Imagine tiny piezoelectric force sensors that can be placed inside the body: they monitor physiological pressure changes in damaged organs, aid precise drug delivery, or promote tissue repair and regeneration. The best part? They require no battery power, and after use, the body absorbs and degrades them, eliminating the need for invasive removal surgery!
However, traditional piezoelectric materials like inorganic ceramics and organic polymers suffer from inadequate degradability and cytotoxicity. Scientists identified amino acid crystals as a promising candidate – they are biocompatible and exhibit excellent piezoelectric properties. The challenge? These crystals are too small, like scattered sand, making it extremely difficult to align them into functional devices.
Researchers Yi Cao and Bin Xue from Nanjing University found a solution: a special technique called "Mechanical Annealing". Using natural amino acid crystals as the piezoelectric material, they engineered fully organic, biodegradable piezoelectric force sensors. When treated with mechanical annealing, the crystals' power generation capability skyrocketed – achieving a piezoelectric coefficient 12 times higher than that of single-crystal powders! Furthermore, the treated crystal films became smooth and flat, like a phone screen protector, significantly improving contact with the electrodes and enabling stronger, more stable electrical signals.
The resulting "absorbable piezoelectric force sensors", once packaged, were implanted in vivo and successfully monitored dynamic movements like muscle contractions and lung respiration continuously for 4 weeks. Afterwards, they gradually degraded without causing inflammation or systemic toxicity. This breakthrough offers new hope for future medicine, providing a pathway to design and manufacture fully organic, biodegradable force sensors for potential clinical applications!
Fabrication of the Packaged Force Sensor:
Preparation of Mechanically Annealed Crystal Films: Isoleucine was dissolved in deionized water to form a solution, heated, then transferred to an ice-water bath to stand, allowing crystal nuclei to form. The crystals were then collected and dried in an oven. The prepared isoleucine crystals were filled into a tablet mold and subjected to the mechanical annealing process, resulting in round, film-like crystals. Other amino acid crystals and their mechanically annealed counterparts were prepared using the same method.
PLA-PAN Electrode Preparation: Polylactic acid (PLA) was dissolved in dichloromethane (DCM) to form PLA films, serving as the sensor's outer "protective membrane". But a membrane alone isn't enough; sensors need electrodes to collect weak electrical signals. The researchers had a clever trick: they immersed one side of the PLA film in a "reactive solution" (containing sulfuric acid and aniline). After processing, the originally insulating PLA film surface became coated with a conductive layer of polyaniline (PAN), transforming it into a dual-function "PLA-PAN composite electrode" – both protective and conductive.
Integrating the "Power Core": Piezoelectric Crystal Film:The mechanically annealed amino acid crystal film is the "heart" of the sensor, responsible for converting pressure into electrical signals. A square PLA film with a hole in the middle was prepared. The crystal film was placed into this hole, creating a sandwich layer. The PLA-PAN electrodes were cut into square films, and the sandwich layer was placed between them. To ensure a complete seal, a special PLA glue (also made from dissolved PLA) was carefully applied to all edges and surfaces, essentially "wrapping the sandwich in cling film". Once the glue dried, a complete, sealed, packaged force sensor was born!
Optimization for In Vivo Use: For the version actually intended for implantation, scientists made key optimizations:To enhance biodegradability, an eight-arm polyethylene glycol amine hydrochloride(8-arm PEG-NH2·HCl (SUC)) was mixed into the PLA solution, and the crystal film was placed directly between the two PLA-PAN electrodes (omitting the middle PLA film layer), sealed with PLA glue. Polyethylene glycol and its derivatives are among the few polymers certified by the U.S. Food and Drug Administration (FDA) for biomedical products, known for their extremely low cytotoxicity and excellent biocompatibility.
Performance and Significance:
Experimental validation confirmed the designed piezoelectric force sensors possess excellent biocompatibility, long-term stability, and biodegradability.This study is the first to propose the mechanical annealing strategy, enabling the fabrication of large-scale, highly ordered isoleucine crystal materials.
This method is versatile and can be extended to design and manufacture other piezoelectric films based on biomaterials. The highly ordered arrangement of the crystalline phase within the film significantly boosts its macroscopic piezoelectric coefficient, while the flat, smooth surface ensures tight connection with the conductive polymer electrodes. Consequently, the packaged force sensors exhibit high sensitivity and a broad force-sensing range.
Recently, researchers have explored various novel biomolecular crystals using peptides and their derivatives, which exhibit piezoelectric coefficients far exceeding those of the materials used in this study. Given the versatility of the mechanical annealing technique, replacing isoleucine with these novel piezoelectric biomaterials holds great promise for further enhancing the performance of amino acid-based biodegradable force sensors.
The 8-arm PEG-NH2·HCl (SUC) used in this study was sourced from Xiamen Sinopeg Biotech Co., Ltd. .Sinopeg offers a variety of eight-arm (SUC) structured products. Inquiries are welcome!
Reference:
Cheng, Yuanqi, et al. "Boosting the piezoelectric sensitivity of amino acid crystals by mechanical annealing for the engineering of fully degradable force sensors." Advanced Science 10.11 (2023): 2207269.
