Patent Publication Number: US-2020276365-A1

Title: Biodegradable piezoelectric ultrasonic transducer system

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a non-provisional of and claims the benefit of U.S. Provisional Patent Application No. 62/812,491, filed on Mar. 1, 2019, the contents of which are incorporated herein by reference. 
    
    
     COLOR DRAWINGS 
     The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawings will be provided by the Office upon request and payment of the necessary fee. 
     BACKGROUND 
     Piezoelectric materials, a type of “smart” material that generates electricity while deforming and vice versa, are used in many important implantable medical devices such as sensors, transducers, and actuators. Piezoelectric sensors have been used along with medical catheters inside the body to monitor important physiological pressures such as intracranial pressure, blood pressure, bladder pressure, etc. 
     More recently, researchers have developed implanted piezoelectric ultrasonic transducers to disrupt the blood-brain barrier (BBB) and facilitate the delivery of drugs into the brain. The BBB, which is composed of tight junctions between the endothelial cells in the blood vessels of the brain, prevents most therapeutics from accessing the brain tissue and thus is a major hurdle for the treatment of brain diseases (e.g., cancers). There are several established methods for opening the BBB, which include solvent, adjuvant, acoustic wave, lipidization, and osmostic pressure; ultrasound (US) or acoustic waves have been extensively studied and shown to be safe and the most effective tool. However, the use of external US is limited to small animals with thin skull bones. Since the human skull is thick and absorbs more than 90% of US energy, it requires a large and bulky array of external US transducers, a complicated energy-focusing operation, and a tedious MM (magnetic resonance imaging) monitoring procedure. This extensive process would be useful for a single treatment like viral gene delivery-based approaches. However, in certain applications such as chemotherapy, research has shown the opening of the BBB requires repetitive treatment. As such, implanted US transducers (e.g., Sonocloud) have emerged as an alternative, which can repeatedly induce low-intensity sonication deep inside brain tissue at a precise location to open the BBB without causing any damage to the surrounding brain tissue. 
     Unfortunately, all of the aforementioned pressure sensors and US transducers rely on conventional piezoelectric materials such as PZT (lead zirconate titanate), PVDF (polyvinylidene fluoride), ZnO (zinc oxide), etc., which are either toxic and/or non-degradable. Thus, these piezoelectric devices pose significant concerns regarding safety after implantation and require a removal surgery, which is invasive and deleterious to directly interfaced organs or tissues. 
     SUMMARY 
     The disclosure provides a powerful biodegradable and biocompatible piezoelectric nanofiber platform for significant medical implant applications, including a highly sensitive, wireless, biodegradable force sensor for the monitoring of physiological pressures, and a biodegradable ultrasonic transducer for the delivery of drugs across the blood-brain barrier. Built upon materials commonly utilized in medical implants, the devices can self-degrade, causing no harm to the body, and avoid any invasive removal surgeries. 
     The disclosed device is completely biodegradable after a controllable lifetime and biocompatible (as it is made of commonly implanted medical materials, such as PLLA, PLA, PCL, PLGA, Mg, Mo, candelilla wax, etc., which have been used extensively in many FDA-approved erodible implants). Therefore, it doesn&#39;t need an invasive removal surgery which is required for other implanted ultrasonic transducers. Other transducers also rely on toxic materials such as PZT (which contains lead) and therefore there is significant concern with potential leakage and toxicity of the currently-used ultrasonic device. 
     The device is an ultrasonic transducer that can be implanted inside the body (e.g., brain, bone, knee, abdomen etc.) and can generate ultrasonic waves or acoustic pressures that are used to stimulate the opening of biological barriers (such as the blood brain barrier, intestinal epithelial barrier, etc.) to facilitate the diffusion of drugs and increase uptake of drugs into organs (e.g., brain, bone, blood, etc.). The ultrasound generated by the device can also be used to disrupt and kill cancerous tissues through heat generated by cavitation. Wireless communication is another possible application of this device. Specifically, the transducer can be used to emit ultrasonic waves, and could therefore serve as a replacement for all non-degradable RF wireless devices (e.g., NFC, Bluetooth, etc.) or non-biodegradable ultrasonic transceivers, which are intensively used for telecommunication in current electronic implants. 
     In one embodiment, the disclosure provides a biodegradable ultrasonic transducer comprising a first biodegradable metal electrode, a second biodegradable metal electrode, a biodegradable piezoelectric material positioned between the first biodegradable metal electrode and the second biodegradable metal electrode, and an encapsulation layer covering the first biodegradable metal electrode, the second biodegradable metal electrode, and the biodegradable piezoelectric material. 
     In another embodiment, the disclosure provides a biodegradable ultrasonic transducer system comprising a biodegradable ultrasonic transducer described above and a coil coupled to the first biodegradable metal electrode and the second biodegradable metal electrode. 
