Soft Tissue and Knee Joint Mechanics
The soft tissues of the knee joint are often assumed to be linear, elastic, isotropic and homogeneous in computational simulations of knee biomechanics, when in fact, most of these tissues are non-linear, viscoelastic, anisotropic, and mechanically heterogeneous. In order to more fully understand the requirements of engineered tissue for applications such as ACL replacement or reconstruction, or to understand the effect of ACL reconstruction on knee biomechanics, we do extensive testing to characterize the non-linear viscoelastic response of both soft tissues and our engineered tissues. We also develop analytical models of tissues and computational simulations of entire anatomical structures, such as the knee, to examine the biomechanics of soft tissue during normal motion and events that result in injury. We demonstrate the effect of neglecting such realities as mechanical heterogeneity and non-linearity on computational predictions of knee biomechanics.
Blast and Impact Mechanics
There has been much work done on the mechanics of blast and its relationship to the optimization of blast-resistant structures. Less work has been done on tailoring material architectures for armor to be used as external protection of delicate structures, such as the brain in a skull. This provides the focus of the current work, in which the mechanics for the mitigation of blast and impact by armor is developed, and then used as the basis for a proposed design for multi-use armor incorporating viscoelastic materials. One high-profile application of this work is the sport helmet. Current helmets used in sports such as football, hockey, and lacrosse do a very good job of reducing the force that is transmitted through the helmet to the skull. This prevents skull fracture and, to a certain extent, it reduces the acceleration of the brain, providing some brain injury protection. However these helmets do very little to mitigate the impulse transmitted to the brain. That is, most of the kinetic energy associated with the impact event is not dissipated by the helmet (or the skull), and the impulse is delivered to the brain causing brain motion relative to the skull and therefore, brain injury. We have developed a design strategy for structures that must dissipate energy over and over and over again using elastic and viscoelastic materials. These impact-resistant structures may be used in helmets, body armor, padding (shoulder pads and shin guards), to better protect wearers from soft tissue injuries, including traumatic brain injury. They may also be used to protect delicate behind armor objects in a wide variety of applications such as electronic packaging, vehicles, and playground surfaces.
Tissue Engineering for Anterior Cruciate Ligament and Rotator Cuff Tendon Repair
A major research area for the Arruda group is the tissue engineering of soft tissues and their interfaces. One of the major challenges in soft tissue engineering of tissues such as muscle, ligament and tendon is the attachment of the engineered tissue to native tissue. In our approach we engineer the tissue interfaces in vitro to provide a structurally viable and biochemically relevant interface at the time the tissue is implanted. A current paradigm in soft tissue engineering is that the mechanical properties of the engineered tissue must match those of the native tissue it is replacing at the time it is implanted. Methods that have used this design paradigm have failed to thrive in vivo, often losing mechanical integrity (stiffness and strength) during the first few weeks and months after implantation. Our approach recognizes that a critical mechanical property that has not been given proper scrutiny in ligament tissue engineering is extensibility, and as long as the engineered tissue can stretch during knee motion, it will not break. This is because ligaments are not loaded by specific weights like the bones of the skeleton. Ligaments must deform to allow movement of joints.
A specific example of what we’re doing is tissue engineering of a bone/ligament/bone construct for use as knee ligament replacement or reconstruction. Anterior cruciate ligament (ACL) reconstruction surgeries using tendon grafts are performed in the US at a rate of nearly 300,000 per year. In addition to the high economic costs associated with knee surgery (6B/yr), acute knee injury is a risk factor for osteoarthritis (OA). Limitations associated with grafting including availability, donor site morbidity, immune rejection, degradation of mechanical properties and incomplete ligamentization, have led investigators to develop strategies to engineer ligament tissue. Current methodologies for engineering ligament using scaffolds suffer similar limitations including immune rejection, degradation, non-physiological intra-articular mechanical properties and incomplete attachment to the bone tunnel. To more closely replicate native intra-articular ligament and promote integration with native bone within the bone tunnel, a multi-phasic engineered ligament with engineered bone at each end and a mechanically viable and biochemically relevant interface between the two tissues would be optimal for replacement. To address this need, our laboratory has developed a scaffold-less method using bone marrow stromal cells (BMSCs) to engineer an in vitro multi-phasic ligament model or bone-ligament-bone (BLB) construct that exhibits the structural and functional interface characteristics of young native tissue. We have demonstrated that our BLB constructs rapidly grow and remodel in vivo to an advanced phenotype, develop a vascular (blood) supply and nerves and physically, mechanically, and biochemically closely resemble the native tissue they have replaced.
Another example application is repair of the supraspinatus tendon of the rotator cuff. This tendon attaches one of the muscles of the rotator cuff to the humerus, providing stability to the shoulder. When torn, the current repair paradigm is to suture it to the humerus. This procedure often results in scarring, pain, and a loss of range of motion. In our laboratory we use a bone-tendon graft to repair the torn tendon. We have preliminary data demonstrating favorable outcomes compared to suture repair.
Mechanics of Polymer Nanocomposites
This area of research deals with the finite deformation mechanics of polymeric (plastic) materials and their composites, which are inherently non-linear viscoelastic materials. (Non-linearity means that the stress response at 20% strain will not be twice that at 10% strain. Viscoelasticity means the response depends on how quickly the polymer is loaded. It also means the response depends on the temperature of loading.) I conduct experiments, develop mathematical models and use numerical approaches to understand how these materials will behave under extreme environments such as high strain rates of loading. I use the same approaches to determine how to make design improvements for the next generation of materials. Applications for this work include automotive crash worthiness and improved armors. Polymers and polymer composites are capable of dissipating energy via several mechanisms, all of which are not fully understood, and dissipation is important because the energy dissipated prior to interaction with a human is not available to cause injury to the human. When riding in an automobile for example, the first thing your various body parts will interact with in the event of a crash is usually a rapidly deforming polymeric material. The challenge in this work is that we cannot predict the high strain rate response of polymers from an understanding of the low strain rate response, even if we understand the strain rate dependence at low strain rates because it changes at high strain rates. At high strain rates of loading the polymer heats up as it deforms because strain energy is dissipated as heat and there is insufficient time for this heat to transfer from the polymer. Therefore we design and conduct experiments and simulations at high strain rates and consider the entire non-linear, thermomechanically coupled response of the materials. Another challenge, especially in extreme rates of loading such as a blast or impact, is to find a way to dissipate the impulse associated with an impact before it reaches the body. Our soldiers and marines are currently surviving encounters with various explosions thanks to current improvements in armor technology, but they often suffer losses of limbs and/or traumatic brain injury as a result. Our goal is to develop the next generation of body and vehicle armor technologies to reduce the effects of blasts and ballistics encounters. The nanocomposites we work with may be applied as tough, clear coatings and interface layers to glasses, and to mitigate the effects of blasts on vehicle and body armors. These same materials will also work to dissipate energy in crash applications.