promote tissue regeneration. Consequently, self-healing injectable hydrogels have been at the forefront of many cutting-edge tissue regeneration strategies. This study provides a critical review of the current state of self-healing injectable hydrogels for tissue regeneration. As key challenges toward further maturation of this exciting research field, we identify (i) the trade-off between the self-healing and injectability of hydrogels vs their physical stability, (ii) the lack of consensus on rheological characterization and quantitative benchmarks for self-healing injectable hydrogels, particularly regarding the capillary flow in syringes, and (iii) practical limitations regarding translation toward therapeutically effective formulations for regeneration of specific tissues. Hence, here we (i) review chemical and physical design strategies for self-healing injectable hydrogels, (ii) provide a practical guide for their rheological analysis, and (iii) showcase their applicability for regeneration of various tissues and 3D printing of complex tissues and organoids.

Hydrogels in Spinal Cord Injury Repair: A Review

Traffic accidents and falling objects are responsible for most spinal cord injuries (SCIs). SCI is characterized by high disability and tends to occur among the young, seriously affecting patients' lives and quality of life. The key aims of repairing SCI include preventing secondary nerve injury, inhibiting glial scarring and inflammatory response, and promoting nerve regeneration. Hydrogels have good biocompatibility and degradability, low immunogenicity, and easy-to-adjust mechanical properties. While providing structural scaffolds for tissues, hydrogels can also be used as slow-release carriers in neural tissue engineering to promote cell proliferation, migration, and differentiation, as well as accelerate the repair of damaged tissue. This review discusses the characteristics of hydrogels and their advantages as delivery vehicles, as well as expounds on the progress made in hydrogel therapy (alone or combined with cells and molecules) to repair SCI. In addition, we discuss the prospects of hydrogels in clinical research and provide new ideas for the treatment of SCI.

Star-Shaped Peptide-Polymer Hybrids as Fast pH-Responsive Supramolecular Hydrogels

Significant challenges have gone into the design of smart hydrogels, with numerous potential applications in the industrial, cosmetic, and biomedical fields. Herein, we report the synthesis of novel 4-arm self-assembling peptide-polyethylene glycol (PEG) hybrid star-shaped polymers and their comprehensive hydrogel properties. β-sheet-forming oligopeptides with alternating hydrophobic Leu/ionizable Glu repeats and Cys residues were successfully conjugated to 4-arm PEG via a thiol-maleimide click reaction. The hybrid star-shaped polymers demonstrated good cytocompatibility and reversible β-sheet (lightly acidic pH)-to-random coil (neutral and basic pH) transition in dilute aqueous solutions. At increasing polymer concentrations up to 0.5 wt %, the star-shaped polymers formed transparent hydrogels with shear-thinning and self-healing behaviors via β-sheet self-assembly, as well as a conformation-dependent gel-sol transition. Interestingly, the star-shaped polymers responded rapidly to pH changes, causing gelation to occur rapidly within a few seconds from the change in pH. Hydrogel characteristics could be modulated by manipulating the length and net charge of the peptide blocks. Furthermore, these star-shaped polymers served as satisfactory network scaffolds that could respond to dynamic environmental changes in the pH-oscillation system, owing to their excellent gelation capability and pH sensitivity. As such, they are highly favorable for diverse applications, such as pH-responsive controlled release.

Poloxamer 407/188 Binary Thermosensitive Gel as a Moxidectin Delivery System: In Vitro Release and In Vivo Evaluation

Moxidectin (MXD) is an antiparasitic drug used extensively in veterinary clinics. In this study, to develop a new formulation of MXD, a thermosensitive gel of MXD (MXD-TG) was prepared based on poloxamer 407/188. Furthermore, the gelation temperature, the stability, in vitro release kinetics and in vivo pharmacokinetics of MXD-TG were evaluated. The results showed that the gelation temperature was approximately 27 °C. MXD-TG was physically stable and can be released continuously for more than 96 h in vitro. The Korsmeyer−Peppas model provided the best fit to the release kinetics, and the release mechanism followed a diffusive erosion style. MXD-TG was released persistently for over 70 days in sheep. Part of pharmacokinetic parameters had a difference in female and male sheep (p < 0.05). It was concluded that MXD-TG had a good stability, and its release followed the characteristics of a diffusive erosion style in vitro and a sustained release pattern in vivo.