     In a further embodiment, the disclosure provides a method of constructing a biodegradable ultrasonic transducer. The method comprises electrospinning PLLA nanofiber to form a nanofiber mesh by rotating a drum at a speed of between 2,000-4,000 rpm, annealing the nanofiber mesh between 100° C.-110° C. for a first period of time, annealing the nanofiber mesh between 155° C.-165 C for a second period of time, sandwiching the annealed nanofiber mesh between a first biodegradable metal electrode and a second biodegradable metal electrode to form a sensor, electrically coupling the sensor to a wire, and encapsulating the sensor and the wire with a biodegradable medical polymer. 
     In another embodiment, the disclosure provides a method of delivering a therapeutic through a blood-brain barrier. The method comprises applying the biodegradable ultrasonic transducer that was constructed by the method described above to a craniotomy defect, transmitting an ultrasonic wave signal through the wire, and delivering a pulsed acoustic pressure to the defect. 
     Other aspects of the invention will become apparent by consideration of the detailed description and accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the Office upon request and payment of the necessary fee. 
         FIG. 1  illustrates PLLA nanofibers with highly controllable and excellent piezoelectric performance for biodegradable implanted piezoelectric devices. The image at Top is a simplified schematic of the treated piezoelectric PLLA nanofibers. The image at Bottom Left is the schematic of a biodegradable pressure sensor and ultrasound (US) transducer. The image at Bottom Right is a schematic illustrating the biodegradable US transducer, implanted inside the brain, which can repeatedly induce US to open the blood-brain barrier (BBB) and facilitate the delivery of drugs into the brain. 
         FIG. 2  illustrates material characterization of the electrospun PLLA. (A) Results from differential scanning calorimetry (DSC) of electrospun PLLA nanofiber films collected at different spin speeds. (B) The 2D X-ray diffraction (2D XRD) images show orientation of crystal domains inside the electrospun PLLA nanofibers, made with different collection speeds. (C) Scanning electron microscopy (SEM) images show PLLA nanofiber alignment with different collection speeds. (Scale bars, 40 μm.) (D) Graphical summary illustrating the trend that, as the PLLA nanofibers are collected at faster speeds, the Herman orientation factor (i.e., crystal alignment) and crystallinity percentage generally increase. 
         FIG. 3  illustrates piezoelectric characterization of the treated PLLA nanofiber films. (A) Charge output from stretched, bulk piezo-PLLA (yellow) and treated electrospun PLLA, collected at different speeds, under the same impact force. (B) Displacement of stretched, bulk PLLA (yellow) and treated electrospun PLLA, collected at different speeds, under the same voltage (20 V pp ) at 1 Hz. (C) Displacement of 300 rpm PLLA negative-control sample (red) and 4,000 rpm PLLA (black), under increasing magnitudes of voltage at 1 Hz. (D) Comparison of the piezoelectric performance for 3,000 rpm electrospun PLLA samples annealed under different conditions over a 14-d period. 
         FIG. 4  illustrates a wireless, biodegradable PLLA-nanofiber force sensor. (A) Comparison of calibration curves for a biodegradable sensor using stretched, bulk piezo-PLLA film (black) and a 4,000 rpm electrospun PLLA nanofiber film (red). Inset shows the optical image of the biodegradable and flexible force sensor, made from the PLLA nanofibers. (Scale bar, 5 mm.) (B) Output from a charge amplifying circuit connected to a 4,000 rpm electrospun, biodegradable PLLA sensor that is subjected to 10,000 cycles of a 10-N force. (C) Simplified schematic of the implanted, wireless pressure sensor in a mouse (Left) and optical image of a mouse receiving the wireless PLLA sensor implanted (Right). NFC, near-field communication chip. (D) Comparison of the simulated abdominal pressure signals, wirelessly recorded from an implanted biodegradable PLLA nanofiber sensor using a 300 rpm negative control (black) and a 4,000 rpm film (red). 
         FIG. 5  illustrates US characterization of the biodegradable PLLA-nanofiber transducer. (A) The output pressure from the transducer with different electrospinning speeds under the same input voltage. The Inset is the simplified schematic of the experiment. (B) Output pressure from a biodegradable US transducer made from 4,000 rpm electrospun PLLA under the same input voltage after different days in PBS at 37° C. 
         FIG. 6  illustrates an in vivo experiment to demonstrate the application of PLLA nanofiber transducer for the BBB opening and drug delivering. (A) The schematic (Left) and optical image (Right) of the in vivo experiment. (B) The optical images of a typical biodegradable US transducer at different days in the buffered solution at an accelerated-degradation temperature of 70° C. (Scale bars, 5 mm.) (C) Representative images showing the autofluorescent signal of blood protein at the coronal section (C2) from the brains of mice that received US from the 4,000 rpm PLLA transducer (Left) and the 300 rpm PLLA negative-control transducer (Right). (D) Representative images show the blood protein signal at different coronal sections of the same mouse brain receiving the US treatment. Section C3 (Right) is closer to the implanted transducer, while section C1 (Left) is far away from the implanted US transducer, serving as an internal control. (Scale bars in C and D, 30 μm.) (E) Representative images show the signal of dextran (FITC) at the coronal sections from the brains of mice that received different treatments and samples. The dashed lines show the boundary between the brain and the biodegradable device. The asterisk (*) shows the position of the implanted device. (Scale bars, 50 μm.) 
