Osteo-inductive effect of piezoelectric stimulation from the poly(l-lactic acid) scaffolds.

Piezoelectric biomaterials can generate piezoelectrical charges in response to mechanical activation. These generated charges can directly stimulate bone regeneration by triggering signaling pathway that is important for regulating osteogenesis of cells seeded on the materials. On the other hand, mechanical forces applied to the biomaterials play an important role in bone regeneration through the process called mechanotransduction. While mechanical force and electrical charges are both important contributing factors to bone tissue regeneration, they operate through different underlying mechanisms. The utilizations of piezoelectric biomaterials have been explored to serve as self-charged scaffolds which can promote stem cell differentiation and the formation of functional bone tissues. However, it is still not clear how mechanical activation and electrical charge act together on such a scaffold and which factors play more important role in the piezoelectric stimulation to induce osteogenesis. In our study, we found Poly(l-lactic acid) (PLLA)-based piezoelectric scaffolds with higher piezoelectric charges had a more pronounced osteoinductive effect than those with lower charges. This provided a new mechanistic insight that the observed osteoinductive effect of the piezoelectric PLLA scaffolds is likely due to the piezoelectric stimulation they provide, rather than mechanical stimulation alone. Our findings provide a crucial guide for the optimization of piezoelectric material design and usage.

Biodegradable piezoelectric skin-wound scaffold.

Electrical stimulation (ES) induces wound healing and skin regeneration. Combining ES with the tissue-engineering approach, which relies on biomaterials to construct a replacement tissue graft, could offer a self-stimulated scaffold to heal skin-wounds without using potentially toxic growth factors and exogenous cells. Unfortunately, current ES technologies are either ineffective (external stimulations) or unsafe (implanted electrical devices using toxic batteries). Hence, we propose a novel wound-healing strategy that integrates ES with tissue engineering techniques by utilizing a biodegradable self-charged piezoelectric PLLA (Poly (l-lactic acid)) nanofiber matrix. This unique, safe, and stable piezoelectric scaffold can be activated by an external ultrasound (US) to produce well-controlled surface-charges with different polarities, thus serving multiple functions to suppress bacterial growth (negative surface charge) and promote skin regeneration (positive surface charge) at the same time. We demonstrate that the scaffold activated by low intensity/low frequency US can facilitate the proliferation of fibroblast/epithelial cells, enhance expression of genes (collagen I, III, and fibronectin) typical for the wound healing process, and suppress the growth of S. aureus and P. aeruginosa bacteria in vitro simultaneously. This approach induces rapid skin regeneration in a critical-sized skin wound mouse model in vivo. The piezoelectric PLLA skin scaffold thus assumes the role of a multi-tasking, biodegradable, battery-free electrical stimulator which is important for skin-wound healing and bacterial infection prevention simultaneuosly.

Highly piezoelectric, biodegradable, and flexible amino acid nanofibers for medical applications.

Amino acid crystals are an attractive piezoelectric material as they have an ultrahigh piezoelectric coefficient and have an appealing safety profile for medical implant applications. Unfortunately, solvent-cast films made from glycine crystals are brittle, quickly dissolve in body fluid, and lack crystal orientation control, reducing the overall piezoelectric effect. Here, we present a material processing strategy to create biodegradable, flexible, and piezoelectric nanofibers of glycine crystals embedded inside polycaprolactone (PCL). The glycine-PCL nanofiber film exhibits stable piezoelectric performance with a high ultrasound output of 334 kPa [under 0.15 voltage root-mean-square (Vrms)], which outperforms the state-of-the-art biodegradable transducers. We use this material to fabricate a biodegradable ultrasound transducer for facilitating the delivery of chemotherapeutic drug to the brain. The device remarkably enhances the animal survival time (twofold) in mice-bearing orthotopic glioblastoma models. The piezoelectric glycine-PCL presented here could offer an excellent platform not only for glioblastoma therapy but also for developing medical implantation fields.

Exercise-induced piezoelectric stimulation for cartilage regeneration in rabbits.