Self-Assembling Peptide Hydrogels as Functional Tools to Tackle Intervertebral Disc Degeneration

Low back pain (LBP), caused by intervertebral disc (IVD) degeneration, is a major contributor to global disability. In its healthy state, the IVD is a tough and well-hydrated tissue, able to act as a shock absorber along the spine. During degeneration, the IVD is hit by a cell-driven cascade of events, which progressively lead to extracellular matrix (ECM) degradation, chronic inflammation, and pain. Current treatments are divided into palliative care (early stage degeneration) and surgical interventions (late-stage degeneration), which are invasive and poorly efficient in the long term. To overcome these limitations, alternative tissue engineering and regenerative medicine strategies, in which soft biomaterials are used as injectable carriers of cells and/or biomolecules to be delivered to the injury site and restore tissue function, are currently being explored. Self-assembling peptide hydrogels (SAPHs) represent a promising class of de novo synthetic biomaterials able to merge the strengths of both natural and synthetic hydrogels for biomedical applications. Inherent features, such as shear-thinning behaviour, high biocompatibility, ECM biomimicry, and tuneable physiochemical properties make these hydrogels appropriate and functional tools to tackle IVD degeneration. This review will describe the pathogenesis of IVD degeneration, list biomaterials requirements to attempt IVD repair, and focus on current peptide hydrogel materials exploited for this purpose.

Biomaterials via peptide assembly: Design, characterization, and application in tissue engineering

A core challenge in biomaterials, with both fundamental significance and technological relevance, concerns the rational design of bioactive microenvironments. Designed properly, peptides can undergo supramolecular assembly into dynamic, physical hydrogels that mimic the mechanical, topological, and biochemical features of native tissue microenvironments. The relatively facile, inexpensive, and automatable preparation of peptides, coupled with low batch-to-batch variability, motivates the expanded use of assembling peptide hydrogels for biomedical applications. Integral to realizing dynamic peptide assemblies as functional biomaterials for tissue engineering is an understanding of the molecular and macroscopic features that govern assembly, morphology, and biological interactions. In this review, we first discuss the design of assembling peptides, including primary structure (sequence), secondary structure (e.g., α-helix and β-sheets), and molecular interactions that facilitate assembly into multiscale materials with desired properties. Next, we describe characterization tools for elucidating molecular structure and interactions, morphology, bulk properties, and biological functionality. Understanding of these characterization methods enables researchers to access a variety of approaches in this ever-expanding field. Finally, we discuss the biological properties and applications of peptide-based biomaterials for engineering several important tissues. By connecting molecular features and mechanisms of assembling peptides to the material and biological properties, we aim to guide the design and characterization of peptide-based biomaterials for tissue engineering and regenerative medicine. STATEMENT OF SIGNIFICANCE: Engineering peptide-based biomaterials that mimic the topological and mechanical properties of natural extracellular matrices provide excellent opportunities to direct cell behavior for regenerative medicine and tissue engineering. Here we review the molecular-scale features of assembling peptides that result in biomaterials that exhibit a variety of relevant extracellular matrix-mimetic properties and promote beneficial cell-biomaterial interactions. Aiming to inspire and guide researchers approaching this challenge from both the peptide biomaterial design and tissue engineering perspectives, we also present characterization tools for understanding the connection between peptide structure and properties and highlight the use of peptide-based biomaterials in neural, orthopedic, cardiac, muscular, and immune engineering applications.

Nerve Growth Factor-Laden Anisotropic Silk Nanofiber Hydrogels to Regulate Neuronal/Astroglial Differentiation for Scarless Spinal Cord Repair

Scarless spinal cord regeneration remains a challenge due to the complicated microenvironment at lesion sites. In this study, the nerve growth factor (NGF) was immobilized in silk protein nanofiber hydrogels with hierarchical anisotropic microstructures to fabricate bioactive systems that provide multiple physical and biological cues to address spinal cord injury (SCI). The NGF maintained bioactivity inside the hydrogels and regulated the neuronal/astroglial differentiation of neural stem cells. The aligned microstructures facilitated the migration and orientation of cells, which further stimulated angiogenesis and neuron extensions both in vitro and in vivo. In a severe rat long-span hemisection SCI model, these hydrogel matrices reduced scar formation and achieved the scarless repair of the spinal cord and effective recovery of motor functions. Histological analysis confirmed the directional regenerated neuronal tissues, with a similar morphology to that of the normal spinal cord. The in vitro and in vivo results showed promising utility for these NGF-laden silk hydrogels for spinal cord regeneration while also demonstrating the feasibility of cell-free bioactive matrices with multiple cues to regulate endogenous cell responses.

Self-assembled di- and tripeptide gels for the passive entrapment and pH-responsive, sustained release of an antidiabetic drug, glimepiride

Diabetes is a global epidemic that poses a severe challenge to public health. The characteristic features of this disease are hyperglycemia and deterioration of the function of pancreatic β-cells, which...