         FIG. 7  is a schematic of a biodegradable ultrasonic transducer system as disclosed herein. 
         FIG. 8  illustrates a biodegradable ultrasonic transducer as disclosed herein. 
         FIG. 9  illustrates neuronal health after 2 weeks implantation inside rat brain. At 2 weeks, no differences in neuronal density or GFAP expression were observed between the PLLA sensor and stainless steel implants, based on the immunofluoresecent images of NeuN/Caspase-3 (A and B) and GFAP/ED-1 (D and E) images and the quantifications of Neuronal counts (C) and GFAP intensity within 100 μm from the implant at the brain surface, normalized to the deeper region of the same images (F). Two animals and 4 sections each were used. Data presented as mean±SEM). 
         FIG. 10  illustrates neuronal health after 4 week implantation inside rat brain. At 4 weeks, no differences in neuronal density or GFAP expression were observed between the PLLA sensors and stainless-steel implants, based on the immunofluorescent images of NeuN/Caspase-3 (A and B) and GFAP/ED-1 (E and F) and the quantification of neuronal counts (C) and GFAP intensity within 100 μm from the implant at the brain surface, normalized to the deeper region of the same images (G). For quantification, two animals and 4 sections each were used. Data presented as mean±SEM. Additionally, examples of non-implanted control region (D and H) showed similar distribution of NeuN, Caspase-3 (D), GFAP and ED-1 positive cells (H) as the implanted regions. These data clearly describe biocompatibility of our PLLA transducer and its degradation byproducts at least for 4 week implantation inside brain. 
         FIG. 11  graphically demonstrates the ability of transmitting and receiving ultrasonic waves of the biodegradable ultrasonic transducer of  FIG. 8  with different number of piezoelectric PLLA layers. In order to characterize the transmitting properties of the transducer, the acoustic pressure generated from the biodegradable transducers are measured from the capsule hydrophone (A and B). Additionally, the output voltages of the multilayers biodegradable transducers, subjected to a 10 kPa acoustic pressure at 1 MHz, are demonstrated (C and D). 
         FIG. 12  shows that the device generates an audible sound at a frequency of 9 kHz before degrading inside a buffer solution. This shows the ability of the device to vibrate and generate acoustic waves under applied electrical voltage. 
     
    
    
     DETAILED DESCRIPTION 
     Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. 
     Piezoelectricity is a phenomenon which allows materials to convert deformation into electricity and vice versa. Piezoelectric materials are often used for force/pressure sensors, transducers, and generators. The materials can be fabricated into nano- and microstructures and interfaced with soft tissues to monitor biological forces. Since piezoelectric materials can generate electricity from mechanical impact, they can serve as appealing sensing materials, alternative to the described passive semiconductors and capacitive polymers, for self-powered force sensors. However, commonly used piezoelectric materials such as lead zirconate titanate (PZT) and polyvinylidene difluoride (PVDF) contain toxic or non-biodegradable components, respectively, and thus are not favorable for implantation inside the human body. 
     Poly-L-lactic acid (PLLA), a biocompatible and biodegradable polymer has recently been found to exhibit piezoelectricity when appropriately processed, thereby offering an excellent platform to construct safer, biodegradable piezoelectric implants, which can avoid problematic removal surgeries. The material exhibits shear piezoelectricity due to electrical polarity present in the carbon-oxygen double-bond branching off from the polymer backbone chain. Although possessing a modest piezoelectric response (5-15 pC/N), PLLA has a low dielectric constant, which allows the material to perform the same energy-conversion efficacy as the common piezoelectric polymer PVDF. By creating a multilayered structure, one can achieve even higher piezoelectricity from PLLA, with an “effective” conversion efficiency, similar to that of ceramic PZT. 
