1. Introduction The development of a method for integrating highly efficient energy conversion materials onto stretchable, biocompatible rubbers could yield breakthroughs in implantable or wearable energy harvesting systems. Being electromechanically coupled, piezoelectric crystals represent a particularly interesting subset of smart materials which can function as both sensors and actuators. Yet, the crystallization of these materials generally requires high temperatures for maximally efficient performance, rendering them incompatible with temperature-sensitive plastics and rubbers. We have demonstrated a key breakthrough in overcoming this dichotomy, and integrating highly efficient piezoelectric nanomaterials onto flexible, stretchable, and biocompatible substrates. Our piezo-flexing devices achieve both flexibility and biocompatibility via a simple printing process, and fundamental experiments show that the energy harvesting capabilities are among the most efficient recorded on a flexible medium. Printing PZT Nanoribbons onto Rubber Substrates Next-generation applications, such as wearable energy harvesting systems, may require the piezoelectric materials to be flexible, lightweight, and biocompatible. Yet, implementing high performance piezoelectric material systems on lightweight, flexible platforms – such as plastic substrates – has proven elusive due to the difficulty of obtaining high quality crystalline materials on temperature- sensitive plastics. In their stead, piezoelectric polymers such as polyvinylidene difluoride (PVDF) have been used for applications such as shoe-sole power generators, and implantable breath harvesting. However, these polymers suffer a number of drawbacks, including low performance and susceptibility to oxidation. Next-generation applications, such as wearable energy harvesting systems, may require the piezoelectric materials to be flexible, lightweight, and biocompatible. Yet, implementing high performance piezoelectric material systems on lightweight, flexible platforms – such as plastic substrates – has proven elusive due to the difficulty of obtaining high quality crystalline materials on temperature- sensitive plastics. In their stead, piezoelectric polymers such as polyvinylidene difluoride (PVDF) have been used for applications such as shoe-sole power generators, and implantable breath harvesting. However, these polymers suffer a number of drawbacks, including low performance and susceptibility to oxidation. Our group has recently shown that piezoelectric nanoribbons of PZT can be printed onto stretchable, biocompatible rubber substrates over large areas. The key steps are as follows: 1) Preparation of crystalline PZT nanoribbons on MgO source wafers; 2) etching of the MgO wafer in phosphoric acid to free the PZT nanoribbons from their host; and 3) transfer printing of PZT ribbons by bringing a piece of PDMS silicone into conformal contact with the wafer, and quickly peeling it back to retrieve the ordered ribbon arrays. Our group has recently shown that piezoelectric nanoribbons of PZT can be printed onto stretchable, biocompatible rubber substrates over large areas. The key steps are as follows: 1) Preparation of crystalline PZT nanoribbons on MgO source wafers; 2) etching of the MgO wafer in phosphoric acid to free the PZT nanoribbons from their host; and 3) transfer printing of PZT ribbons by bringing a piece of PDMS silicone into conformal contact with the wafer, and quickly peeling it back to retrieve the ordered ribbon arrays. Fundamental Measurement of Piezoelectric Charge Constant Particularly useful for characterizing our piezo-rubber is determining whether the fundamental piezoelectric performance is preserved after transfer of the PZT ribbons to PDMS. In this regard, the most critical indicator is the piezoelectric constant d 33, as the conversion efficiency in piezoelectrics is directly proportional to the piezoelectric charge constant. Particularly useful for characterizing our piezo-rubber is determining whether the fundamental piezoelectric performance is preserved after transfer of the PZT ribbons to PDMS. In this regard, the most critical indicator is the piezoelectric constant d 33, as the conversion efficiency in piezoelectrics is directly proportional to the piezoelectric charge constant. We conducted d 33 measurements before and after transfer printing using piezoforce microscopy (PFM). Our results show that d 33 is 145 pm/V for PZT nanoribbons after poling. Significantly, this value represents the largest charge constant measured in any piezoelectric NW system to date. Measurements of d 33 were also directly performed on PZT NWs transferred onto flexible PDMS substrates. The measured d 33 values for PZT nanoribbons on PDMS represent a ~6-fold improvement over similar values for other flexible piezoelectric systems, including PVDF polymers (d 33, PVDF = -26 pm/V), ZnO nanowires (d 33 = 25 pm/V), and PZT-PVDF hybrid composites (d 33 = 15-25 pm/V). We conducted d 33 measurements before and after transfer printing using piezoforce microscopy (PFM). Our results show that d 33 is 145 pm/V for PZT nanoribbons after poling. Significantly, this value represents the largest charge constant measured in any piezoelectric NW system to date. Measurements of d 33 were also directly performed on PZT NWs transferred onto flexible PDMS substrates. The measured d 33 values for PZT nanoribbons on PDMS represent a ~6-fold improvement over similar values for other flexible piezoelectric systems, including PVDF polymers (d 33, PVDF = -26 pm/V), ZnO nanowires (d 33 = 25 pm/V), and PZT-PVDF hybrid composites (d 33 = 15-25 pm/V). Integrated Flexible Energy Harvesting Devices Efficient, highly portable energy sources have attracted increased interest due to the proliferation of handheld consumer electronics. Decreasing power requirements for mobile electronics open the possibility of augmenting batteries with systems that continuously scavenge otherwise wasted energy from the environment. Most intriguing