Designed protein- and peptide-based hydrogels for biomedical sciences

Proteins are fundamentally the most important macromolecules for biochemical, mechanical, and structural functions in living organisms. Therefore, they provide us with diverse structural building blocks for constructing various types of biomaterials, including an important class of such materials, hydrogels. Since natural peptides and proteins are biocompatible and biodegradable, they have features advantageous for their use as the building blocks of hydrogels for biomedical applications. They display constitutional and mechanical similarities with the native extracellular matrix (ECM), and can be easily bio-functionalized via genetic and chemical engineering with features such as bio-recognition, specific stimulus-reactivity, and controlled degradation. This review aims to give an overview of hydrogels made up of recombinant proteins or synthetic peptides as the structural elements building the polymer network. A wide variety of hydrogels composed of protein or peptide building blocks with different origins and compositions - including β-hairpin peptides, α-helical coiled coil peptides, elastin-like peptides, silk fibroin, and resilin - have been designed to date. In this review, the structures and characteristics of these natural proteins and peptides, with each of their gelation mechanisms, and the physical, chemical, and mechanical properties as well as biocompatibility of the resulting hydrogels are described. In addition, this review discusses the potential of using protein- or peptide-based hydrogels in the field of biomedical sciences, especially tissue engineering.

The design criteria and therapeutic strategy of functional scaffolds for spinal cord injury repair

Spinal cord injury (SCI) remains a therapeutic challenge in clinic. Current drug and cell therapeutics have obtained significant efficacy but are still in the early stages for complete neural and functional recovery. In the past few decades, functional scaffolds (FSs) have been rapidly developed to bridge the lesion and provide a framework for tissue regeneration in SCI repair. Moreover, a FS can act as an adjuvant for locally delivering drugs in the lesion with a designed drug release profile, and supplying a biomimetic environment for implanted cells. In this review, the design criteria of FSs for SCI treatment are summarized according to their biocompatibility, mechanical properties, morphology, architecture, and biodegradability. Subsequently, FSs designed for SCI repair in the scope of drug delivery, cell implantation and combination therapy are introduced, respectively. And how a FS promotes their therapeutic efficacy is analyzed. Finally, the challenges, perspectives, and potential of FSs for SCI treatment are discussed. Hopefully, this review may inspire the future development of potent FSs to facilitate SCI repair in clinic.

Hydrogel-based local drug delivery strategies for spinal cord repair

Spinal cord injury results in significant loss of motor, sensory, and autonomic functions. Although a wide range of therapeutic agents have been shown to attenuate secondary injury or promote regeneration/repair in animal models of spinal cord injury, clinical translation of these strategies has been limited, in part due to difficulty in safely and effectively achieving therapeutic concentrations in the injured spinal cord tissue. Hydrogel-based drug delivery systems offer unique opportunities to locally deliver drugs to the injured spinal cord with sufficient dose and duration, while avoiding deleterious side effects associated with systemic drug administration. Such local drug delivery systems can be readily fabricated from biocompatible and biodegradable materials. In this review, hydrogel-based strategies for local drug delivery to the injured spinal cord are extensively reviewed, and recommendations are made for implementation.

Recent trends in the development of peptide and protein-based hydrogel therapeutics for the healing of CNS injury

Traumatic brain injury (TBI) and spinal cord injury (SCI) cause millions of deaths and permanent or prolonged physical disabilities around the globe every year. It generally happens due to various incidents, such as accidents during sports, war, physical assault, and strokes which result in severe damage to brain and spinal cord. If this remains untreated, traumatic CNS injuries may lead to early development of several neurodegenerative diseases like Alzheimer's, Parkinson, multiple sclerosis, and other mental illnesses. The initial physical reaction, which is also termed as the primary phase, includes swelling, followed by inflammation as a result of internal haemorrhage causing damage to indigenous tissue, i.e., axonal shear injury, rupture of blood vessels, and partial impaired supply of oxygen and essential nutrients in the neurons, thereby initiating a cascade of events causing secondary injuries such as hypoxia, hypotension, cognitive impairment, seizures, imbalanced calcium homeostasis and glutamate-induced excitotoxicity resulting in concomitant neuronal cell death and cumulative permanent tissue damage. In the modern era of advanced biomedical technology, we are still living with scarcity of the clinically applicable comparative non-invasive therapeutic strategies for regeneration or functional recovery of neurons or neural networks after a massive CNS injury. One of the key reasons for this scarcity is the limited regenerative ability of neurons in CNS. Growth-impermissive glial scar and the lack of a synthetic biocompatible platform for proper neural tissue engineering and controlled supply of drugs further retard the healing process. Injectable or implantable hydrogel materials, consisting majorly of water in its porous three-dimensional (3D) structure, can serve as an excellent drug delivery platform as well as a transplanted cell-supporting scaffold medium. Among the various neuro-compatible bioinspired materials, we are limiting our discussion to the recent advancement of engineered biomaterials comprising mainly of peptides and proteins due to their growing demand, low immunogenicity and versatility in the fabrication of neuro regenerative medicine. In this article, we try to explore all the recent scientific avenues that are developing gradually to make peptide and peptide-conjugated biomaterial hydrogels as a therapeutic and supporting scaffold for treating CNS injuries.