     Previously, thermally stretched, compression-molded PLLA bulk films were employed to create a biodegradable piezoelectric force sensor. However, stretched PLLA bulk films pose several problems, including low reproducibility, film rigidity, and modest piezoelectric constants (˜5 to 12 pC/N) (20, 21), which render the bulk PLLA films useless for actuators, transducers or highly sensitive pressure sensors. Recently, biodegradable amino acid crystals (e.g., glycine) have been reported with an excellent piezoelectric constant. However, it is challenging to fabricate these powder-based materials into functional films and orient the crystals in a repeatable manner to obtain a controllable piezoelectric performance for device applications. A few researchers have utilized electrospinning to create flexible PLLA piezoelectric nanofiber films, but the reported works struggle with major limitations. First, these reports lack appropriate material processing to stabilize the nanomaterial or utilize the shear-piezoelectric mode (i.e., d14) of PLLA for an optimal piezoelectric performance. Consequently, the PLLA nanofibers can only produce small, unstable electrical signals under applied force. Second, the measured electrical signals are often mixed with other noises caused by friction between the rough nanofiber film and metal electrodes, commonly known as the triboelectric effect. Third, there is no report on the ability to control the piezoelectric performance of the PLLA nanofibers. These major drawbacks collectively restrict applications of this nanomaterial. As a result, there are only a few reported applications of piezoelectric PLLA nanofibers for non-degradable and non-implantable force sensors or energy harvesters. 
     The disclosure provides a strategy for materials processing, device assembly, and electronic integration to 1) achieve biodegradable and biocompatible piezoelectric PLLA nanofibers with a highly controllable, efficient, and stable piezoelectric performance, and 2) demonstrate biodegradable, safe piezoelectric devices built upon this powerful nanomaterial ( FIG. 1 ). First, it is demonstrated that a biodegradable force sensor, made with the PLLA nanofiber film, possesses higher sensitivity and flexibility than that of the reported thermally stretched PLLA bulk film and can be used to wirelessly monitor vital physiological pressures. Second, it is demonstrated that the same PLLA nanofiber sensor acts as a biodegradable ultrasonic transducer that can be implanted into the brain to open the BBB and safely self-degrade, causing no harm to the body. Despite several achievements in the field of biodegradable electronics, this report introduces a biodegradable, highly efficient piezoelectric US transducer, which is only made of materials commonly utilized in medical implants to facilitate the BBB opening for the delivery of drugs into the brain. 
     In order to improve the piezoelectric response of PLLA, the two major material properties that need to be addressed are the crystallinity and orientation of the polymer chains. By improving these properties, the carbon-oxygen double bonds (C═O) present in the helical PLLA backbone become aligned resulting in an inherent net polarization, and a well-documented shear piezoelectric response under applied force. The PLLA nanofibers are made using an electrospinning process. The speed of the rotating drum was varied from 300 to 4,000 rpm, while other parameters such as the voltage applied to the needle, distance to collector, needle gauge, flow rate, and solution concentrations were held constant. This resulted in PLLA nanofiber mats with different levels of fiber orientation. The nanofiber mat samples initially made by the electrospinning setup are highly amorphous and unstable, as seen by the DSC (differential scanning calorimetry). Therefore, the samples were carefully annealed and slowly cooled down in two serial steps at 105° C. and 160.1° C. to improve the crystallinity. After these annealing processes, the crystallinities of the processed nanofiber samples appear to be in about the same range of 70% to 88% (see DSC data of  FIG. 2  (at A)). The nanofiber films, collected at smaller spin speeds, have lower levels of fiber alignment. Therefore, the 300 rpm electrospun PLLA sample was selected as a negative control due to its lower crystallinity and poor fiber orientation, which results in little-to-no piezoelectric effect. X-ray diffraction (XRD) data show that all of the samples are predominantly (200) and (110) (miller index for crystal planes) crystal phases, indicating the presence of (3-form crystal structures, which is the piezoelectric phase of PLLA. Additionally, as seen in the 2D XRD images of  FIG. 2  (at B), electrospinning with a faster collector speed improves the orientation of the crystal domains in each nanofiber. The fiber alignment over the entire film also appeared to increase with faster collector speeds, as seen in the scanning electron microscopy (SEM) images ( FIG. 2  (at C)). However, macroscopic orientation of the PLLA fibers and the molecular alignment are also related to the jet speed (dictated by applied voltage used for electrospinning). By tuning the jet speed to match the drum speed, the optimal piezoelectric PLLA film can be generated. 
     Estimation of the crystallinity (using DSC) and Herman&#39;s orientation factor (using 2D XRD) for the electrospun PLLA samples is described in  FIG. 2  (at D), which shows that improving the collector drum speed generally results in higher crystallinity and crystal alignment in the nanofibers. Thus, by tailoring the collector and jet speeds, the piezoelectricity of the nanofibers can be controlled. 
     The piezoelectric performance of the PLLA nanofiber films was assessed through an impact test (i.e., generation of voltage under impact force) and an actuation test (i.e., displacement under an applied electric field). To create the PLLA sensor for these tests, the PLLA film was annealed and cut at a 45° angle relative to the fiber direction to utilize shear piezoelectricity by maximizing shear force under an applied normal force. The fully treated and cut PLLA films possess a stable, efficient, and highly controllable piezoelectric performance, which has not been achieved by previous reports for the PLLA nanofibers. The films were then sandwiched between aluminum foil electrodes and Kapton tape. For impact testing, the PLLA sensor was subjected to a consistent force induced by an actuator, which was integrated with a dynamic force sensor and driven by a defined voltage waveform. The charge output from the PLLA sample was measured with an electrometer. All of the sensors had the same area of 161.29 mm 2  and thicknesses in the range of 19 to 28 Additionally, prior to fabrication of the sensors, all of the films are soaked in deionized water to minimize the influence of the triboelectric effect. 
