AFRL-AFOSR-JP-TR-2017-0014 Bio-inspired Nano-capillary Self-powered Fluid Transport in Nanocomposite (NBIT III)

Inspired by composite materials in nature, e.g. bone, with both sufficient mechanical strength and permeability, we designed and fabricated hierarchically organized channels with tapered cross-sections inside a bioceramic. A novel pressurized sintering process was developed to enable the fabrication. The tapering of the cross-sections of the channels was along the channel length and controlled by a pressure gradient during sintering. The resultant composite showed a mechanical strength of the bioceramics with nano-channels built-in similar to that of human bone and an ability to self-power the transport of fluids and nutrients. Notably, by tracing transport of radioactive fludeoxyglucose (18F-FDG), we directly demonstrated sustained cellular growth by self-powered uptake of nutrient via the nano-channels, replicating the expected function of nano-channels in natural system. This design and process can be adapted to a multitude of different composite materials to enhance both their permeability and strength for industrial applications. An additional functionality realized was self-lubrication. It is known that ceramics with a modulus of elasticity higher than that of high carbon chrome bearing steel, ceramic axes and ball-bearing turbos exhibit less deformation at contact points and therefore a greater stress under the same load. Combined with a lower thermal conductivity, their working temperature increases and their load capacity decreases more rapidly in operation. In nature, however, a ceramic composite system, bone, is known for its superior load-bearing capacity, and its self-protection at the high-stress moving-contact points (joints). Less known but no less important is the fact that it does so with self-lubrication, through a built-in capillary networks for self-powered supply of lubricants and coolants, as well as nutrient and growth factor. This is another example from Nature that inspired our efforts and led to designs and demonstrations of multifunctional ceramic composites capable of self-powered supplies of fluidics while retaining and even enhancing the mechanical load-bearing capability for engineering applications. In this annual report, we highlight our attempt to develop a method for generating networks of microand nano-capillaries within a ceramic composite during the sintering process at atmospheric pressure. We then present test results of the multifunctional composites for self-powered supply of fluids to the contact (load-bearing) surface via the capillary networks from a fluid reservoir. As a further extension, a self-regulation mechanism is added into the design to enable temperature-controlled self-powered lubrication, and tested in a model system. The method is adaptable to various structural shapes, scalable in size, and applicable to both bio-physiologic and mechanic composite systems. DISTRIBUTION A. Approved for public release: distribution unlimited. Approach: Permeability and mechanical strength are both essential for bone materials to enable optimized growth and recovery. Bone implants with efficient permeability can provide nutrients to induce fast tissue regeneration. On the other hand, bone implant material should also have sufficient mechanical strength to support body weight, motion, and mechanical impact. However, until now, permeability and mechanical strength were considered as in conflict for engineering micrometer-scale porous materials. To overcome this problem, we designed and fabricated nano-channel built-in ceramic structure to pursue permeability while retaining, and even enhancing, the desired mechanical strength. For validation, we tested the capillarity of various designs of ceramic composites with nano-channel built-in, and observed nutrient delivery in a fabricated ceramic composite sample in situ, by utilizing Positron Emission Tomography-Computed Tomography (PET-CT). Also, fluid transport tests were conducted quantitatively among various designs of nano-channel structure. Moreover, we confirmed the functionality of sustaining cellular activities at the end facet of the ceramic sample enabled by the nano-channels’ self-powered nutrient delivery. The proliferation level and bone mineralization related activity of human bone cells were evaluated. Interestingly, by adding radioactive fludeoxyglucose (18F-FDG) in the fluid at the reservoir end of the nano-channels, we directly demonstrated that actual cellular uptake of 18F-FDG was happening in real time. These results provided the first and conclusive proof that small organisms can proliferate solely by the self-powered supply of nutrients via the nano-channels. One can further expect that the self-powered nano-channel delivery can find applications in other engineering fields, such as ionic charge transport (e.g. distributed battery), heat pipe for efficient cooling, and desalination device with enhanced evaporation/condensation process. Cross-disciplinary Pursuit and Significance: In man-made systems, ceramic load-bearing composites have enabled functional devices, e.g. rotors and valves, to operate in high-temperature, light-weight, and corrosion-resistant applications. However, unlike bones, they are designed and structured so far mainly for load-bearing in these mechanical applications. Separately, there have been biomedical engineering pursuits for composite materials, e.g. synthetic hydroxyapatite (HAP), for bio-physicochemical in vitro and in vivo applications, such as middle ear implants, reconstructive joint replacement, and bone implants etc.). [1-4]. DISTRIBUTION A. Approved for public release: distribution unlimited. It seems only natural for the developments in