Synthetic peptide hydrogels as 3D scaffolds for tissue engineering

The regeneration of tissues and organs poses an immense challenge due to the extreme complexity in the research work involved. Despite the tissue engineering approach being considered as a promising strategy for more than two decades, a key issue impeding its progress is the lack of ideal scaffold materials. Nature-inspired synthetic peptide hydrogels are inherently biocompatible, and its high resemblance to extracellular matrix makes peptide hydrogels suitable 3D scaffold materials. This review covers the important aspects of peptide hydrogels as 3D scaffolds, including mechanical properties, biodegradability and bioactivity, and the current approaches in creating matrices with optimized features. Many of these scaffolds contain peptide sequences that are widely reported for tissue repair and regeneration and these peptide sequences will also be discussed. Furthermore, 3D biofabrication strategies of synthetic peptide hydrogels and the recent advances of peptide hydrogels in tissue engineering will also be described to reflect the current trend in the field. In the final section, we will present the future outlook in the design and development of peptide-based hydrogels for translational tissue engineering applications.

Self-assembling Peptides in Current Nanomedicine: Versatile Nanomaterials for Drug Delivery

BACKGROUND

The development of modern nanomedicine greatly depends on the involvement of novel materials as drug delivery system. In order to maximize the therapeutic effects of drugs and minimize their side effects, a number of natural or synthetic materials have been widely investigated for drug delivery. Among these materials, biomimetic self-assembling peptides (SAPs) have received more attention in recent years. Considering the rapidly growing number of SAPs designed for drug delivery, a summary of how SAPs-based drug delivery systems were designed, would be beneficial.

METHOD

We outlined research works on different SAPs that have been investigated as carriers for different drugs, focusing on the design of SAPs nanomaterials and how they were used for drug delivery in different strategies.

RESULTS

Based on the principle rules of chemical complementarity and structural compatibility, SAPs such as ionic self-complementary peptide, peptide amphiphile and surfactant-like peptide could be designed. Determined by the features of peptide materials and the drugs to be delivered, different strategies such as hydrogel embedding, hydrophobic interaction, electrostatic interaction, covalent conjugation or the combination of them could be employed to fabricate SAPs-drug complex, which could achieve slow release, targeted or environment-responsive delivery of drugs. Furthermore, some SAPs could also be combined with other types of materials for drug delivery, or even act as drug by themselves.

CONCLUSION

Various types of SAPs have been designed and used for drug delivery following various strategies, suggesting that SAPs as a category of versatile nanomaterials have promising potential in the field of nanomedicine.

Coassembly Generates Peptide Hydrogel with Wound Dressing Material Properties

Multicomponent self-assembly of peptides is a powerful strategy to fabricate novel functional materials with synergetic properties that can be used for several nanobiotechnological applications. In the present study, we used a coassembly strategy to generate an injectable ultrashort bioactive peptide hydrogel formed by mixing a dipeptide hydrogelator with a macrophage attracting short chemotactic peptide ligand. Coassembly does not impede hydrogelation as shown by cryo-transmission electron microscopy (cryo-TEM), scanning electron microscopy, and rheology. Biocompatibility was shown by cytotoxicity assays and confocal microscopy. The hydrogels release the entrapped skin antibiotic ciprofloxacin, among others, in a slow and continuous manner. Such bioinspired advanced functional materials can find applications as wound dressing materials to treat chronic wound conditions like diabetic foot ulcer.

Peptide gels for controlled release of proteins

Treatment strategies in clinics have been shifting from small molecules to protein drugs due to the promising results of a highly specific mechanism of action and reduced toxicity. Despite their prominent roles in disease treatment, delivery of the protein therapeutics is challenging due to chemical instability, immunogenicity and biological barriers. Peptide hydrogels with spatiotemporally tunable properties have shown an outstanding potential to deliver complex protein therapeutics, maintain drug efficacy and stability over time, mimicking the extracellular matrix, and responding to external stimuli. In this review, we present recent advances in peptide hydrogel design strategies, protein release kinetics and mechanisms for protein drug delivery in cellular engineering, tissue engineering, immunotherapy and disease treatments.