       FIG. 3  (at A) illustrates charge outputs from the PLLA samples subjected to a 30-N impact force. The signals generated from the 6 treated PLLA samples clearly show the electrospun sample collected at 4,000 rpm has the largest charge output of about 0.9 nC while the 300 rpm sample exhibits little-to no charge output (˜0.1 nC). The highly aligned nanofiber film, collected at 3,000 and 4,000 rpm drum speeds, noticeably outperforms the bulk piezoelectric PLLA film [annealed and stretched with a 3.5 draw ratio (DR)]. All open-circuit voltage outputs for these piezoelectric nanofiber films were reversible when the electrode connections were swapped, indicating that the PLLA is truly polarized, and that the measured signal is minimally influenced by triboelectricity. 
     The impact measurement was also repeated using dry films, and the resulting data was used to estimate the shear piezoelectric coefficient (d 14 ) for all of the samples. Using the measured mechanical properties of the PLLA films, the piezoelectric constant of the samples was roughly estimated; the 4,000 rpm sample appears to exhibit a d 14  of −19 pC/N, while the conventional bulk PLLA film only exhibits a d 14  of −12 pC/N. This indicates that the processing of PLLA nanofibers significantly improves the material&#39;s shear piezoelectric response. Furthermore, cutting the PLLA films at 45° angles to utilize shear piezoelectricity was also justified by comparing the charge outputs of a 0° and 45° cut film under the same applied force. For the actuation measurement, a treated PLLA film (1.27 cm×1.27 cm) was sandwiched in the center of aluminum foil electrodes (9.53 mm×9.53 mm). A controlled voltage waveform was then applied to the sensor, and the displacement in the exposed right corner of the sample was measured using a laser displacement sensor. As seen in  FIG. 3  (at B), the treated PLLA nanofiber samples vibrate with the same frequency (1 Hz) as the applied sinusoidal voltage waveform (20 V pp ). The 4,000 rpm electrospun sample again exhibits the greatest displacement (˜4.5 μm), while the stretched 3.5 DR bulk film and 300 rpm electrospun samples exhibited no measurable displacement. This result confirms the superior piezoelectric performance of the highly aligned nanofiber film. In addition, as the amplitude of the applied voltage increases, the amplitude of displacement for the electrospun films also increases, and the displacement is frequency dependent ( FIG. 3  (at C). Piezoelectric performance in the treated PLLA nanofiber film is also stable. This advantage is significant as there has been little research to avoid depolarization of the PLLA nanofibers over time. Indeed, as seen in  FIG. 3  (at D), only an electrospun sample (3,000 rpm) that underwent the full annealing processes (i.e., annealed at both 105° C. and 160.1° C.) has a stable piezoelectric output under the same applied force (˜30 N) for 7 d, with a marginal loss (˜6%) in signal at 14 d. In contrast, the untreated (i.e., not annealed) and partially treated (i.e., annealed only at 105° C.) samples rapidly lose their performance and are therefore not stable for long-term implant applications. 
     After verifying the piezoelectric effect of the PLLA nanofibers, a biodegradable force sensor was created by using the nanofibers, molybdenum (Mo) electrodes, and encapsulating untreated PLLA layers ( FIG. 1 ). PLLA or PLA are common biodegradable polymers used in Food and Drug Administration (FDA)-approved implanted tissue scaffolds, bone screws, and drug carriers. Molybdenum is a common nutrient and a biodegradable metal. A biodegradable piezoelectric force sensor was previously reported, however, the device was based on the stretched PLLA bulk film, which is less flexible, exhibits much lower piezoelectric performance, and consequently offers lower sensitivity for force detection. 
       FIG. 4  (at A) clearly illustrates this by showing that the slope of a calibration curve for a biodegradable sensor made with a treated 4,000 rpm electrospun PLLA film is 1.8 times steeper than that of a sensor, using a conventional thermally stretched, bulk PLLA film (DR=3.5). However, when compared to a sensor made of a common non-degradable piezoelectric PVDF-TrFE film (which exhibits a higher d 33  of approximately 34 pC/N), the biodegradable 4,000 rpm electrospun PLLA sensor appears to produce lower-amplitude signals under the same applied force. It is also shown that the charge output from the same 4,000 rpm sensor is stable for over 10,000 cycles of the same impact force (10 N). Not only was the 4,000 rpm electrospun film more sensitive, but its higher crystallinity did not appear to result in any significant changes to the degradation rate when compared to the bulk PLLA piezoelectric film. Without being encapsulated, the PLLA films exhibited a reduction in piezoelectricity after being exposed to aqueous environments due to plastic deformation under an applied load. It was then demonstrated that the nanofiber sensor can be used to monitor intra-abdominal pressure in a mouse. 