the two fields to cross paths and benefit from each other’s advances. All the key elements – the concept, theory, design, and base materials are in place and can be co-developed for the interests in developing next generation functional composites for both bio-physiologic and mechanical applications. The project enabled such experimental tests of the concept, theory, and design of self-powered fluid transport in a composite material as a model platform that mimics the bone in functionality, with fluids and nutrients constantly supplied through microand nano-porous networks by capillary forces and/or osmosis [1]. And, like the bone, the primary structural material of body, these porous composites manifest a superior load-bearing capability but can be greater in load-bearing capacity and yet also is cable of self-powered and self-regulated cooling, lubrication, even elf-sustained growth and healing. For this project, we primarily experimented with the hydroxyapatite [HAP: Ca10(PO4)6(OH)2] for structuring the model platform. We performed a number of tests, guided largely by requirements for bone implants. The findings are however generally applicable to other ceramic composites and to engineering applications. HAP, as a most stable calcium phosphate salt at pH between 4 and 12, have been used in catalysis, fertilizers and pharmaceutical, protein chromatography, water treatment processes, and preparation of biocompatible materials. It is the main inorganic component in calcified hard tissues (e.g., bone and teeth) of vertebrates [5, 6]. Pathologically as a result of functional irregularities, it also can form cartilage arthritis, renal, bladder, and bile stones, atheromatic plaque, and calcification of transplanted cardiac valves [7]. As such, HAP has been employed as a model compound to study bio-mineralization phenomena [8-12]. In many ways, targeting bone implant functions in this project represents a greater challenge. That is because bone implant should satisfy a larger set and often more stringent requirements than ceramic parts in conventional rotors and valves. In addition, the implant should be biocompatible, bioactive, or biodegradable [13]. The implant should have a porous three-dimensional architecture capable of supporting cell and tissue infiltration, transport of nutrients, and development of capillaries [14]. It must have the requisite mechanical properties for supporting loads experienced by the bone to be replaced [15]. Considerable improvements in bone implants are necessary to meet these stringent requirements, particularly for applications in the repair of load-bearing bone, one of which would be in self-lubrication and healing. Like the moving parts in a ceramic rotor or valve, bone joints are lubricated. They, a marvel of bio-engineering, are structured to maintain their lubrication with lubricin – a DISTRIBUTION A. Approved for public release: distribution unlimited. principal boundary lubricating and anti-adhesion protein found in a thick colorless liquid that surrounds joints (synovial fluid). These fluids are self-supplied over the parts of the joint not reached by blood vessels that transport nutrition and lubrication, allowing the bones to glide over each other. However this self-lubrication mechanism could be disrupted, damaged, or simply absent, for example in cases of arthritis or the use of today’s implants. Nevertheless, bone grafts are necessary in orthopaedic surgery for filling bone cavities, treatment of nonunion and replacement of bone lost during trauma and tumor removal. Incorporating fluidic, nutrient, and lubricant supply and management into bone implants would represent a significant step forward in bone implant material development. One outcome of this project is that we have made inroads with our HAP composites by structuring within them distributed networks of gradient capillaries, resembling that found in natural bone [16-18], and conducted a series of experiments and tests to show that it is possible to achieve self-powered supply of nutrients in HAP composite structures. In the following, we report on details of the advances we made in building capillary networks within an HAP composite and in enabling continuous and spatially distributed supply of lubricants through load-bearing structures, and the findings as a model platform for future extension to ceramic engine. As a starting point, the HAP composites were formed from a homogeneous mixture of HAP powders and polymers (e.g. PEG or agarose gel). The distributed networks of capillaries in the composite were produced during the sintering process by taking advantage of the phase-separation between the HAP and polymers as well as the pressure-dependent liquid to vapor phase transition. For implant applications, the polymers (e.g. PEG) can be chosen from those that are biocompatible and molecular weight (size) scalable. With this method, high porosity and networked capillaries that are density-variable and size-variable by pressure can be controllably produced in the composite. In year-3, a further innovation was introduced in the process by which a linear or radial gradient of the porosity and pore size can be obtained. It was accomplished by stacking HAP-agarose gel composites of gradually changing weight percentage of the agarose gel. The following describes the collaborative efforts between the two teams and the facile synthesis of HAP composite structures of various shapes, all with built-in capillary networks, and the temperature-controlled self-regulation and self-powered supply of lubricants via these networks. DISTRIBUTION A. Approved for public release: distribution unlimited. Korean Side: led by Seoul National University, aided in conceptualization, design, and theory from the Brown team