Stimuli-Responsive Delivery of Growth Factors for Tissue Engineering

Growth factors (GFs) play a crucial role in directing stem cell behavior and transmitting information between different cell populations for tissue regeneration. However, their utility as therapeutics is limited by their short half-life within the physiological microenvironment and significant side effects caused by off-target effects or improper dosage. "Smart" materials that can not only sustain therapeutic delivery over a treatment period but also facilitate on-demand release upon activation are attracting significant interest in the field of GF delivery for tissue engineering. Three properties are essential in engineering these "smart" materials: 1) the cargo vehicle protects the encapsulated therapeutic; 2) release is targeted to the site of injury; 3) cargo release can be modulated by disease-specific stimuli. The aim of this review is to summarize the current research on stimuli-responsive materials as intelligent vehicles for controlled GF delivery; Five main subfields of tissue engineering are discussed: skin, bone and cartilage, muscle, blood vessel, and nerve. Challenges in achieving such "smart" materials and perspectives on future applications of stimuli-responsive GF delivery for tissue regeneration are also discussed.

A review on recent advances in polymer and peptide hydrogels

In this review, we focus on the very recent developments on the use of the stimuli responsive properties of polymer hydrogels for targeted drug delivery, tissue engineering, and biosensing utilizing their different optoelectronic properties. Besides, the stimuli-responsive hydrogels, the conducting polymer hydrogels are discussed, with specific attention to the energy generation and storage behavior of the xerogel derived from the hydrogel. The electronic and ionic conducting gels have been discussed that have applications in various electronic devices, e.g., organic field effect transistors, soft robotics, ionic skins, and sensors. The properties of polymer hybrid gels containing carbon nanomaterials have been exemplified here giving attention to applications in supercapacitors, dye sensitized solar cells, photocurrent switching, etc. Recent trends in the properties and applications of some natural polymer gels to produce thermal and acoustic insulating materials, drug delivery vehicles, self-healing material, tissue engineering, etc., are discussed. Besides the polymer gels, peptide gels of different dipeptides, tripeptides, oligopeptides, polypeptides, cyclic peptides, etc., are discussed, giving attention mainly to biosensing, bioimaging, and drug delivery applications. The properties of peptide-based hybrid hydrogels with polymers, nanoparticles, nucleotides, fullerene, etc., are discussed, giving specific attention to drug delivery, cell culture, bio-sensing, and bioimaging properties. Thus, the present review delineates, in short, the preparation, properties, and applications of different polymer and peptide hydrogels prepared in the past few years.

Decoupling the effects of hydrophilic and hydrophobic moieties at the neuron-nanofibre interface

Peptide-based nanofibres are a versatile class of tunable materials with applications in optoelectronics, sensing and tissue engineering. However, the understanding of the nanofibre surface at the molecular level is limited. Here, a series of homologous dilysine–diphenylalnine tetrapeptides were synthesised and shown to self-assemble into water-soluble nanofibres. Despite the peptide nanofibres displaying similar morphologies, as evaluated through atomic force microscopy and neutron scattering, significant differences were observed in their ability to support sensitive primary neurons. Contact angle and labelling experiments revealed that differential presentation of lysine moieties at the fibre surface did not affect neuronal viability; however the mobility of phenylalanine residues at the nanofibre surface, elucidated through solid- and gel-state NMR studies and confirmed through tethered bilayer lipid membrane experiments, was found to be the determining factor in governing the suitability of a given peptide as a scaffold for primary neurons. This work offers new insights into characterising and controlling the nanofibre surface at the molecular level.

The mobility of hydrophobic moieties at a peptide nanofibre surface determines its suitability as a scaffold for sensitive primary cells.

Design of a Peptide-Based Electronegative Hydrogel for the Direct Encapsulation, 3D Culturing, in Vivo Syringe-Based Delivery, and Long-Term Tissue Engraftment of Cells

Soft materials that facilitate the three-dimensional (3D) encapsulation, proliferation, and facile local delivery of cells to targeted tissues will aid cell-based therapies, especially those that depend on the local engraftment of implanted cells. Herein, we develop a negatively charged fibrillar hydrogel based on the de novo-designed self-assembling peptide AcVES3-RGDV. Cells are easily encapsulated during the triggered self-assembly of the peptide leading to gel formation. Self-assembly is induced by adjusting the ionic strength and/or temperature of the solution, while avoiding large changes in pH. The AcVES3-RGDV gel allows cell-material attachment enabling both two-dimensional and 3D cell culture of adherent cells. Gel-cell constructs display shear-thin/recovery rheological properties enabling their syringe-based delivery. In vivo cellular fluorescence as well as tissue resection experiments show that the gel supports the long-term engraftment of cells delivered subcutaneously into mice.