     The sensor (5 mm×5 mm) was fully implanted into the abdominal cavity of a mouse and connected to a small printed circuit board (PCB) via a subcutaneous (s.c.) biodegradable wire made of Mo and coated in PLLA. The PCB contains a charge amplifying circuit, a wireless near-field communication (NFC) chip and a commercial antenna. The entire PCB was sealed inside an 18 mm×14 mm PDMS (polydimethylsiloxane) box and subcutaneously implanted at the back of the animal ( FIG. 4  (at C)). Thus, the abdominal sensor and the connecting wires can self-degrade, while the non-degradable PCB could be easily removed at the end of the sensor&#39;s lifetime in a minimally invasive manner. The mouse&#39;s abdomen, filled with saline solution, was then manually stimulated to generate an internal fluid pressure, which mimicked a change in intra-abdominal pressure. A clearly distinguishable signal ( FIG. 4  (at D)) was wirelessly measured while the mouse&#39;s abdomen was periodically depressed and relaxed. The measured pressure signal was then compared to the signal generated by a 300 rpm PLLA sensor (negative control) to verify the signal was not generated by triboelectricity and motion artifacts of the wires ( FIG. 4  (at D)). These results clearly demonstrate the potential of the biodegradable PLLA sensor for monitoring vital physiological pressures inside the body. 
     In addition to monitoring intra-abdominal pressure, it was demonstrated that the same PLLA nanofiber sensor can also be used as a biodegradable ultrasound (US) transducer. The PLLA nanofibers&#39; ability to transmit or receive ultrasonic waves was tested. During US transmission testing ( FIG. 5  (at A, Inset)), a capsule hydrophone was used to measure the acoustic pressure. The PLLA device was driven by a function generator to produce a continuous ultrasonic wave at 1 MHz. As seen in  FIG. 5  (at A), there was no signal detected when the function generator was “off” When the generator was “on,” all PLLA transducers generated distinct acoustic waves while the 300 rpm sample (negative-control sample, non-piezoelectric) resulted in only noise. The trend is similar in the US receiving test; in all of these experiments, the highly aligned 4,000 rpm sample provided the highest conversion signals. Interestingly, the PLLA transducers can act as speakers to generate audible sounds and even play music. 
     A degradation experiment was conducted and demonstrated that a transducer, using encapsulating layers of untreated PLLA (100 μm thick), can have a lifetime of up to 8 d in phosphate buffer saline (PBS) at 37° C. ( FIG. 5  (at B)). Longer functional lifetimes can certainly be achieved by engineering the properties [i.e., thickness or molecular weight (MW)] of the encapsulating PLLA layers or using other biodegradable encapsulating polymers. 
     As proof-of-concept on a potential application of the biodegradable transducer, the PLLA device was employed, made of 4,000 rpm nanofiber samples, for disruption of the BBB in vivo. The experiment is illustrated in  FIG. 6  (at A). A 5 mm×5 mm biodegradable US transducer, which was connected to flexible, biodegradable PLLA-encapsulated Mo wires, was placed on a craniotomy defect in a mouse skull. The spatial pressure field of the biodegradable transducer was recorded. The transducer was operated at 1 MHz to generate an acoustic pressure of 0.3 MPa (rarefaction pressure value) in a series of 2 shots lasting 30 s, with a 30-s break between each shot. The device functioned well in its predefined lifetime and eventually self-degraded ( FIG. 6  (at B)). The brains were processed for fluorescence analysis of bloodborne elements to gauge leakage of the BBB. Two indicators of leakage were intentionally chosen in order to reflect the relative degree of BBB disruption. Tissue autofluorescence at 488 nm was associated with the presence of the 64.5 kDa (in MW) blood protein hemoglobin, which has been suggested to leak across a disrupted BBB. As seen in  FIG. 6  (at C), a noticeable halo of autofluorescence (green stain) could be seen around various microvessels (red stain) in the brains of mice sonicated by the 4,000 rpm transducer. In contrast, no similar signal was observed from the same coronal sections (C2) of the control mouse, sonicated by the 300 rpm non-piezoelectric control sample. Additional brain sections of the control mouse (receiving the non-piezoelectric device) were documented. As further illustrated in  FIG. 6  (at D), for the mice that received US treatment, the closer the coronal sections were to the implanted transducer, the more disrupted vessels were associated with autofluorescent signals. This serves as an internal control and clearly shows the local US-induced BBB opening. The BBB opening was again confirmed by immunofluorescence analysis on the leakage of the serum protein IgG (˜150 kDa). 