More than 32.5 million American adults suffer from osteoarthritis, and current treatments including pain medicines and anti-inflammatory drugs only alleviate symptoms but do not cure the disease. Here, we have demonstrated that a biodegradable piezoelectric poly(L-lactic acid) (PLLA) nanofiber scaffold under applied force or joint load could act as a battery-less electrical stimulator to promote chondrogenesis and cartilage regeneration. The PLLA scaffold under applied force or joint load generated a controllable piezoelectric charge, which promoted extracellular protein adsorption, facilitated cell migration or recruitment, induced endogenous TGF-ß via calcium signaling pathway, and improved chondrogenesis and cartilage regeneration both in vitro and in vivo. Rabbits with critical-sized osteochondral defects receiving the piezoelectric scaffold and exercise treatment experienced hyaline-cartilage regeneration and completely healed cartilage with abundant chondrocytes and type II collagen after 1 to 2 months of exercise (2 to 3 months after surgery including 1 month of recovery before exercise), whereas rabbits treated with nonpiezoelectric scaffold and exercise treatment had unfilled defect and limited healing. The approach of combining biodegradable piezoelectric tissue scaffolds with controlled mechanical activation (via physical exercise) may therefore be useful for the treatment of osteoarthritis and is potentially applicable to regenerating other injured tissues.

Electrical Stimulation for Immune Modulation in Cancer Treatments.

Immunotherapy is becoming a very common treatment for cancer, using approaches like checkpoint inhibition, T cell transfer therapy, monoclonal antibodies and cancer vaccination. However, these approaches involve high doses of immune therapeutics with problematic side effects. A promising approach to reducing the dose of immunotherapeutic agents given to a cancer patient is to combine it with electrical stimulation, which can act in two ways; it can either modulate the immune system to produce the immune cytokines and agents in the patient's body or it can increase the cellular uptake of these immune agents via electroporation. Electrical stimulation in form of direct current has been shown to reduce tumor sizes in immune-competent mice while having no effect on tumor sizes in immune-deficient mice. Several studies have used nano-pulsed electrical stimulations to activate the immune system and drive it against tumor cells. This approach has been utilized for different types of cancers, like fibrosarcoma, hepatocellular carcinoma, human papillomavirus etc. Another common approach is to combine electrochemotherapy with immune modulation, either by inducing immunogenic cell death or injecting immunostimulants that increase the effectiveness of the treatments. Several therapies utilize electroporation to deliver immunostimulants (like genes encoded with cytokine producing sequences, cancer specific antigens or fragments of anti-tumor toxins) more effectively. Lastly, electrical stimulation of the vagus nerve can trigger production and activation of anti-tumor immune cells and immune reactions. Hence, the use of electrical stimulation to modulate the immune system in different ways can be a promising approach to treat cancer.

Biodegradable Nanofiber Bone-Tissue Scaffold as Remotely-Controlled and Self-Powering Electrical Stimulator.

Electrical stimulation (ES) has been shown to induce and enhance bone regeneration. By combining this treatment with tissue-engineering approaches (which rely on biomaterial scaffolds to construct artificial tissues), a replacement bone-graft with strong regenerative properties can be achieved while avoiding the use of potentially toxic levels of growth factors. Unfortunately, there is currently a lack of safe and effective methods to induce electrical cues directly on cells/tissues grown on the biomaterial scaffolds. Here, we present a novel bone regeneration method which hybridizes ES and tissue-engineering approaches by employing a biodegradable piezoelectric PLLA (Poly(L-lactic acid)) nanofiber scaffold which, together with externally-controlled ultrasound (US), can generate surface-charges to drive bone regeneration. We demonstrate that the approach of using the piezoelectric scaffold and US can enhance osteogenic differentiation of different stem cells in vitro, and induce bone growth in a critical-sized calvarial defect in vivo. The biodegradable piezoelectric scaffold with applied US could significantly impact the field of tissue engineering by offering a novel biodegradable, battery-free and remotely-controlled electrical stimulator.

Biodegradable nanofiber-based piezoelectric transducer.