Implementation of a High-Throughput Pilot Screen in Peptide Hydrogel-Based Three-Dimensional Cell Cultures

Cell-based high-throughput drug screening (HTS) is a common starting point for the drug discovery and development process. Currently, there is a push to combine complex cell culture systems with HTS to provide more clinically applicable results. However, there are mechanistic requirements inherent to HTS as well as material limitations that make this integration challenging. Here, we used the peptide-based shear-thinning hydrogel MAX8 tagged with the RGDS sequence to create a synthetic extracellular scaffold to culture cells in three dimensions and showed a preliminary implementation of the scaffold within an automated HTS setup using a pilot drug screen targeting medulloblastoma, a pediatric brain cancer. A total of 2202 compounds were screened in the 384-well format against cells encapsulated in the hydrogel as well as cells growing on traditional two-dimensional (2D) plastic. Eighty-two compounds passed the first round of screening at a single point of concentration. Sixteen-point dose-response was done on those 82 compounds, of which 17 compounds were validated. Three-dimensional (3D) cell-based HTS could be a powerful screening tool that allows researchers to finely tune the cell microenvironment, getting more clinically applicable data as a result. Here, we have shown the successful integration of a peptide-based hydrogel into the high-throughput format.

Amyloid-Based Injectable Hydrogel Derived from Hydrolyzed Hen Egg White Lysozyme

Injectable hydrogels based on synthetic peptides have shown great promise in many biomedical applications. Yet, the high cost generally associated with synthetic peptides hinders the practical use of such peptide-based injectable hydrogel. To overcome this drawback, here, we propose to use the peptides from hydrolyzed low-cost natural protein as an economical and convenient peptide source to prepare an injectable hydrogel. We demonstrate the effectiveness of this alternative strategy using hen egg white lysozyme (HEWL) as an example. We used the peptide fragments from hydrolyzed HEWL as the gelator, and the magnesium ion as the performance enhancer to prepare the injectable hydrogel. We showed that the hydrogel is an amyloid gel as it was formed by a dense network of amyloid fibrils. We also showed that the hydrogel possesses a thixotropic property and displays a low cytotoxicity. The hydrolysis extent of HEWL was found to be a critical factor that influences the performance of the hydrogel. A fluorescence assay based on 8-anilinonaphthalene-1-sulfonic acid was proposed as a mean to precisely and conveniently control the hydrolysis extent of HEWL to enable the best injectability performance. At last, using doxorubicin as a model compound, we explored the potential of this amyloid-based hydrogel as an injectable drug carrier.

Self-assembling injectable peptide hydrogels for emerging treatment of ischemic stroke

Ischaemic stroke remains one of the leading causes of death and disability worldwide, without any long-term effective treatments targeted at regeneration. This has led to developments of novel, biomaterial-based strategies using self-assembling peptide hydrogels.

Peptide Nanofiber Substrates for Long-Term Culturing of Primary Neurons

The culturing of primary neurons represents a central pillar of neuroscience research. Primary neurons are derived directly from brain tissue and recapitulate key aspects of neuronal development in an in vitro setting. Unlike neural stem cells, primary neurons do not divide; thus, initial attachment of cells to a suitable substrate is critical. Commonly used polylysine substrates can suffer from batch variability owing to their polymeric nature. Herein, we report the use of chemically well-defined, self-assembling tetrapeptides as substrates for primary neuronal culture. These water-soluble peptides assemble into fibers which facilitate adhesion and development of primary neurons, their long-term survival (>40 days), synaptic maturation, and electrical activity. Furthermore, these substrates are permissive toward neuronal transfection and transduction which, coupled with their uniformity and reproducible nature, make them suitable for a wide variety of applications in neuroscience.

Emerging properties of hydrogels in tissue engineering

Hydrogels are three-dimensional polymeric networks filled with water and mimic tissue environments. Therefore, they are considered optimal to deliver cells and engineer damaged tissues. The hydrogel networks have been significantly modified to endow biochemical functionality with adhesive ligands, growth factors, or degradable sites that are helpful to drive proper cell functions. Recently, some of the biophysical properties of hydrogels have emerged as key players in dictating cell fate. Beyond static stiffness, time-dependent stress/strain changes in the interaction with cells and the cell-mediated degradation and matrix synthesis have been demonstrated to shape cell status and tissue repair process. We highlight here the emerging biophysical properties of hydrogels that can motivate tissue engineers to design and develop hydrogels optimally for tissue regeneration.