     To further certify the potential application of the biodegradable device for delivering therapeutics or pharmaceutical agents through the BBB, another in vivo animal model was performed. The procedure of this experiment was similar to the previous experiment except that the dextran (3 kDa, FITC, Lysine Fixable; Thermo Fisher) as a drug model was retro-orbitally injected into the mice after the sonication process. Additionally, another control group in which mice did not receive the microbubbles before sonication was added to this experiment in order to validate the effect of microbubbles in the BBB opening. As seen in  FIG. 6  (at E, Left), a remarkable level of green signal (FITC) was found around the microvessels in the brain of mice that received the treatments by the 4,000 rpm transducer and microbubbles. It is noticeable that the intensity of the FITC signal is reduced at deeper areas of the brain. On the other hand, no green signal was detected from the same coronal sections of the two control samples,  FIG. 6  (at E, Center and Right). If higher output acoustic pressure and wireless powering are needed for the US transducer, the device can be fabricated with multiple layers of PLLA nanofiber films and utilize the inductive coupling effect. 
     Finally, to demonstrate the biocompatibility of the PLLA nanofiber devices, these devices were implanted subcutaneously into the backs of mice and the intracranial cavity of rats for histology analysis. The histological images from both experiments showed that the device elicits minimal fibrosis and immune response after implantation for 2 and 4 wk. Collectively, these results illustrate that the biodegradable PLLA transducer can be implanted safely into the brain to locally and effectively open the BBB, which could facilitate the delivery of drugs into the brain for the treatment of various brain diseases or disorders. Built upon materials commonly utilized in medical implants, the transducer can self-degrade, causing no harm to the body, and avoid any invasive brain surgery for removal. 
       FIG. 7  illustrates a schematic of a biodegradable piezoelectric ultrasonic transducer system  10 . The biodegradable piezoelectric ultrasonic transducer system  10  includes a transducer  14 , a link  18 , and a coil  22 .  FIG. 8  also illustrates a constructed biodegradable piezoelectric ultrasonic transducer  14  and a link  18 . In one construction, the size of the ultrasonic patch is 5 mm×5 mm. The transducer  14  comprises a first biodegradable metal electrode  26  and a second biodegradable metal electrode  30 . A biodegradable piezoelectric material  34  is positioned between the first electrode  26  and the second electrode  30 . 
     The biodegradable piezoelectric material  34  is positioned between the electrodes  26 ,  30  can be PLLA, silk, glycine, etc., which are all biodegradable and safe for use inside the body. The piezoelectric material  34  may have an area greater than or equal to the area of the electrodes  26 ,  30 . As illustrated in  FIGS. 7-8 , the piezoelectric material  34  includes an area slightly greater than the area of the electrodes  26 ,  30  as the piezoelectric material  34  extends beyond the edges of the electrodes  26 ,  30 . In one construction, the piezoelectric material PLLA can be treated by mechanical stretching and thermal annealing to obtain stable piezoelectric properties (Eli Curry et al. Biodegradable piezoelectric force sensor, PNAS, 2018). The PLLA can also be processed into a stable piezoelectric material through electrospinning and thermal treatment. The piezoelectric film can be prepared by electrospinning a 4% w/v solution of PLLA dissolved in a 1:4 mixture of N,N-Dimethylformamide (DMF) and dichloromethane (DCM). The solution is pumped at a constant rate of 2 ml/hr through a 22-gauge needle with a 14 kV (kilovolts) DC voltage applied to it (Eli Curry et al. Biodegradable piezoelectric nanofiber based transducer, PNAS Jan. 7, 2020 117 (1) 214-220). This electrified solution is then sprayed at a ground aluminum drum rotating at speeds from 300-4,500 rpm (rotations per minute). This results in a nanofiber mat of PLLA (diameter ˜300 nm) with varying degrees of alignment based on rotating drum speed. These fibrous mats are then annealed at 105° C. for 10 hr and allowed to cool to room temperature. They are then annealed at 160° C. for 10 hr and allowed to cool to room temperature. Finally, the electrospun films are cut at a 45° angle relative to the oriented direction in order to harvest the shear piezoelectric signal of the film. 
     The metal electrodes  26 ,  30  can comprise different biodegradable metals, including: Molybdenum (Mo), Magnesium (Mg), Iron (Fe), Zinc (Zn) conducting polymers, etc. or an alloy of any of the previously mentioned metals. The electrodes  26 ,  30  and piezoelectric material  34  are covered in an encapsulation layer  38  with a biodegradable medical polymer. The encapsulation layer  38  can comprise poly(lactic acid) (PLA), poly(lactic-co-glycolic acid) (PLGA), candelilla wax, polycaprolactone (PCL), metals such as Mo or other suitable biodegradable polymer. 