is the possibility of utilizing work produced by the human body via everyday activities, such as breathing or walking. For example, the heel strike during walking is a particularly rich source of energy, with 67 watts of power available from a brisk walker. Harvesting even 1-5% of that power would be sufficient to run many body-worn devices such as mobile phones. Similarly, lung motion by breathing can generate up to 1 W of power. If this power were harvested into charging a pacemaker battery (~5 W), it may increase the time required between battery replacement surgeries for patients. Efficient, highly portable energy sources have attracted increased interest due to the proliferation of handheld consumer electronics. Decreasing power requirements for mobile electronics open the possibility of augmenting batteries with systems that continuously scavenge otherwise wasted energy from the environment. Most intriguing is the possibility of utilizing work produced by the human body via everyday activities, such as breathing or walking. For example, the heel strike during walking is a particularly rich source of energy, with 67 watts of power available from a brisk walker. Harvesting even 1-5% of that power would be sufficient to run many body-worn devices such as mobile phones. Similarly, lung motion by breathing can generate up to 1 W of power. If this power were harvested into charging a pacemaker battery (~5 W), it may increase the time required between battery replacement surgeries for patients. Body Power: For Biomedical Devices? Large-Scale Nanowire Nanomanufacturing Single-crystal perovskites, such as PZT, are exceptionally efficient piezoelectric energy conversion materials, with demonstrated mechanical to electrical energy conversion efficiencies above 80%. Single-crystal perovskites, such as PZT, are exceptionally efficient piezoelectric energy conversion materials, with demonstrated mechanical to electrical energy conversion efficiencies above 80%. We have recently developed a new nanowire (NW) fabrication approach for patterning wafer-scale PZT NW arrays, which we term photolithography and etching for nanoscale lithography (PENCiL). Our approach employs only low- resolution photolithography and etching to define sub-100 nm diameter NWs. Significantly, the NWs can be generated over an an entire 3 wafer, in parallel, and at precisely defined locations. We have recently developed a new nanowire (NW) fabrication approach for patterning wafer-scale PZT NW arrays, which we term photolithography and etching for nanoscale lithography (PENCiL). Our approach employs only low- resolution photolithography and etching to define sub-100 nm diameter NWs. Significantly, the NWs can be generated over an an entire 3 wafer, in parallel, and at precisely defined locations. Buckled PZT Ribbons for Stretchability Buckled PZT ribbons can also be generated by applying a pre-strain to the PDMS silicone, and then releasing the strain after printing. The competition between elastic bending and stretching energies vs. adhesion energies (van der Waals) of the PZT ribbons with the PDMS substrate result in an energetically favorable detachment of the PZT ribbons into sinusoidal buckled ribbons on the surface. Buckled PZT ribbons can also be generated by applying a pre-strain to the PDMS silicone, and then releasing the strain after printing. The competition between elastic bending and stretching energies vs. adhesion energies (van der Waals) of the PZT ribbons with the PDMS substrate result in an energetically favorable detachment of the PZT ribbons into sinusoidal buckled ribbons on the surface. Applying a subsequent strain to the PDMS/buckled PZT ribbon hybrid structure results in a stretching of the piezoelectric ribbons up to 14% strain. This process is completely reversible, and can be contrasted with the flat ribbons which can stretch only up to ca. 4% strain. The buckled structure thus opens avenues for generating stretchable and biocompatible energy harvesting materials. Applying a subsequent strain to the PDMS/buckled PZT ribbon hybrid structure results in a stretching of the piezoelectric ribbons up to 14% strain. This process is completely reversible, and can be contrasted with the flat ribbons which can stretch only up to ca. 4% strain. The buckled structure thus opens avenues for generating stretchable and biocompatible energy harvesting materials. Key to realizing the ultimate potential of this new paradigm is the integration of these hybrid materials into fully functional flexible energy harvesters. We have developed a method for accessing the d 33 mode to generate electrical power from the piezo-nanoribbons on flexible plastic substrates, by using interdigitated electrodes to pole the PZT NWs in the plane of the device. Key to realizing the ultimate potential of this new paradigm is the integration of these hybrid materials into fully functional flexible energy harvesters. We have developed a method for accessing the d 33 mode to generate electrical power from the piezo-nanoribbons on flexible plastic substrates, by using interdigitated electrodes to pole the PZT NWs in the plane of the device. Measurements of the short-circuit current and open-circuit voltage from the device while tapping on the chip show, encouragingly, that power can be generated simply from small applied biomechanical stresses, such as finger tapping. Due to strain rate effects, the response depends on the frequency: as the tapping frequency is increased to 3.2 Hz, the current and voltage increase to maximal values of ~40 nA and 0.25 V, respectively. Measurements of the short-circuit current and open-circuit voltage from the device while tapping on the chip show, encouragingly, that power can be generated simply from small applied biomechanical stresses, such as finger tapping. Due to strain rate effects, the response depends on the frequency: as the tapping frequency is increased to 3.2 Hz, the current and voltage increase to maximal values of ~40 nA and 0.25 V, respectively.