Effects of Transplanted Heparin-Poloxamer Hydrogel Combining Dental Pulp Stem Cells and bFGF on Spinal Cord Injury Repair

Spinal cord injury (SCI) is one of serious traumatic diseases of the central nervous system and has no effective treatment because of its complicated pathophysiology. Tissue engineering strategy which contains scaffolds, cells, and growth factors can provide a promising treatment for SCI. Hydrogel that has 3D network structure and biomimetic microenvironment can support cellular growth and embed biological macromolecules for sustaining release. Dental pulp stem cells (DPSCs), derived from cranial neural crest, possess mesenchymal stem cell (MSC) characteristics and have an ability to provide neuroprotective and neurotrophic properties for SCI treatment. Basic fibroblast growth factor (bFGF) is able to promote cell survival and proliferation and also has beneficial effect on neural regeneration and functional recovery after SCI. Herein, a thermosensitive heparin-poloxamer (HP) hydrogel containing DPSCs and bFGF was prepared, and the effects of HP-bFGF-DPSCs on neuron restoration after SCI were evaluated by functional recovery tests, western blotting, magnetic resonance imaging (MRI), histology evaluation, and immunohistochemistry. The results suggested that transplanted HP hydrogel containing DPSCs and bFGF had a significant impact on spinal cord repair and regeneration and may provide a promising strategy for neuron repair, functional recovery, and tissue regeneration after SCI.

Nanoengineered Colloidal Inks for 3D Bioprinting

Nanoengineered hydrogels offer the potential to design shear-thinning bioinks for three-dimensional (3D) bioprinting. Here, we have synthesized colloidal bioinks composed of disk-shaped two-dimensional (2D) nanosilicates (Laponite) and poly(ethylene glycol) (PEG). The addition of Laponite reinforces the PEG network and increases viscosity, storage modulus, and network stability. PEG-Laponite hydrogels display shear-thinning and self-recovery characteristics due to rapid internal phase rearrangement. As a result, a range of complex patterns can be printed using PEG-Laponite bioinks. The 3D bioprinted structure has similar mechanical properties compared to the as-casted structure. In addition, encapsulated cells within the PEG-Laponite bioink show high viability after bioprinting. Overall, this study introduces a new class of PEG-Laponite colloidal inks for bioprinting and cell delivery.

Responsive Hybrid (Poly)peptide-Polymer Conjugates

(Poly)peptide-polymer conjugates continue to garner significant interest in the production of functional materials given their composition of natural and synthetic building blocks that confer select and synergistic properties. Owing to opportunities to design predefined architectures and structures with different morphologies, these hybrid conjugates enable new approaches for producing micro- or nanomaterials. Their modular design enables the incorporation of multiple responsive properties into a single conjugate. This review presents recent advances in (poly)peptide-polymer conjugates for drug-delivery applications, with a specific focus on the utility of the (poly)peptide component in the assembly of particles and nanogels, as well as the role of the peptide in triggered drug release.

Toward the development of biomimetic injectable and macroporous biohydrogels for regenerative medicine

Repairing or replacing damaged human tissues has been the ambitious goal of regenerative medicine for over 25years. One promising approach is the use of hydrated three-dimensional scaffolds, known as hydrogels, which have had good results repairing tissues in pre-clinical trials. Benefiting from breakthrough advances in the field of biology, and more particularly regarding cell/matrix interactions, these hydrogels are now designed to recapitulate some of the fundamental cues of native environments to drive the local tissue regeneration. We highlight the key parameters that are required for the development of smart and biomimetic hydrogels. We also review the wide variety of polymers, crosslinking methods, and manufacturing processes that have been developed over the years. Of particular interest is the emergence of supramolecular chemistries, allowing for the development of highly functional and reversible biohydrogels. Moreover, advances in computer assisted design and three-dimensional printing have revolutionized the production of macroporous hydrogels and allowed for more complex designs than ever before with the opportunity to develop fully reconstituted organs. Today, the field of biohydrogels for regenerative medicine is a prolific area of research with applications for most bodily tissues. On top of these applications, injectable hydrogels and macroporous hydrogels (foams) were found to be the most successful. While commonly associated with cells or biologics as drug delivery systems to increase therapeutic outcomes, they are steadily being used in the emerging fields of organs-on-chip and hydrogel-assisted cell therapy. To highlight these advances, we review some of the recent developments that have been achieved for the regeneration of tissues, focusing on the articular cartilage, bone, cardiac, and neural tissues. These biohydrogels are associated with improved cartilage and bone defects regeneration, reduced left ventricular dilation upon myocardial infarction and display promising results repairing neural lesions. Combining the benefits from each of these areas reviewed above, we envision that an injectable biohydrogel foam loaded with either stem cells or their secretome is the most promising hydrogel solution to trigger tissue regeneration. A paradigm shift is occurring where the combined efforts of fundamental and applied sciences head toward the development of hydrogels restoring tissue functions, serving as drug screening platforms or recreating complex organs.

