Papers by Author: David C.C. Lam

Abstract: Drug dosage delivered by drug-coated microneedle is dependent on needle insertion behavior. The insertion length and gripping force at varied insertion speeds are determined quantitatively using a precision test frame. The ratio of inserted depth to pressed depth was found to rise asymptotically to a plateau, but decreased rapidly to zero insertion when the needles are pressed less than 1000 microns deep for both silicone rubber and porcine skin. No insertion was observed when the needles are pressed less than 200 microns. The gripping force exerted onto the inserted needle by the skin decreased by 0.1N per mm of needle diameter and insertion depth. The short insertion depth and low force suggest that drug delivery using short 300 micron microneedles would be tenuous. High insertion speeds can help to improve drug delivery, but the improvement is limited to large needles since the results from this study showed that insertions become speedindependent when the needle diameter is less than 130 microns.

Abstract: Porous solids are less stiff than the solid, but its stiffness can be increased if the elastic properties of the struts within are sterically stiffened. The stiffening behavior cannot be modeled by conventional strain-based finite element method but can be modeled using couple stress mechanics. In this investigation, a new finite element modeling approach (FEM) based on strain field similitude is developed. The conventional displacement field was used to determine the steric rotation gradients and the additional high-order deformation energy. The mechanics basis underpinning the methodology and the conditions of applicability were detailed, and the results were benchmarked with analytical solution and experimental data. Finally, the methodology was applied to a periodic cellular structure to demonstrate the effect of steric stiffening in nanoporous solids.

Abstract: Structures are assemblies of planar and three-dimensional objects. Planar components and parts are commonly because the deformation behaviors of plates and beams can be analyzed within the plane problem framework. For micron-scale structures, patterning processes in microfabrications are intrinsically planar and the resulting fabricated structures are also planar. These planar micron-scale structures have been designed and analyzed using conventional mechanics, but increasingly as the sizes of these structures become smaller, higher order effects become significant. In nanometer-scale, surfaces were recognized to play significant roles in affecting the physical behavior. Size dependent elastic and plastic deformation behaviors in micron-scale structures were also observed. Size dependence is an intrinsic part of higher order theory of mechanics and has been used successfully to explain scale dependent behavior in threedimensional structures. In this paper, two-dimensional higher order elastic relations in plane stress and plane strain for compressible solids are developed. The difference between the higher order and conventional elasticity theories is compared

Abstract: Conventional strain-based mechanics theory does not account for contributions from strain gradients. Failure to include the strain gradient contributions can lead to underestimates of stresses and size-dependent behaviors in small-scaled structure [1]. This paper focus on the structural size effects on torsion of cylinders. The torsional stiffness of cylinders can be higher than conventional expectation when the cylinder size is in the nanometer - or micron-scale. Following the Saint-Venant theory of torsion, we established the equation of torsion in terms of the warping function on the basis of the nano-mechanical theory of elasticity. The torsional equations contain two higher order material length scale parameters and two conventional Lame constants. The equilibrium equation is a fourth order partial differential equation which can be reduced to two second order equations. Two formulations in terms of pseudo warping function and stress function are presented. Closed-form solutions for circular and thin wall section and series solutions for rectangular microbars have been obtained. The total torque depends only on the stresses conjugated to the strain and is only implicitly dependent on the higher order stress metrics. The solution reveals that the torsional rigidity is dependent on the higher order length scale parameters and strain gradients and increases asymptotically upward when the cylinder size is reduced to the size of the higher order length scale material parameters. The increase is most marked for thin walled cylinders, stiffening to more then 10 times the conventional value when the cylinder size is near that of the higher order length scaled parameters.