Piezoelectric materials, a type of "smart" material that generates electricity while deforming and vice versa, have been used extensively for many important implantable medical devices such as sensors, transducers, and actuators. However, commonly utilized piezoelectric materials are either toxic or nondegradable. Thus, implanted devices employing these materials raise a significant concern in terms of safety issues and often require an invasive removal surgery, which can damage directly interfaced tissues/organs. Here, we present a strategy for materials processing, device assembly, and electronic integration to 1) create biodegradable and biocompatible piezoelectric PLLA [poly(l-lactic acid)] nanofibers with a highly controllable, efficient, and stable piezoelectric performance, and 2) demonstrate device applications of this nanomaterial, including a highly sensitive biodegradable pressure sensor for monitoring vital physiological pressures and a biodegradable ultrasonic transducer for blood-brain barrier opening that can be used to facilitate the delivery of drugs into the brain. These significant applications, which have not been achieved so far by conventional piezoelectric materials and bulk piezoelectric PLLA, demonstrate the PLLA nanofibers as a powerful material platform that offers a profound impact on various medical fields including drug delivery, tissue engineering, and implanted medical devices.

Piezoelectric Biomaterials for Sensors and Actuators.

Recent advances in materials, manufacturing, biotechnology, and microelectromechanical systems (MEMS) have fostered many exciting biosensors and bioactuators that are based on biocompatible piezoelectric materials. These biodevices can be safely integrated with biological systems for applications such as sensing biological forces, stimulating tissue growth and healing, as well as diagnosing medical problems. Herein, the principles, applications, future opportunities, and challenges of piezoelectric biomaterials for medical uses are reviewed thoroughly. Modern piezoelectric biosensors/bioactuators are developed with new materials and advanced methods in microfabrication/encapsulation to avoid the toxicity of conventional lead-based piezoelectric materials. Intriguingly, some piezoelectric materials are biodegradable in nature, which eliminates the need for invasive implant extraction. Together, these advancements in the field of piezoelectric materials and microsystems can spark a new age in the field of medicine.

Physical Stimulations for Bone and Cartilage Regeneration.

A wide range of techniques and methods are actively invented by clinicians and scientists who are dedicated to the field of musculoskeletal tissue regeneration. Biological, chemical, and physiological factors, which play key roles in musculoskeletal tissue development, have been extensively explored. However, physical stimulation is increasingly showing extreme importance in the processes of osteogenic and chondrogenic differentiation, proliferation and maturation through defined dose parameters including mode, frequency, magnitude, and duration of stimuli. Studies have shown manipulation of physical microenvironment is an indispensable strategy for the repair and regeneration of bone and cartilage, and biophysical cues could profoundly promote their regeneration. In this article, we review recent literature on utilization of physical stimulation, such as mechanical forces (cyclic strain, fluid shear stress, etc.), electrical and magnetic fields, ultrasound, shock waves, substrate stimuli, etc., to promote the repair and regeneration of bone and cartilage tissue. Emphasis is placed on the mechanism of cellular response and the potential clinical usage of these stimulations for bone and cartilage regeneration.

Thermally tailored gradient topography surface on elastomeric thin films.

We report a simple method for creating a nanopatterned surface with continuous variation in feature height on an elastomeric thin film. The technique is based on imprinting the surface of a film of thermo-curable elastomer (Sylgard 184), which has continuous variation in cross-linking density introduced by means of differential heating. This results in variation of viscoelasticity across the length of the surface and the film exhibits differential partial relaxation after imprinting with a flexible stamp and subjecting it to an externally applied stress for a transient duration. An intrinsic perfect negative replica of the stamp pattern is initially created over the entire film surface as long as the external force remains active. After the external force is withdrawn, there is partial relaxation of the applied stresses, which is manifested as reduction in amplitude of the imprinted features. Due to the spatial viscoelasticity gradient, the extent of stress relaxation induced feature height reduction varies across the length of the film (L), resulting in a surface with a gradient topography with progressively varying feature heights (hF). The steepness of the gradient can be controlled by varying the temperature gradient as well as the duration of precuring of the film prior to imprinting. The method has also been utilized for fabricating wettability gradient surfaces using a high aspect ratio biomimetic stamp. The use of a flexible stamp allows the technique to be extended for creating a gradient topography on nonplanar surfaces as well. We also show that the gradient surfaces with regular structures can be used in combinatorial studies related to pattern directed dewetting.