Beta-hairpin hydrogels as scaffolds for high-throughput drug discovery in three-dimensional cell culture

Automated cell-based high-throughput screening (HTS) is a powerful tool in drug discovery, and it is increasingly being recognized that three-dimensional (3D) models, which more closely mimic in vivo-like conditions, are desirable screening platforms. One limitation hampering the development of 3D HTS is the lack of suitable 3D culture scaffolds that can readily be incorporated into existing HTS infrastructure. We now show that β-hairpin peptide hydrogels can serve as a 3D cell culture platform that is compatible with HTS. MAX8 β-hairpin peptides can physically assemble into a hydrogel with defined porosity, permeability and mechanical stability with encapsulated cells. Most importantly, the hydrogels can then be injected under shear-flow and immediately reheal into a hydrogel with the same properties exhibited prior to injection. The post-injection hydrogels are cell culture compatible at physiological conditions. Using standard HTS equipment and medulloblastoma pediatric brain tumor cells as a model system, we show that automatic distribution of cell-peptide mixtures into 384-well assay plates results in evenly dispensed, viable MAX8-cell constructs suitable for commercially available cell viability assays. Since MAX8 peptides can be functionalized to mimic the microenvironment of cells from a variety of origins, MAX8 peptide gels should have broad applicability for 3D HTS drug discovery.

A Thermosensitive Heparin-Poloxamer Hydrogel Bridges aFGF to Treat Spinal Cord Injury

Acidic fibroblast growth factor (aFGF) exerts a protective effect on spinal cord injury (SCI) but is limited by the lack of physicochemical stability and the ability to cross the blood spinal cord barrier (BSCB). As promising biomaterials, hydrogels contain substantial amounts of water and a three-dimensional porous structure and are commonly used to load and deliver growth factors. Heparin can not only enhance growth factor loading onto hydrogels but also can stabilize the structure and control the release behavior. Herein, a novel aFGF-loaded thermosensitive heparin-poloxamer (aFGF-HP) hydrogel was developed and applied to provide protection and regeneration after SCI. To assess the effects of the aFGF-HP hydrogel, BSCB restoration, neuron and axonal rehabilitation, glial scar inhibition, inflammatory response suppression, and motor recovery were studied both in vivo and in vitro. The aFGF-HP hydrogels exhibited sustained release of aFGF and protected the bioactivity of aFGF in vitro. Compared to groups intravenously administered either drug-free HP hydrogel or aFGF alone, the aFGF-HP hydrogel group revealed prominent and attenuated disruption of the BSCB, reduced neuronal apoptosis, reactive astrogliosis, and increased neuron and axonal rehabilitation both in vivo and in vitro. This work provides an effective approach to enhance recovery after SCI and provide a successful strategy for SCI protection.

Biodegradable Neuro-Compatible Peptide Hydrogel Promotes Neurite Outgrowth, Shows Significant Neuroprotection, and Delivers Anti-Alzheimer Drug

A novel neuro-compatible peptide-based hydrogel has been designed and developed, which contains microtubule stabilizing and neuroprotective short peptide. This hydrogel shows strong three-dimensional cross-linked fibrillary networks, which can capture water molecules. Interestingly, this hydrogel serves as excellent biocompatible soft material for 2D and 3D (neurosphere) neuron cell culture and provides stability of key cytoskeleton filaments such as microtubule and actin. Remarkably, it was observed that this hydrogel slowly enzymatically degrades and releases neuroprotective peptide, which promotes neurite outgrowth of neuron cell as well as exhibits excellent neuroprotection against anti-NGF-induced toxicity in neuron cells. Further, it can encapsulate anti-Alzheimer and anticancer hydrophobic drug curcumin, releases slowly, and inhibits significantly the growth of a 3D spheroid of neuron cancer cells. Thus, this novel neuroprotective hydrogel can be used for both neuronal cell transplantation for repairing brain damage as well as a delivery vehicle for neuroprotective agents, anti-Alzheimer, and anticancer molecules.