     The link  18  includes a first wire  42  and a second wire  46  coupled to the transducer  14 . The first wire  42  is coupled to the first biodegradable metal electrode  26  and the second wire  46  is coupled to the second biodegradable metal electrode  30 . The first wire  42  and the second wire  46  comprise Mo in one construction and is encapsulated inside a flexible biodegradable encapsulation layers made of PLA, poly(glycerol sebacate) (PGS), poly(octamethylene maleate (anhydride) citrate) (POMaC), PLGA, or other suitable biodegradable polymer. As illustrated in  FIG. 7 , the transducer  14  and the link  18  are encapsulated with the same encapsulation layer  38 . By controlling the thickness, molecular weight, or by using different polymers, the functional-lifetime of the transducer  14  can be engineered and pre-defined prior to implantation. 
     The inductive coil  22  is coupled to the link  18 . The inductive coil  22  includes a first end  50  coupled to the second wire  46  and a second end  54  coupled to the first wire  46 . The inductive coil  22  comprises Mg or Mo which can receive power supplied through a resonant inductive coupling effect from an outside transmitting coil to provide power to the ultrasonic transducer  14 . The inductive coil  22  is encapsulated inside a biodegradable polymer of PLA or PGS or PoMac or PLGA, or another suitable biodegradable polymer. The inductive coil  22  also is biodegradable, similar as the transducer  14 . As illustrated in  FIG. 7 , the transducer  14 , the link  18 , and the coil  22  are encapsulated with the same encapsulation layer  38 . 
     In an alternative construction, the transducer  14  can be connected to a non-degradable link and a non-degradable coil or other electronics to receive power. During the implantation of such a system, the transducer  14  can be implanted into the tissue that it needs to target (e.g., inside the skull, close to dura mater to open the blood brain barrier) while the non-degradable electronics (in replacement of the inductive coil  22  in  FIG. 7 ) can be implanted subcutaneously and far away from the delicate tissue to be targeted. After the transducer  14  is used, the non-degradable electronics can be removed in a minimally-invasive manner while the transducer  14  will self-degrade without the need to be removed thus minimizing surgical risk. 
     In another alternative construction, a transcutaneous wire can be connected to the transducer  14  and an external power source can be used to power the transducer. The wire will be very small. After the functional lifetime of the transducer  14 , the transcutaneous wires can be removed through a minimally invasive surgery while leaving the biodegradable transducer intact within the delicate/important tissue. 
     The biocompatibility of the transducer  14  (including piezoelectric PLLA, encapsulating layer PLA, and electrode Mo) inside the brain has been tested. The result after one month shows minimal immune rejection and an excellent biocompatibility of the device. 
     To examine if implantation of the transducer  14  has caused any inflammatory or damaging reactions on the underlying cortical tissues, the neuronal density and health underneath the transducer and stainless steel samples at week  2  and week  4  were compared (see  FIG. 9  and  FIG. 10 ). The distribution of neurons is similar between the control and the PLLA device ( FIG. 9  (at A and B) and  FIG. 10  (at A and B)), with no significant difference in neuronal density ( FIG. 9  (at C) and  FIG. 10  (at C)). 
     Moreover, all neurons appear healthy, based on the lack of NeuN/Caspase-3 co-localization. Qualitatively, the distribution of GFAP positive cells (astrocytes) and ED-1 positive cells (macrophage in the meningeal layer and activated microglia in the brain) also appear similar between the stainless steel and the sensor-implanted regions, and also between the sensor-implanted regions and non-implanted control regions (i.e., regions without any implants). Quantification of the GFAP intensity showed no significant difference between the two implanted areas ( FIG. 9  (at F) and  FIG. 10  (at F)), and also between the implanted areas and the areas without any implant. Taken together, this histological study suggests that the implanted PLLA-transducer is benign and does not cause harmful host tissue response inside the brain for the periods examined. 
     In another biocompatibility test, the PLLA transducer  10  (with the same structure and materials including piezoelectric PLLA sandwiched between Mo electrodes and encapsulated in PLA) was implanted inside a subcutaneous area of mice and the results show a very minimal inflammation or mild immune response to the implant. 
     It has been shown that the transducer  14  can generate as well as receive ultrasonic wave in a wide range of frequencies.  FIG. 11  shows that a transducer  14  with a varying number of piezoelectric PLLA layers can generate different acoustic pressures under the same applied input voltage at 1 MHz or provide different output voltage values (Vpp) when being subjected to the same applied acoustic pressure (generated by a commercial ultrasonic transmitter) at 1 MHz frequency in water. Increasing the number of PLLA layers boosts the sensitivity or the power of the transducer. 
     It is also shown that the transducer  10  can generate sound under an applied electrical signal. Under an AC input voltage at 9 kHz, the device can generate an audible sound. The transducer  10  degrades afterward as illustrated in  FIG. 12 . 
     Various features and advantages of the invention are set forth in the following claims.